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  <entry>
    <title>Java 学习指南_学习Java：接口总结</title>
    <link href="http://yoursite.com/2018/03/28/java-tutorial-learning_interface_summary/"/>
    <id>http://yoursite.com/2018/03/28/java-tutorial-learning_interface_summary/</id>
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    <updated>2018-03-28T07:38:16.891Z</updated>
    
    <content type="html"><![CDATA[<h1 id="接口总结"><a href="#接口总结" class="headerlink" title="接口总结"></a>接口总结</h1><p>接口定义可以包含方法签名，默认方法，静态方法以及常量定义。只有默认方法和静态方法可以有实现。</p>
<p>类如果实现了接口必须实现接口里定义的所有抽象方法。</p>
<p>使用有类型名字的地方都可以使用接口名。</p>
]]></content>
    
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      &lt;h1 id=&quot;接口总结&quot;&gt;&lt;a href=&quot;#接口总结&quot; class=&quot;headerlink&quot; title=&quot;接口总结&quot;&gt;&lt;/a&gt;接口总结&lt;/h1&gt;&lt;p&gt;接口定义可以包含方法签名，默认方法，静态方法以及常量定义。只有默认方法和静态方法可以有实现。&lt;/p&gt;
&lt;p&gt;类如果实现了接口
    
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  <entry>
    <title>Java 学习指南_学习Java：默认方法</title>
    <link href="http://yoursite.com/2018/03/27/java-tutorial-learning_interface_default_methods/"/>
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    <published>2018-03-27T08:50:59.000Z</published>
    <updated>2018-03-28T07:42:06.037Z</updated>
    
    <content type="html"><![CDATA[<h1 id="默认方法Default-Methods"><a href="#默认方法Default-Methods" class="headerlink" title="默认方法Default Methods"></a>默认方法Default Methods</h1><p>接口章节 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html" target="_blank" rel="external">Interfaces</a> 描述了一个例子，关于电脑控制汽车生产厂商发布工业标准接口，调用其中的方法可以对他们的汽车进行操控。如果电脑控制汽车厂商为汽车增加了新的功能，例如飞行的功能，会怎么样呢？这些厂商需要明确新的方法是其他的公司（例如电子制导系统公司）调整他们的软件来控制汽车飞行。这些厂商要在哪里声明这些飞行相关的方法呢？如果他们吧方法添加在原有的接口中，那么已经实现了这些接口的开发者就必须重写他们的实现。如果作为静态方法添加，开发者会将其作为工具方法，而不是必要的和兴方法来对待。</p>
<p>默认方法可以让你为库里的接口添加新的功能，并且在重写旧版本接口的代码时确保两部分功能兼容。</p>
<p>考虑下面的接口, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/TimeClient.java" target="_blank" rel="external"><code>TimeClient</code></a>, 在之前的联系中出现的 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/QandE/interfaces-answers.html" target="_blank" rel="external">Answers to Questions and Exercises: Interfaces</a>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">import</span> java.time.*; </div><div class="line"> </div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setTime</span><span class="params">(<span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setDate</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setDateAndTime</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year,</span></span></div><div class="line">                               <span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second);</div><div class="line">    <span class="function">LocalDateTime <span class="title">getLocalDateTime</span><span class="params">()</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>下面的类, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/SimpleTimeClient.java" target="_blank" rel="external"><code>SimpleTimeClient</code></a>, 实现了上面的接口 <code>TimeClient</code>:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div></pre></td><td class="code"><pre><div class="line">package defaultmethods;</div><div class="line"></div><div class="line">import java.time.*;</div><div class="line">import java.lang.*;</div><div class="line">import java.util.*;</div><div class="line"></div><div class="line">public class SimpleTimeClient implements TimeClient &#123;</div><div class="line">    </div><div class="line">    private LocalDateTime dateAndTime;</div><div class="line">    </div><div class="line">    public SimpleTimeClient() &#123;</div><div class="line">        dateAndTime = LocalDateTime.now();</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    public void setTime(int hour, int minute, int second) &#123;</div><div class="line">        LocalDate currentDate = LocalDate.from(dateAndTime);</div><div class="line">        LocalTime timeToSet = LocalTime.of(hour, minute, second);</div><div class="line">        dateAndTime = LocalDateTime.of(currentDate, timeToSet);</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    public void setDate(int day, int month, int year) &#123;</div><div class="line">        LocalDate dateToSet = LocalDate.of(day, month, year);</div><div class="line">        LocalTime currentTime = LocalTime.from(dateAndTime);</div><div class="line">        dateAndTime = LocalDateTime.of(dateToSet, currentTime);</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    public void setDateAndTime(int day, int month, int year,</div><div class="line">                               int hour, int minute, int second) &#123;</div><div class="line">        LocalDate dateToSet = LocalDate.of(day, month, year);</div><div class="line">        LocalTime timeToSet = LocalTime.of(hour, minute, second); </div><div class="line">        dateAndTime = LocalDateTime.of(dateToSet, timeToSet);</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    public LocalDateTime getLocalDateTime() &#123;</div><div class="line">        return dateAndTime;</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    public String toString() &#123;</div><div class="line">        return dateAndTime.toString();</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    public static void main(String... args) &#123;</div><div class="line">        TimeClient myTimeClient = new SimpleTimeClient();</div><div class="line">        System.out.println(myTimeClient.toString());</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>Suppose that you want to add new functionality to the <code>TimeClient</code> interface, such as the ability to specify a time zone through a <a href="https://docs.oracle.com/javase/8/docs/api/java/time/ZonedDateTime.html" target="_blank" rel="external"><code>ZonedDateTime</code></a> object (which is like a<a href="https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html" target="_blank" rel="external"><code>LocalDateTime</code></a> object except that it stores time zone information):</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setTime</span><span class="params">(<span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setDate</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setDateAndTime</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year,</span></span></div><div class="line">        <span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second);</div><div class="line">    <span class="function">LocalDateTime <span class="title">getLocalDateTime</span><span class="params">()</span></span>;                           </div><div class="line">    <span class="function">ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>下面对于接口 <code>TimeClient</code>的修改，你也必须修改 类 <code>SimpleTimeClient</code> 并实现其中新增的方法 <code>getZonedDateTime</code>. 不过, 与其将 <code>getZonedDateTime</code> 作为 <code>abstract</code> 抽象的,不如将它替换为默认方法 <em>default implementation</em>. (记住 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html" target="_blank" rel="external">abstract method</a> 是没有实现的方法.)</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"> </div><div class="line"><span class="keyword">import</span> java.time.*;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setTime</span><span class="params">(<span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setDate</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">setDateAndTime</span><span class="params">(<span class="keyword">int</span> day, <span class="keyword">int</span> month, <span class="keyword">int</span> year,</span></span></div><div class="line">                               <span class="keyword">int</span> hour, <span class="keyword">int</span> minute, <span class="keyword">int</span> second);</div><div class="line">    <span class="function">LocalDateTime <span class="title">getLocalDateTime</span><span class="params">()</span></span>;</div><div class="line">    </div><div class="line">    <span class="function"><span class="keyword">static</span> ZoneId <span class="title">getZoneId</span> <span class="params">(String zoneString)</span> </span>&#123;</div><div class="line">        <span class="keyword">try</span> &#123;</div><div class="line">            <span class="keyword">return</span> ZoneId.of(zoneString);</div><div class="line">        &#125; <span class="keyword">catch</span> (DateTimeException e) &#123;</div><div class="line">            System.err.println(<span class="string">"Invalid time zone: "</span> + zoneString +</div><div class="line">                <span class="string">"; using default time zone instead."</span>);</div><div class="line">            <span class="keyword">return</span> ZoneId.systemDefault();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">        </div><div class="line">    <span class="function"><span class="keyword">default</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>在方法签名前使用 <code>default</code> 关键字来明确接口中的方法是默认方法。接口中定义的所有方法包括默认方法，都是<code>public</code>的，因此你可以省略<code>public</code>修饰符。</p>
<p>这样定义接口的话，你就不需要修改类 <code>SimpleTimeClient</code>, 这个类 (以及所有实现了接口 <code>TimeClient</code>的类),都会已定义的 <code>getZonedDateTime</code> 方法.下面的例子, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/TestSimpleTimeClient.java" target="_blank" rel="external"><code>TestSimpleTimeClient</code></a>,调用了<code>SimpleTimeClient</code>一个实例的 <code>getZonedDateTime</code> 的方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"> </div><div class="line"><span class="keyword">import</span> java.time.*;</div><div class="line"><span class="keyword">import</span> java.lang.*;</div><div class="line"><span class="keyword">import</span> java.util.*;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TestSimpleTimeClient</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String... args)</span> </span>&#123;</div><div class="line">        TimeClient myTimeClient = <span class="keyword">new</span> SimpleTimeClient();</div><div class="line">        System.out.println(<span class="string">"Current time: "</span> + myTimeClient.toString());</div><div class="line">        System.out.println(<span class="string">"Time in California: "</span> +</div><div class="line">            myTimeClient.getZonedDateTime(<span class="string">"Blah blah"</span>).toString());</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<h2 id="扩展包含有默认方法的接口Extending-Interfaces-That-Contain-Default-Methods"><a href="#扩展包含有默认方法的接口Extending-Interfaces-That-Contain-Default-Methods" class="headerlink" title="扩展包含有默认方法的接口Extending Interfaces That Contain Default Methods"></a><a href="">扩展包含有默认方法的接口Extending Interfaces That Contain Default Methods</a></h2><p>当你从一个包含有默认方法的接口进行扩展时，你可以有一下做法：</p>
<ul>
<li>不提及默认方法，让扩展的几口继承原有的默认方法</li>
<li>重新定义默认方法，声明为<code>abstract</code></li>
<li>重新定义默认方法，复写它</li>
</ul>
<p>假设你按照下面的写法扩展 <code>TimeClient</code> 接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">AnotherTimeClient</span> <span class="keyword">extends</span> <span class="title">TimeClient</span> </span>&#123; &#125;</div></pre></td></tr></table></figure>
<p>任何实现了 <code>AnotherTimeClient</code> 接口的类都有默认方法 <code>TimeClient.getZonedDateTime</code>的实现.</p>
<p>假设你按照如下写法扩展 `TimeClient接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">AbstractZoneTimeClient</span> <span class="keyword">extends</span> <span class="title">TimeClient</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">public</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>任何实现接口 <code>AbstractZoneTimeClient</code>的类都必须重新实现方法 <code>getZonedDateTime</code>;这个方法现在是抽象方法 <code>abstract</code> ，就像接口中其他的非默认和非静态方法一样 。</p>
<p>假设你按照如下写法扩展 `TimeClient接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">HandleInvalidTimeZoneClient</span> <span class="keyword">extends</span> <span class="title">TimeClient</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">default</span> <span class="keyword">public</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span> </span>&#123;</div><div class="line">        <span class="keyword">try</span> &#123;</div><div class="line">            <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(),ZoneId.of(zoneString)); </div><div class="line">        &#125; <span class="keyword">catch</span> (DateTimeException e) &#123;</div><div class="line">            System.err.println(<span class="string">"Invalid zone ID: "</span> + zoneString +</div><div class="line">                <span class="string">"; using the default time zone instead."</span>);</div><div class="line">            <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(),ZoneId.systemDefault());</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>任何实现 <code>HandleInvalidTimeZoneClient</code> 接口的类都会用其中的 <code>getZonedDateTime</code> 方法实现类替代原有接口 <code>TimeClient</code>中的默认方法实现.</p>
<h2 id="静态方法Static-Methods"><a href="#静态方法Static-Methods" class="headerlink" title="静态方法Static Methods"></a><a href="">静态方法Static Methods</a></h2><p>默认方法之外，你也可以在接口中定义静态方法 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html" target="_blank" rel="external">static methods </a>.（静态方法属于类而不是其他的对象。类的每个实例都可以分享类它的静态方法）这样能让你更加容易的组织你库里的辅助方法；你可以将静态方法定义在统一接口中而不是其他的类里面。下面的例子定义了一个静态方法，获取一个 <a href="https://docs.oracle.com/javase/8/docs/api/java/time/ZoneId.html" target="_blank" rel="external"><code>ZoneId</code></a> 对象先关的时区标识符；如果该对象没有给出相关的时区标识符则使用系统默认的时区标识符。（这样你就可以简化方法 <code>getZonedDateTime</code>）:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TimeClient</span> </span>&#123;</div><div class="line">    <span class="comment">// ...</span></div><div class="line">    <span class="function"><span class="keyword">static</span> <span class="keyword">public</span> ZoneId <span class="title">getZoneId</span> <span class="params">(String zoneString)</span> </span>&#123;</div><div class="line">        <span class="keyword">try</span> &#123;</div><div class="line">            <span class="keyword">return</span> ZoneId.of(zoneString);</div><div class="line">        &#125; <span class="keyword">catch</span> (DateTimeException e) &#123;</div><div class="line">            System.err.println(<span class="string">"Invalid time zone: "</span> + zoneString +</div><div class="line">                <span class="string">"; using default time zone instead."</span>);</div><div class="line">            <span class="keyword">return</span> ZoneId.systemDefault();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    <span class="function"><span class="keyword">default</span> <span class="keyword">public</span> ZonedDateTime <span class="title">getZonedDateTime</span><span class="params">(String zoneString)</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));</div><div class="line">    &#125;    </div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>就像类中定义的静态方法一样，在接口中定义静态方法同样是在方法签名前面加上<code>static</code>关键字。因为接口中定义的所有方法都是<code>public</code>的，所以你可以省略<code>public</code>修饰符。</p>
<h2 id="在已有的代码库中融入默认方法Integrating-Default-Methods-into-Existing-Libraries"><a href="#在已有的代码库中融入默认方法Integrating-Default-Methods-into-Existing-Libraries" class="headerlink" title="在已有的代码库中融入默认方法Integrating Default Methods into Existing Libraries"></a><a href="">在已有的代码库中融入默认方法Integrating Default Methods into Existing Libraries</a></h2><p>默认方法可以你是你在已有的接口中添加新的功能，并同时确保兼容老版本接口写的代码。特别的是，默认方法允许你添加接受lambda表达式作为参数。本节演示了如何使用默认方法和静态方法增强 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html" target="_blank" rel="external"><code>Comparator</code></a>接口.</p>
<p>考虑 <code>Card</code> 以及 <code>Deck</code> 类，在之前的联系中出现过 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/creating-questions.html" target="_blank" rel="external">Questions and Exercises: Classes</a>.本例重写了两个类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/Card.java" target="_blank" rel="external"><code>Card</code></a> 以及 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/Deck.java" target="_blank" rel="external"><code>Deck</code></a> 作为接口. <code>Card</code> 接口包含了两个枚举类型 <code>enum</code> (<code>Suit</code> 和 <code>Rank</code>) 以及两个抽象方法(<code>getSuit</code> 和 <code>getRank</code>):</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Card</span> <span class="keyword">extends</span> <span class="title">Comparable</span>&lt;<span class="title">Card</span>&gt; </span>&#123;</div><div class="line">    </div><div class="line">    <span class="keyword">public</span> <span class="keyword">enum</span> Suit &#123; </div><div class="line">        DIAMONDS (<span class="number">1</span>, <span class="string">"Diamonds"</span>), </div><div class="line">        CLUBS    (<span class="number">2</span>, <span class="string">"Clubs"</span>   ), </div><div class="line">        HEARTS   (<span class="number">3</span>, <span class="string">"Hearts"</span>  ), </div><div class="line">        SPADES   (<span class="number">4</span>, <span class="string">"Spades"</span>  );</div><div class="line">        </div><div class="line">        <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">int</span> value;</div><div class="line">        <span class="keyword">private</span> <span class="keyword">final</span> String text;</div><div class="line">        Suit(<span class="keyword">int</span> value, String text) &#123;</div><div class="line">            <span class="keyword">this</span>.value = value;</div><div class="line">            <span class="keyword">this</span>.text = text;</div><div class="line">        &#125;</div><div class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">value</span><span class="params">()</span> </span>&#123;<span class="keyword">return</span> value;&#125;</div><div class="line">        <span class="function"><span class="keyword">public</span> String <span class="title">text</span><span class="params">()</span> </span>&#123;<span class="keyword">return</span> text;&#125;</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    <span class="keyword">public</span> <span class="keyword">enum</span> Rank &#123; </div><div class="line">        DEUCE  (<span class="number">2</span> , <span class="string">"Two"</span>  ),</div><div class="line">        THREE  (<span class="number">3</span> , <span class="string">"Three"</span>), </div><div class="line">        FOUR   (<span class="number">4</span> , <span class="string">"Four"</span> ), </div><div class="line">        FIVE   (<span class="number">5</span> , <span class="string">"Five"</span> ), </div><div class="line">        SIX    (<span class="number">6</span> , <span class="string">"Six"</span>  ), </div><div class="line">        SEVEN  (<span class="number">7</span> , <span class="string">"Seven"</span>),</div><div class="line">        EIGHT  (<span class="number">8</span> , <span class="string">"Eight"</span>), </div><div class="line">        NINE   (<span class="number">9</span> , <span class="string">"Nine"</span> ), </div><div class="line">        TEN    (<span class="number">10</span>, <span class="string">"Ten"</span>  ), </div><div class="line">        JACK   (<span class="number">11</span>, <span class="string">"Jack"</span> ),</div><div class="line">        QUEEN  (<span class="number">12</span>, <span class="string">"Queen"</span>), </div><div class="line">        KING   (<span class="number">13</span>, <span class="string">"King"</span> ),</div><div class="line">        ACE    (<span class="number">14</span>, <span class="string">"Ace"</span>  );</div><div class="line">        <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">int</span> value;</div><div class="line">        <span class="keyword">private</span> <span class="keyword">final</span> String text;</div><div class="line">        Rank(<span class="keyword">int</span> value, String text) &#123;</div><div class="line">            <span class="keyword">this</span>.value = value;</div><div class="line">            <span class="keyword">this</span>.text = text;</div><div class="line">        &#125;</div><div class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">value</span><span class="params">()</span> </span>&#123;<span class="keyword">return</span> value;&#125;</div><div class="line">        <span class="function"><span class="keyword">public</span> String <span class="title">text</span><span class="params">()</span> </span>&#123;<span class="keyword">return</span> text;&#125;</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    <span class="keyword">public</span> Card.<span class="function">Suit <span class="title">getSuit</span><span class="params">()</span></span>;</div><div class="line">    <span class="keyword">public</span> Card.<span class="function">Rank <span class="title">getRank</span><span class="params">()</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p> <code>Deck</code> 接口包含了许多操作cards的方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods; </div><div class="line"> </div><div class="line"><span class="keyword">import</span> java.util.*;</div><div class="line"><span class="keyword">import</span> java.util.stream.*;</div><div class="line"><span class="keyword">import</span> java.lang.*;</div><div class="line"> </div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Deck</span> </span>&#123;</div><div class="line">    </div><div class="line">    <span class="function">List&lt;Card&gt; <span class="title">getCards</span><span class="params">()</span></span>;</div><div class="line">    <span class="function">Deck <span class="title">deckFactory</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">int</span> <span class="title">size</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">addCard</span><span class="params">(Card card)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">addCards</span><span class="params">(List&lt;Card&gt; cards)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">addDeck</span><span class="params">(Deck deck)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">shuffle</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">sort</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">sort</span><span class="params">(Comparator&lt;Card&gt; c)</span></span>;</div><div class="line">    <span class="function">String <span class="title">deckToString</span><span class="params">()</span></span>;</div><div class="line"></div><div class="line">    <span class="function">Map&lt;Integer, Deck&gt; <span class="title">deal</span><span class="params">(<span class="keyword">int</span> players, <span class="keyword">int</span> numberOfCards)</span></span></div><div class="line">        <span class="keyword">throws</span> IllegalArgumentException;</div><div class="line"></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/PlayingCard.java" target="_blank" rel="external"><code>PlayingCard</code></a> 实现了 接口 <code>Card</code>, 类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/StandardDeck.java" target="_blank" rel="external"><code>StandardDeck</code></a> 实现了接口 <code>Deck</code>.</p>
<p>类 <code>StandardDeck</code> 实现了类的 <code>Deck.sort</code> 方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">StandardDeck</span> <span class="keyword">implements</span> <span class="title">Deck</span> </span>&#123;</div><div class="line">    </div><div class="line">    <span class="keyword">private</span> List&lt;Card&gt; entireDeck;</div><div class="line">    </div><div class="line">    <span class="comment">// ...</span></div><div class="line">    </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">sort</span><span class="params">()</span> </span>&#123;</div><div class="line">        Collections.sort(entireDeck);</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    <span class="comment">// ...</span></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p> <code>Collections.sort</code> 方法讲一个List实例进行了排序，List实例的元素类型是实现了 <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html" target="_blank" rel="external"><code>Comparable</code></a>接口.成员变量 <code>entireDeck</code> 是一个元素为<code>Card</code>的 <code>List</code>实例。 <code>Card</code>实现了<code>Comparable</code> 接口. 类 <code>PlayingCard</code> 实现了 <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html#compareTo-T-" target="_blank" rel="external"><code>Comparable.compareTo</code></a> 方法，内容如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">hashCode</span><span class="params">()</span> </span>&#123;</div><div class="line">    <span class="keyword">return</span> ((suit.value()-<span class="number">1</span>)*<span class="number">13</span>)+rank.value();</div><div class="line">&#125;</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compareTo</span><span class="params">(Card o)</span> </span>&#123;</div><div class="line">    <span class="keyword">return</span> <span class="keyword">this</span>.hashCode() - o.hashCode();</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p> 方法 <code>StandardDeck.sort()</code> 通过 <code>compareTo</code> 方法是扑克先按照花色再按照点数排序。如果你想要让扑克先按照点数，再按照花色排序该怎么做呢？你可能需要实现<code>Comparator</code>接口来明确新的排序规则，并使用方法<a href="https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#sort-java.util.List-java.util.Comparator-" target="_blank" rel="external"><code>sort(List list, Comparator c)</code></a> ( <code>sort</code>方法包含<code>Comparator</code>参数的版本).你可以在类<code>StandardDeck</code>中定义如下方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">sort</span><span class="params">(Comparator&lt;Card&gt; c)</span> </span>&#123;</div><div class="line">    Collections.sort(entireDeck, c);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>有了这个方法你可以，你是可以使用 <code>Collections.sort</code> 明确 <code>Card</code> 类实例的排序规则.一种方法是实现 <code>Comparator</code> 接口来明确扑克排序的规则.下面的类 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/examples/defaultmethods/SortByRankThenSuit.java" target="_blank" rel="external"><code>SortByRankThenSuit</code></a> 进行了示例:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">package</span> defaultmethods;</div><div class="line"></div><div class="line"><span class="keyword">import</span> java.util.*;</div><div class="line"><span class="keyword">import</span> java.util.stream.*;</div><div class="line"><span class="keyword">import</span> java.lang.*;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">SortByRankThenSuit</span> <span class="keyword">implements</span> <span class="title">Comparator</span>&lt;<span class="title">Card</span>&gt; </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compare</span><span class="params">(Card firstCard, Card secondCard)</span> </span>&#123;</div><div class="line">        <span class="keyword">int</span> compVal =</div><div class="line">            firstCard.getRank().value() - secondCard.getRank().value();</div><div class="line">        <span class="keyword">if</span> (compVal != <span class="number">0</span>)</div><div class="line">            <span class="keyword">return</span> compVal;</div><div class="line">        <span class="keyword">else</span></div><div class="line">            <span class="keyword">return</span> firstCard.getSuit().value() - secondCard.getSuit().value(); </div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>下面的调用时扑克先按照点数再按照花色排序:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">StandardDeck myDeck = <span class="keyword">new</span> StandardDeck();</div><div class="line">myDeck.shuffle();</div><div class="line">myDeck.sort(<span class="keyword">new</span> SortByRankThenSuit());</div></pre></td></tr></table></figure>
<p>不过，这种方法太冗长verbose；如果你可以明确按照什么排序而不是怎么排序会更好。假设你是写<code>Compatator</code>接口 开发者。你会在<code>Comparator</code>接口中添加什么默认方法或者静态方法来是其他开发者更加简单的明确排序规则？</p>
<p>一开始，你想要将这些扑克按照点数排序而不管花色。你可以这样调用 `StandardDeck.sort方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line">StandardDeck myDeck = <span class="keyword">new</span> StandardDeck();</div><div class="line">myDeck.shuffle();</div><div class="line">myDeck.sort(</div><div class="line">    (firstCard, secondCard) -&gt;</div><div class="line">        firstCard.getRank().value() - secondCard.getRank().value()</div><div class="line">);</div></pre></td></tr></table></figure>
<p>因为接口 <code>Comparator</code> 是一个功能接口 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#approach6" target="_blank" rel="external">functional interface</a>, 所以你可以使用lambda表达式作为<code>sort</code>方法的参数。本例中，lambda表达式比较了两个整数值。</p>
<p>这样可以是开发者通过只调用<code>Card.getRank</code>方法就简单的创建一个<code>Comparator</code>实例。并且，他可以付诸开发者<code>Comparator</code>实例来比较任何可以通过入住<code>getValue</code> <code>hashCode</code>等方法返回数字值的对象。<code>Comparator</code>接口已经通过静态方法 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparing-java.util.function.Function-java.util.Comparator-" target="_blank" rel="external"><code>comparing</code></a>增强而拥有这个能力:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(Comparator.comparing((card) -&gt; card.getRank()));</div></pre></td></tr></table></figure>
<p>这个例子中，你也可以通过方法引用来代替 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html" target="_blank" rel="external">method reference</a>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(Comparator.comparing(Card::getRank));</div></pre></td></tr></table></figure>
<p>这种调用很好的演示了用什么排序而不是怎么排序.</p>
<p> <code>Comparator</code> 接口同样有其他类似<code>comparing</code>版本的静态方法增强如  <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparingDouble-java.util.function.ToDoubleFunction-java.util.Comparator-" target="_blank" rel="external"><code>comparingDouble</code></a> 和 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparingLong-java.util.function.ToLongFunction-" target="_blank" rel="external"><code>comparingLong</code></a> 可以用来创建通过其他数据类型比较的 <code>Comparator</code> 实例.</p>
<p>假设你的开发者想要创建一个 <code>Comparator</code> 实例，使用不止一个条件比较对象。例如，你如何将扑克先按照点数再按照花色排序？像前面的例子一样，你可以使用lambda表达式来明确这些排序规则:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line">StandardDeck myDeck = <span class="keyword">new</span> StandardDeck();</div><div class="line">myDeck.shuffle();</div><div class="line">myDeck.sort(</div><div class="line">    (firstCard, secondCard) -&gt; &#123;</div><div class="line">        <span class="keyword">int</span> compare =</div><div class="line">            firstCard.getRank().value() - secondCard.getRank().value();</div><div class="line">        <span class="keyword">if</span> (compare != <span class="number">0</span>)</div><div class="line">            <span class="keyword">return</span> compare;</div><div class="line">        <span class="keyword">else</span></div><div class="line">            <span class="keyword">return</span> firstCard.getSuit().value() - secondCard.getSuit().value();</div><div class="line">    &#125;      </div><div class="line">);</div></pre></td></tr></table></figure>
<p>如果开发者可以通过一系列的 <code>Comparator</code> 实例来构造 <code>Comparator</code> 实例，将会更加简单. <code>Comparator</code> 接口已经通过默认方法  <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparing-java.util.Comparator-" target="_blank" rel="external"><code>thenComparing</code></a>增强而拥有了这个能力:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(</div><div class="line">    Comparator</div><div class="line">        .comparing(Card::getRank)</div><div class="line">        .thenComparing(Comparator.comparing(Card::getSuit)));</div></pre></td></tr></table></figure>
<p> <code>Comparator</code> 接口同样有其他版本的 <code>thenComparing</code>方法 (例如  <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparingDouble-java.util.function.ToDoubleFunction-" target="_blank" rel="external"><code>thenComparingDouble</code></a> 以及 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparingLong-java.util.function.ToLongFunction-" target="_blank" rel="external"><code>thenComparingLong</code></a>) 这样你可以构造 <code>Comparator</code> 实例来比较其他的数据类型.</p>
<p>假设你的开发者想要创建 <code>Comparator</code> 实例，倒序排列一个集合，例如你想要将扑克按照点数降序排列，从A到2.像之前一样你可以定义lambda表达式来实现。不过，如果可以通过已有的<code>Comparator</code>实例调用方法将会更简单。<code>Comparator</code>接口已经预知了默认方法 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#reversed--" target="_blank" rel="external"><code>reversed</code></a>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">myDeck.sort(</div><div class="line">    Comparator.comparing(Card::getRank)</div><div class="line">        .reversed()</div><div class="line">        .thenComparing(Comparator.comparing(Card::getSuit)));</div></pre></td></tr></table></figure>
<p>这个例子演示了 <code>Comparator</code> 接口是如何通过默认方法，静态方法，lambda表达式，以及方法引用来增强的，这样就创建了更有表现力的库和方法，功能开发者可以快速的推断出该如何调用其中的方法。用同样的构造来创建你自己的库里的接口。</p>
]]></content>
    
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      &lt;h1 id=&quot;默认方法Default-Methods&quot;&gt;&lt;a href=&quot;#默认方法Default-Methods&quot; class=&quot;headerlink&quot; title=&quot;默认方法Default Methods&quot;&gt;&lt;/a&gt;默认方法Default Methods&lt;/h1&gt;&lt;p&gt;接口
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：进化接口</title>
    <link href="http://yoursite.com/2018/03/27/java-tutorial-learning_interface_evolving/"/>
    <id>http://yoursite.com/2018/03/27/java-tutorial-learning_interface_evolving/</id>
    <published>2018-03-27T08:40:59.000Z</published>
    <updated>2018-03-27T08:38:28.780Z</updated>
    
    <content type="html"><![CDATA[<h1 id="进化接口-Interfaces"><a href="#进化接口-Interfaces" class="headerlink" title="进化接口 Interfaces"></a>进化接口 Interfaces</h1><p>考虑一下你之前开发的一个名为 <code>DoIt</code>的接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoIt</span> </span>&#123;</div><div class="line">   <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>假设，过了一段时间以后，你想要在接口中添加一个方法，那么接口会变成下面的样子:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoIt</span> </span>&#123;</div><div class="line"></div><div class="line">   <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line">   <span class="function"><span class="keyword">boolean</span> <span class="title">didItWork</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x, String s)</span></span>;</div><div class="line">   </div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>如果要做这个改变，那么所有实现了旧的<code>DoIt</code>接口的类都不能再使用了，因为他们没有实现接口的所有方法。依赖这个接口的开发者将会大声抗议。</p>
<p>试着在开始就预见你的借口的所有用处并做完整的定义。如果你想要在接口中添加额外的方法，可以有几种选择。你可以创建一个 <code>DoItPlus</code> 接口来扩展 <code>DoIt</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoItPlus</span> <span class="keyword">extends</span> <span class="title">DoIt</span> </span>&#123;</div><div class="line"></div><div class="line">   <span class="function"><span class="keyword">boolean</span> <span class="title">didItWork</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x, String s)</span></span>;</div><div class="line">   </div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>现在你代码的用户可以选择继续使用旧的接口，或者升级到新的接口。</p>
<p>同样，也可以定义新的方法为默认方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html" target="_blank" rel="external">default methods</a>. 下面的列子定义了一个名为<code>didItWork</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">DoIt</span> </span>&#123;</div><div class="line"></div><div class="line">   <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line">   <span class="function"><span class="keyword">default</span> <span class="keyword">boolean</span> <span class="title">didItWork</span><span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x, String s)</span> </span>&#123;</div><div class="line">       <span class="comment">// Method body </span></div><div class="line">   &#125;</div><div class="line">   </div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>注意，你必须为默认方法提供一个实现。你也可以在现存从接口中定义一个新的静态方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html#static" target="_blank" rel="external">static methods</a>。实现了这个接口的用户可以通过默认方法或者静态方法类增强他们的功能，而不需要为适应新增的方法修改和重新编译所有的类。</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;进化接口-Interfaces&quot;&gt;&lt;a href=&quot;#进化接口-Interfaces&quot; class=&quot;headerlink&quot; title=&quot;进化接口 Interfaces&quot;&gt;&lt;/a&gt;进化接口 Interfaces&lt;/h1&gt;&lt;p&gt;考虑一下你之前开发的一个名为 &lt;co
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：使用接口作为一个类型</title>
    <link href="http://yoursite.com/2018/03/26/java-tutorial-learning_interface_use_as_type/"/>
    <id>http://yoursite.com/2018/03/26/java-tutorial-learning_interface_use_as_type/</id>
    <published>2018-03-26T09:40:59.000Z</published>
    <updated>2018-03-28T07:41:07.079Z</updated>
    
    <content type="html"><![CDATA[<h1 id="使用接口作为一个类型"><a href="#使用接口作为一个类型" class="headerlink" title="使用接口作为一个类型"></a>使用接口作为一个类型</h1><p>当你定义一个新的接口，你定义了一个新的引用数据类型。使用其他引用类型名的地方都可以使用接口名。如果你定义的一个接口的引用类型变量，赋值必须为实现了这个接口的类的实例。</p>
<p>下面的例子，定义了一个找到size比较大的对象的方法，对象可以是任何实现了<code>Relatable</code>接口的类的实例：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> Object <span class="title">findLargest</span><span class="params">(Object object1, Object object2)</span> </span>&#123;</div><div class="line">   Relatable obj1 = (Relatable)object1;</div><div class="line">   Relatable obj2 = (Relatable)object2;</div><div class="line">   <span class="keyword">if</span> ((obj1).isLargerThan(obj2) &gt; <span class="number">0</span>)</div><div class="line">      <span class="keyword">return</span> object1;</div><div class="line">   <span class="keyword">else</span> </div><div class="line">      <span class="keyword">return</span> object2;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>通过转换 <code>object1</code> 类型为 <code>Relatable</code> 类型, 就可以调用其 <code>isLargerThan</code> 方法.</p>
<p>如果你注意到很多的类都实现了 <code>Relatable</code>接口，任何这些类的实例都可以通过方法 <code>findLargest()</code>进行比较—只要他们是同一个类的对象.同样,他们都可以通过下面的方法进行比较:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> Object <span class="title">findSmallest</span><span class="params">(Object object1, Object object2)</span> </span>&#123;</div><div class="line">   Relatable obj1 = (Relatable)object1;</div><div class="line">   Relatable obj2 = (Relatable)object2;</div><div class="line">   <span class="keyword">if</span> ((obj1).isLargerThan(obj2) &lt; <span class="number">0</span>)</div><div class="line">      <span class="keyword">return</span> object1;</div><div class="line">   <span class="keyword">else</span> </div><div class="line">      <span class="keyword">return</span> object2;</div><div class="line">&#125;</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">isEqual</span><span class="params">(Object object1, Object object2)</span> </span>&#123;</div><div class="line">   Relatable obj1 = (Relatable)object1;</div><div class="line">   Relatable obj2 = (Relatable)object2;</div><div class="line">   <span class="keyword">if</span> ( (obj1).isLargerThan(obj2) == <span class="number">0</span>)</div><div class="line">      <span class="keyword">return</span> <span class="keyword">true</span>;</div><div class="line">   <span class="keyword">else</span> </div><div class="line">      <span class="keyword">return</span> <span class="keyword">false</span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>这个方法对所有的”relatable”对象都有效,不管他们的继承成了什么，当他们实现了 <code>Relatable</code>接口,他们同时可以是谈么本身（或者其父类）的类型， 也可以是一个 <code>Relatable</code> 类型.这就给了他们多重继承的优势，他们同时可以表现出其父类和接口的行为.</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;使用接口作为一个类型&quot;&gt;&lt;a href=&quot;#使用接口作为一个类型&quot; class=&quot;headerlink&quot; title=&quot;使用接口作为一个类型&quot;&gt;&lt;/a&gt;使用接口作为一个类型&lt;/h1&gt;&lt;p&gt;当你定义一个新的接口，你定义了一个新的引用数据类型。使用其他引用类型名的地方
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：实现接口</title>
    <link href="http://yoursite.com/2018/03/26/java-tutorial-learning_interface_implements/"/>
    <id>http://yoursite.com/2018/03/26/java-tutorial-learning_interface_implements/</id>
    <published>2018-03-26T08:21:59.000Z</published>
    <updated>2018-03-26T10:22:29.576Z</updated>
    
    <content type="html"><![CDATA[<h1 id="实现一个接口Implementing-an-Interface"><a href="#实现一个接口Implementing-an-Interface" class="headerlink" title="实现一个接口Implementing an Interface"></a>实现一个接口Implementing an Interface</h1><p>定义一个实现接口的类，需要在类定义时包含 <code>implements</code> ，一个类可以事多个接口所以，<code>implements</code> 关键字后可以跟一个逗号分隔的实现接口的列表. 按照惯例, <code>implements</code> 引用跟随在 <code>extends</code> 扩展引用之后如果有的话.</p>
<h2 id="一个简单的接口，相关联的Relatable"><a href="#一个简单的接口，相关联的Relatable" class="headerlink" title="一个简单的接口，相关联的Relatable"></a>一个简单的接口，相关联的Relatable</h2><p>考虑定义一个比较对象size的接口.</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line">public interface Relatable &#123;</div><div class="line">        </div><div class="line">    // this (object calling isLargerThan)</div><div class="line">    // and other must be instances of </div><div class="line">    // the same class returns 1, 0, -1 </div><div class="line">    // if this is greater than, </div><div class="line">    // equal to, or less than other</div><div class="line">    public int isLargerThan(Relatable other);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>如果你想过要比较相似的对象的size，不管他们是什么类型的对象，可实例化类必须要实现这个接口 <code>Relatable</code>.</p>
<p>任何类都可以实现这个<code>Relatable</code>接口，如果有方法可以比较实例化对象的相对size.例如字符串，可以通过字符数来比较，例如书本，可以通过页数来比较，例如学生，可以通过体重类比较等。例如平面几何图形，面积会是一个好的选择。(参考，下面的 <code>RectanglePlus</code>类),立体形状，则可以通过体积来比较.所有的这些类都可以实现 <code>isLargerThan()</code>方法.</p>
<p>如果你知道一个类实现了 <code>Relatable</code>接口,那么你就可以比较这个类的实例化对象的size.</p>
<h2 id="实现Relatable接口"><a href="#实现Relatable接口" class="headerlink" title="实现Relatable接口"></a>实现Relatable接口</h2><p>一下是矩形类 <code>Rectangle</code>，在 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html" target="_blank" rel="external">Creating Objects</a> 出现过, 重写这个类来实现 <code>Relatable</code>接口.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">RectanglePlus</span> </span></div><div class="line">    <span class="keyword">implements</span> <span class="title">Relatable</span> &#123;</div><div class="line">    <span class="keyword">public</span> <span class="keyword">int</span> width = <span class="number">0</span>;</div><div class="line">    <span class="keyword">public</span> <span class="keyword">int</span> height = <span class="number">0</span>;</div><div class="line">    <span class="keyword">public</span> Point origin;</div><div class="line"></div><div class="line">    <span class="comment">// four constructors</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="title">RectanglePlus</span><span class="params">()</span> </span>&#123;</div><div class="line">        origin = <span class="keyword">new</span> Point(<span class="number">0</span>, <span class="number">0</span>);</div><div class="line">    &#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="title">RectanglePlus</span><span class="params">(Point p)</span> </span>&#123;</div><div class="line">        origin = p;</div><div class="line">    &#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="title">RectanglePlus</span><span class="params">(<span class="keyword">int</span> w, <span class="keyword">int</span> h)</span> </span>&#123;</div><div class="line">        origin = <span class="keyword">new</span> Point(<span class="number">0</span>, <span class="number">0</span>);</div><div class="line">        width = w;</div><div class="line">        height = h;</div><div class="line">    &#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="title">RectanglePlus</span><span class="params">(Point p, <span class="keyword">int</span> w, <span class="keyword">int</span> h)</span> </span>&#123;</div><div class="line">        origin = p;</div><div class="line">        width = w;</div><div class="line">        height = h;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    <span class="comment">// a method for moving the rectangle</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">move</span><span class="params">(<span class="keyword">int</span> x, <span class="keyword">int</span> y)</span> </span>&#123;</div><div class="line">        origin.x = x;</div><div class="line">        origin.y = y;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    <span class="comment">// a method for computing</span></div><div class="line">    <span class="comment">// the area of the rectangle</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getArea</span><span class="params">()</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> width * height;</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    <span class="comment">// a method required to implement</span></div><div class="line">    <span class="comment">// the Relatable interface</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">isLargerThan</span><span class="params">(Relatable other)</span> </span>&#123;</div><div class="line">        RectanglePlus otherRect </div><div class="line">            = (RectanglePlus)other;</div><div class="line">        <span class="keyword">if</span> (<span class="keyword">this</span>.getArea() &lt; otherRect.getArea())</div><div class="line">            <span class="keyword">return</span> -<span class="number">1</span>;</div><div class="line">        <span class="keyword">else</span> <span class="keyword">if</span> (<span class="keyword">this</span>.getArea() &gt; otherRect.getArea())</div><div class="line">            <span class="keyword">return</span> <span class="number">1</span>;</div><div class="line">        <span class="keyword">else</span></div><div class="line">            <span class="keyword">return</span> <span class="number">0</span>;               </div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>因为矩形类 <code>RectanglePlus</code> 实现了 <code>Relatable</code>,任一两个 <code>RectanglePlus</code> 对象的size事可以比较的.</p>
<hr>
<p><strong>注意</strong>:<code>isLargerThan</code>方法，是在 <code>Relatable</code>接口中定义的，接口参数类型为<code>Relatable</code>.将<code>other</code>装换为 <code>RectanglePlus</code>实例，类型转换告诉编译器<code>other</code>对象本身是什么类型，直接在<code>other</code>对象上调用<code>other.getArea()</code>方法将会编译失败。</p>
<hr>
]]></content>
    
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      &lt;h1 id=&quot;实现一个接口Implementing-an-Interface&quot;&gt;&lt;a href=&quot;#实现一个接口Implementing-an-Interface&quot; class=&quot;headerlink&quot; title=&quot;实现一个接口Implementing an Interfac
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：定义接口</title>
    <link href="http://yoursite.com/2018/03/26/java-tutorial-learning_interface_define/"/>
    <id>http://yoursite.com/2018/03/26/java-tutorial-learning_interface_define/</id>
    <published>2018-03-26T07:40:59.000Z</published>
    <updated>2018-03-26T08:18:50.151Z</updated>
    
    <content type="html"><![CDATA[<h1 id="定义一个接口Defining-an-Interface"><a href="#定义一个接口Defining-an-Interface" class="headerlink" title="定义一个接口Defining an Interface"></a>定义一个接口Defining an Interface</h1><p>接口的定义包含了锌锭修饰符，关键字<code>interface</code>，接口名称，逗号分割的父接口（如果有的话）,以及接口体,例如:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">GroupedInterface</span> <span class="keyword">extends</span> <span class="title">Interface1</span>, <span class="title">Interface2</span>, <span class="title">Interface3</span> </span>&#123;</div><div class="line"></div><div class="line">    <span class="comment">// constant declarations</span></div><div class="line">    </div><div class="line">    <span class="comment">// base of natural logarithms</span></div><div class="line">    <span class="keyword">double</span> E = <span class="number">2.718282</span>;</div><div class="line"> </div><div class="line">    <span class="comment">// method signatures</span></div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">doSomething</span> <span class="params">(<span class="keyword">int</span> i, <span class="keyword">double</span> x)</span></span>;</div><div class="line">    <span class="function"><span class="keyword">int</span> <span class="title">doSomethingElse</span><span class="params">(String s)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p><code>public</code>修饰符表明了接口可以被任何包中的任何类使用。如果你不加这个修饰符，接口就只能被定义在用一个包中的类作为接口实现。</p>
<p>接口可以扩展自其它接口，与类扩展其它的类一样。不过，区别是类值能扩展一个类，但是一个几口可以扩展任何数量的接口，接口定义中包含了他扩展的逗号分割的父接口。</p>
<h2 id="接口体The-Interface-Body"><a href="#接口体The-Interface-Body" class="headerlink" title="接口体The Interface Body"></a>接口体The Interface Body</h2><p>接口体可以包含 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html" target="_blank" rel="external">抽象方法abstract methods</a>, <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html" target="_blank" rel="external">默认default methods</a>, 以及 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html#static" target="_blank" rel="external">静态static methods</a>. 接口中的抽象方法由分号结尾，但是不包括大括号包围的方法体（即没有方法的实现）。默认方法有<code>default</code>修饰符定义，静态方法由 <code>static</code>关键字。接口中所有的抽象方法，默认方法，静态方法 都是<code>public</code>,所以你可以省略 <code>public</code>修饰符.</p>
<p>另外，几口可以包含常量声明。接口中定义的所有常量都是 <code>public</code>, <code>static</code>, 并且 <code>final</code>的.所有你可以省略这些限定修饰符.</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;定义一个接口Defining-an-Interface&quot;&gt;&lt;a href=&quot;#定义一个接口Defining-an-Interface&quot; class=&quot;headerlink&quot; title=&quot;定义一个接口Defining an Interface&quot;&gt;&lt;/a&gt;定义一个接
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：接口</title>
    <link href="http://yoursite.com/2018/03/26/java-tutorial-learning_interface/"/>
    <id>http://yoursite.com/2018/03/26/java-tutorial-learning_interface/</id>
    <published>2018-03-26T07:40:59.000Z</published>
    <updated>2018-03-28T07:43:09.443Z</updated>
    
    <content type="html"><![CDATA[<h1 id="接口"><a href="#接口" class="headerlink" title="接口"></a>接口</h1><p>软件工程中的很多情况写，让各个独立的开发小组遵循一个”合约”，来详细说明他们的软件是如何相互作用的。每个小组应该能再完全不知道其他小组写的代码的情况下编写自己的代码。广泛的说，接口<code>interfaces</code>就是这样的合约。</p>
<p>例如，想象一下未来社会，电脑控制的机器人汽车运载乘客穿过城市的街道而不需要人为的操作。汽车制造商编写软件（当然，用java语言）来控制他们的汽车，停止，启动，加速，左转等等。另外一个工业小组，电子制导设备制造商，制造电脑系统来接受GPS（Global Positioning System）位置信息以及无线传输的交通状况信息，并根据这些信息驾驶汽车。</p>
<p>汽车制造商必须发布一个工业标准详细说明调用什么方法可以让汽车移动（任何一家厂商的任何一辆车）。这时候制导系统厂商就可以编写软件调用接口所规定的方法命令汽车运行。这样工业组并不需要了解其方法具体是怎么实现的，汽车厂商也高度保留了其所有权并可以随时修改其实现，同时又遵循与已经公布的接口标准。</p>
<h2 id="Java中的接口"><a href="#Java中的接口" class="headerlink" title="Java中的接口"></a>Java中的接口</h2><p>Java编程语言中，接口interface是一个引用类型，与类class相似，不过只可以包含常量，方法签名，默认方法，静态方法，嵌套类型。只有默认方法和静态方法可以存在方法体。接口不能被实例化——只能被类实现或者被其他的借口扩展。后续的借口将会讲到扩展的内容。</p>
<p>定义一个接口与定义一个类非常类似:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">OperateCar</span> </span>&#123;</div><div class="line"></div><div class="line">   <span class="comment">// constant declarations, if any</span></div><div class="line"></div><div class="line">   <span class="comment">// method signatures</span></div><div class="line">   </div><div class="line">   <span class="comment">// An enum with values RIGHT, LEFT</span></div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">turn</span><span class="params">(Direction direction,</span></span></div><div class="line">            <span class="keyword">double</span> radius,</div><div class="line">            <span class="keyword">double</span> startSpeed,</div><div class="line">            <span class="keyword">double</span> endSpeed);</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">changeLanes</span><span class="params">(Direction direction,</span></span></div><div class="line">                   <span class="keyword">double</span> startSpeed,</div><div class="line">                   <span class="keyword">double</span> endSpeed);</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">signalTurn</span><span class="params">(Direction direction,</span></span></div><div class="line">                  <span class="keyword">boolean</span> signalOn);</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">getRadarFront</span><span class="params">(<span class="keyword">double</span> distanceToCar,</span></span></div><div class="line">                     <span class="keyword">double</span> speedOfCar);</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">getRadarRear</span><span class="params">(<span class="keyword">double</span> distanceToCar,</span></span></div><div class="line">                    <span class="keyword">double</span> speedOfCar);</div><div class="line">         ......</div><div class="line">   <span class="comment">// more method signatures</span></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>注意方法签名没有大括号包围的方法体有分号终结。</p>
<p>要使用接口，你学写一个类来实现implements这个接口。当一个科实例化的类实现一个接口是，他提供接口中定义的方法的方法体，例如：,</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">OperateBMW760i</span> <span class="keyword">implements</span> <span class="title">OperateCar</span> </span>&#123;</div><div class="line"></div><div class="line">    <span class="comment">// the OperateCar method signatures, with implementation --</span></div><div class="line">    <span class="comment">// for example:</span></div><div class="line">    <span class="function"><span class="keyword">int</span> <span class="title">signalTurn</span><span class="params">(Direction direction, <span class="keyword">boolean</span> signalOn)</span> </span>&#123;</div><div class="line">       <span class="comment">// code to turn BMW's LEFT turn indicator lights on</span></div><div class="line">       <span class="comment">// code to turn BMW's LEFT turn indicator lights off</span></div><div class="line">       <span class="comment">// code to turn BMW's RIGHT turn indicator lights on</span></div><div class="line">       <span class="comment">// code to turn BMW's RIGHT turn indicator lights off</span></div><div class="line">    &#125;</div><div class="line"></div><div class="line">    <span class="comment">// other members, as needed -- for example, helper classes not </span></div><div class="line">    <span class="comment">// visible to clients of the interface</span></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>上面是机器人汽车例子中，需要实现接口的事汽车制造商。雪佛兰Chevrolet的实现可能与丰田Toyota的实现相当的不同，当然两家厂商都会遵循同样的接口。制导系统厂商，作为接口的消费者，创建系统用来根据汽车的GPS数据，数字地图，你及交通数据驾驶汽车。这么做的做，制导系统厂商只需调用接口中的方法: turn, change lanes, brake, accelerate等等。</p>
<h2 id="接口做为API"><a href="#接口做为API" class="headerlink" title="接口做为API"></a>接口做为API</h2><p>机器人汽车的例子展示了接口作为工业标准<em>Application Programming Interface (API)</em>. APIs在商业软件中也同样常见。通常，一个公司出售的软件包包含了其他公司想要在他们的产品中使用的复杂方法。一个数字图像处理方法的软件包出售给制作终端用户平面设计软件的公司。图形处理公司写一些类来实现一个接口，这个类对他们的客户开放。平面设计软件公司通过使用接口中定义的方法签名和返回类型使用图形处理方法。这样在图形处理公司的API是公开的，但是API的实现作为私密信息得到保护——事实上他们可以在后续长期工作中根据其客户依赖的原有接口不断的改进其实现方法。</p>
]]></content>
    
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      &lt;h1 id=&quot;接口&quot;&gt;&lt;a href=&quot;#接口&quot; class=&quot;headerlink&quot; title=&quot;接口&quot;&gt;&lt;/a&gt;接口&lt;/h1&gt;&lt;p&gt;软件工程中的很多情况写，让各个独立的开发小组遵循一个”合约”，来详细说明他们的软件是如何相互作用的。每个小组应该能再完全不知道其他小组写的代
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：接口与继承</title>
    <link href="http://yoursite.com/2018/03/26/java-tutorial-learning_interface&amp;inheritance/"/>
    <id>http://yoursite.com/2018/03/26/java-tutorial-learning_interface&amp;inheritance/</id>
    <published>2018-03-26T07:26:59.000Z</published>
    <updated>2018-03-26T07:08:41.149Z</updated>
    
    <content type="html"><![CDATA[<h1 id="课程-接口与继承"><a href="#课程-接口与继承" class="headerlink" title="课程: 接口与继承"></a>课程: 接口与继承</h1><h2 id="接口Interface"><a href="#接口Interface" class="headerlink" title="接口Interface"></a><a href="https://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html" target="_blank" rel="external">接口Interface</a></h2><p>上一个课程中你已经见到过如果实现一个接口。本节将学到更多关于接口的内容-接口是什么，为什么需要写接口，以及如何写接口。</p>
<h2 id="继承Inheritance"><a href="#继承Inheritance" class="headerlink" title="继承Inheritance"></a><a href="https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html" target="_blank" rel="external">继承Inheritance</a></h2><p>该部分讲述了你如何从一个类衍生出新的类。也就是，一个子类<em>subclass</em>如何继承父类/超类<em>superclass</em>的方法和字段。将学习如果自<code>Object</code>类衍生出新的类，以及如何修改子类从父类继承的方法。同样会覆盖到类似接口的抽象类<em>abstarct class</em></p>
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  <entry>
    <title>Java 学习指南_学习Java：注解-问题与练习</title>
    <link href="http://yoursite.com/2018/03/26/java-tutorial-learning_annotation_q&amp;e/"/>
    <id>http://yoursite.com/2018/03/26/java-tutorial-learning_annotation_q&amp;e/</id>
    <published>2018-03-26T04:26:59.000Z</published>
    <updated>2018-03-26T10:20:29.287Z</updated>
    
    <content type="html"><![CDATA[<h1 id="问题与练习-注解"><a href="#问题与练习-注解" class="headerlink" title="问题与练习: 注解"></a>问题与练习: 注解</h1><h2 id="问题"><a href="#问题" class="headerlink" title="问题"></a>问题</h2><ol>
<li><p>问题: 线面的接口有哪些错误:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">House</span> </span>&#123;</div><div class="line">    <span class="meta">@Deprecated</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>​</p>
</li>
<li><p><strong>问题</strong>: 考虑下面的 <code>House</code> 类，实现了问题1中的借口.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MyHouse</span> <span class="keyword">implements</span> <span class="title">House</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>如果你编译这个程序，编译器会产生警告，因为<code>open</code>方法是反对的（在接口中），你应该做什么来避免警告？</p>
</li>
<li><p>下面的代码在编译时是否会发生错误？为什么？</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> Meal &#123; ... &#125;</div><div class="line"></div><div class="line"><span class="meta">@Meal</span>(<span class="string">"breakfast"</span>, mainDish=<span class="string">"cereal"</span>)</div><div class="line"><span class="meta">@Meal</span>(<span class="string">"lunch"</span>, mainDish=<span class="string">"pizza"</span>)</div><div class="line"><span class="meta">@Meal</span>(<span class="string">"dinner"</span>, mainDish=<span class="string">"salad"</span>)</div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">evaluateDiet</span><span class="params">()</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
</li>
</ol>
<h2 id="练习"><a href="#练习" class="headerlink" title="练习"></a>练习</h2><ol>
<li>练习: 定义一个增强请求的注解类型，包含元素<code>id</code>,<code>synopsis</code>,<code>engineer</code>, 以及<code>date</code>. 设定engineer默认值为<code>unassigned</code>，date默认值为<code>unknown</code>.</li>
</ol>
<h1 id="问题与练习答案-注解"><a href="#问题与练习答案-注解" class="headerlink" title="问题与练习答案: 注解"></a>问题与练习答案: 注解</h1><a id="more"></a>
<h2 id="问题答案"><a href="#问题答案" class="headerlink" title="问题答案"></a>问题答案</h2><ol>
<li><p>问题: 线面的接口有哪些错误:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">House</span> </span>&#123;</div><div class="line">    <span class="meta">@Deprecated</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p><strong>答案</strong> 文档应该反映出 <code>open</code> 为什么是反对的以及用什么来代替，例如:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">House</span> </span>&#123; </div><div class="line">    <span class="comment">/**</span></div><div class="line">     * <span class="doctag">@deprecated</span> use of open </div><div class="line">     * is discouraged, use</div><div class="line">     * openFrontDoor or </div><div class="line">     * openBackDoor instead.</div><div class="line">     */</div><div class="line">    <span class="meta">@Deprecated</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span></span>; </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span></span>;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
</li>
<li><p><strong>问题</strong>: 考虑下面的 <code>House</code> 类，实现了问题1中的借口.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MyHouse</span> <span class="keyword">implements</span> <span class="title">House</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>如果你编译这个程序，编译器会产生警告，因为<code>open</code>方法是反对的（在接口中），你应该做什么来避免警告？</p>
<p><strong>答案</strong>: 你可以为<code>open</code>方法的实现添加deprecate注解:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MyHouse</span> <span class="keyword">implements</span> <span class="title">House</span> </span>&#123; </div><div class="line">    <span class="comment">// The documentation is </span></div><div class="line">    <span class="comment">// inherited from the interface.</span></div><div class="line">    <span class="meta">@Deprecated</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span> </span>&#123;&#125; </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>同样，你也可以通过<code>@SuppressWarings</code>注解镇压这个警告:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MyHouse</span> <span class="keyword">implements</span> <span class="title">House</span> </span>&#123; </div><div class="line">    <span class="meta">@SuppressWarnings</span>(<span class="string">"deprecation"</span>)</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">open</span><span class="params">()</span> </span>&#123;&#125; </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openFrontDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">openBackDoor</span><span class="params">()</span> </span>&#123;&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
</li>
<li><p>下面的代码在编译时是否会发生错误？为什么？</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> Meal &#123; ... &#125;</div><div class="line"></div><div class="line"><span class="meta">@Meal</span>(<span class="string">"breakfast"</span>, mainDish=<span class="string">"cereal"</span>)</div><div class="line"><span class="meta">@Meal</span>(<span class="string">"lunch"</span>, mainDish=<span class="string">"pizza"</span>)</div><div class="line"><span class="meta">@Meal</span>(<span class="string">"dinner"</span>, mainDish=<span class="string">"salad"</span>)</div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">evaluateDiet</span><span class="params">()</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p><strong>答案</strong>:代码将会编译失败。JDK8之前，重复注解是不支持的。JDK8中，同样会编译失败，因为<code>Meal</code>注解类型并没有声明为可重复的。可以通过添加 <code>@Repeatable</code> 元注解，并定义一个注解容器类型来修复这个问题:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@java</span>.lang.annotation.Repeatable(MealContainer.class)</div><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> Meal &#123; ... &#125;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> MealContainer &#123;</div><div class="line">    Meal[] value();</div><div class="line">&#125;</div></pre></td></tr></table></figure>
</li>
</ol>
<h2 id="练习答案"><a href="#练习答案" class="headerlink" title="练习答案"></a>练习答案</h2><ol>
<li><p>练习: 定义一个增强请求的注解类型，包含元素<code>id</code>,<code>synopsis</code>,<code>engineer</code>, 以及<code>date</code>. 设定engineer默认值为<code>unassigned</code>，date默认值为<code>unknown</code>.</p>
<p>答案:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="comment">/**</span></div><div class="line"> * Describes the Request-for-Enhancement (RFE) annotation type.</div><div class="line"> */</div><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> RequestForEnhancement &#123;</div><div class="line">    <span class="function"><span class="keyword">int</span> <span class="title">id</span><span class="params">()</span></span>;</div><div class="line">    <span class="function">String <span class="title">synopsis</span><span class="params">()</span></span>;</div><div class="line">    <span class="function">String <span class="title">engineer</span><span class="params">()</span> <span class="keyword">default</span> "[unassigned]"</span>;</div><div class="line">    <span class="function">String <span class="title">date</span><span class="params">()</span> <span class="keyword">default</span> "[unknown]"</span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure></li>
</ol>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;问题与练习-注解&quot;&gt;&lt;a href=&quot;#问题与练习-注解&quot; class=&quot;headerlink&quot; title=&quot;问题与练习: 注解&quot;&gt;&lt;/a&gt;问题与练习: 注解&lt;/h1&gt;&lt;h2 id=&quot;问题&quot;&gt;&lt;a href=&quot;#问题&quot; class=&quot;headerlink&quot; title=&quot;问题&quot;&gt;&lt;/a&gt;问题&lt;/h2&gt;&lt;ol&gt;
&lt;li&gt;&lt;p&gt;问题: 线面的接口有哪些错误:&lt;/p&gt;
&lt;figure class=&quot;highlight java&quot;&gt;&lt;table&gt;&lt;tr&gt;&lt;td class=&quot;gutter&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;1&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;2&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;3&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;4&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;5&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;6&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;td class=&quot;code&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;class&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;interface&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;House&lt;/span&gt; &lt;/span&gt;&amp;#123;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;meta&quot;&gt;@Deprecated&lt;/span&gt;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;open&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt;&lt;/span&gt;;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;openFrontDoor&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt;&lt;/span&gt;;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;openBackDoor&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt;&lt;/span&gt;;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&amp;#125;&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/figure&gt;
&lt;p&gt;​&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;: 考虑下面的 &lt;code&gt;House&lt;/code&gt; 类，实现了问题1中的借口.&lt;/p&gt;
&lt;figure class=&quot;highlight java&quot;&gt;&lt;table&gt;&lt;tr&gt;&lt;td class=&quot;gutter&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;1&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;2&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;3&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;4&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;5&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;td class=&quot;code&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;class&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;class&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;MyHouse&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;implements&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;House&lt;/span&gt; &lt;/span&gt;&amp;#123;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;open&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt; &lt;/span&gt;&amp;#123;&amp;#125;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;openFrontDoor&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt; &lt;/span&gt;&amp;#123;&amp;#125;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;    &lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;openBackDoor&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt; &lt;/span&gt;&amp;#123;&amp;#125;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&amp;#125;&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/figure&gt;
&lt;p&gt;如果你编译这个程序，编译器会产生警告，因为&lt;code&gt;open&lt;/code&gt;方法是反对的（在接口中），你应该做什么来避免警告？&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;下面的代码在编译时是否会发生错误？为什么？&lt;/p&gt;
&lt;figure class=&quot;highlight java&quot;&gt;&lt;table&gt;&lt;tr&gt;&lt;td class=&quot;gutter&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;1&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;2&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;3&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;4&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;5&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;6&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;td class=&quot;code&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;meta&quot;&gt;@interface&lt;/span&gt; Meal &amp;#123; ... &amp;#125;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;meta&quot;&gt;@Meal&lt;/span&gt;(&lt;span class=&quot;string&quot;&gt;&quot;breakfast&quot;&lt;/span&gt;, mainDish=&lt;span class=&quot;string&quot;&gt;&quot;cereal&quot;&lt;/span&gt;)&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;meta&quot;&gt;@Meal&lt;/span&gt;(&lt;span class=&quot;string&quot;&gt;&quot;lunch&quot;&lt;/span&gt;, mainDish=&lt;span class=&quot;string&quot;&gt;&quot;pizza&quot;&lt;/span&gt;)&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;meta&quot;&gt;@Meal&lt;/span&gt;(&lt;span class=&quot;string&quot;&gt;&quot;dinner&quot;&lt;/span&gt;, mainDish=&lt;span class=&quot;string&quot;&gt;&quot;salad&quot;&lt;/span&gt;)&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;function&quot;&gt;&lt;span class=&quot;keyword&quot;&gt;public&lt;/span&gt; &lt;span class=&quot;keyword&quot;&gt;void&lt;/span&gt; &lt;span class=&quot;title&quot;&gt;evaluateDiet&lt;/span&gt;&lt;span class=&quot;params&quot;&gt;()&lt;/span&gt; &lt;/span&gt;&amp;#123; ... &amp;#125;&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/figure&gt;
&lt;/li&gt;
&lt;/ol&gt;
&lt;h2 id=&quot;练习&quot;&gt;&lt;a href=&quot;#练习&quot; class=&quot;headerlink&quot; title=&quot;练习&quot;&gt;&lt;/a&gt;练习&lt;/h2&gt;&lt;ol&gt;
&lt;li&gt;练习: 定义一个增强请求的注解类型，包含元素&lt;code&gt;id&lt;/code&gt;,&lt;code&gt;synopsis&lt;/code&gt;,&lt;code&gt;engineer&lt;/code&gt;, 以及&lt;code&gt;date&lt;/code&gt;. 设定engineer默认值为&lt;code&gt;unassigned&lt;/code&gt;，date默认值为&lt;code&gt;unknown&lt;/code&gt;.&lt;/li&gt;
&lt;/ol&gt;
&lt;h1 id=&quot;问题与练习答案-注解&quot;&gt;&lt;a href=&quot;#问题与练习答案-注解&quot; class=&quot;headerlink&quot; title=&quot;问题与练习答案: 注解&quot;&gt;&lt;/a&gt;问题与练习答案: 注解&lt;/h1&gt;
    
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      <category term="java" scheme="http://yoursite.com/categories/java/"/>
    
    
      <category term="java" scheme="http://yoursite.com/tags/java/"/>
    
      <category term="tutorial" scheme="http://yoursite.com/tags/tutorial/"/>
    
      <category term="教程" scheme="http://yoursite.com/tags/%E6%95%99%E7%A8%8B/"/>
    
      <category term="注解" scheme="http://yoursite.com/tags/%E6%B3%A8%E8%A7%A3/"/>
    
      <category term="annotation" scheme="http://yoursite.com/tags/annotation/"/>
    
  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：类型注解以及可插拔类型系统</title>
    <link href="http://yoursite.com/2018/03/23/java-tutorial-learning_annotation_type&amp;pluggable/"/>
    <id>http://yoursite.com/2018/03/23/java-tutorial-learning_annotation_type&amp;pluggable/</id>
    <published>2018-03-23T04:26:59.000Z</published>
    <updated>2018-03-26T04:24:20.245Z</updated>
    
    <content type="html"><![CDATA[<h1 id="类型注解以及可插拔的类型系统"><a href="#类型注解以及可插拔的类型系统" class="headerlink" title="类型注解以及可插拔的类型系统"></a>类型注解以及可插拔的类型系统</h1><p>Java SE 8版本之前，注解只能被应用于声明或定义。Java 8中，注解可以应用于任何<em>type use</em>使用类行的地方。例如，类型实例创建表达式 (<code>new</code>),类型转换 casts,实现引用 <code>implements</code>,以及抛出异常 <code>throws</code> 引用.这种格式的注解叫做类型注解 <em>type annotation</em>。注解基础 <a href="https://docs.oracle.com/javase/tutorial/java/annotations/basics.html" target="_blank" rel="external">Annotations Basics</a>里提供了几个相关的例子.</p>
<p>类型注解的创建，支持提高确保java程序更强的类型检查的分析方法。Java 8 并没有提供类型检查框架。但是它允许你写或者下载一个类型检查框架，实现一个或者多个可插拔的模块，来与Java编译器结合使用。</p>
<p>例如，你想要确保你程序中一个特定的变量不允许为null；你想要避免触发 <code>NullPointerException</code>. 你可以写一个自定义的插件检查这个规则.你现在需要修改你的代码并给对应的变量添加注解,表明其不允许被赋值为null。变量的定义可能如下所示：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@NonNull</span> String str;</div></pre></td></tr></table></figure>
<p>当你编译代码时，包含 <code>NonNull</code> 模块。编译器侦测出潜在的问题就会打印出一个警告，告知你修改代码以避免错误。当你修改代码移除所有的警告之后，这个特定的错误在程序运行时将不会发生.</p>
<p>你也可以使用多个类型检查模块，每个模块检查不同类型的错误。这样，你就可以在Java类型系统之上，添加任何你想要的特定的检查。</p>
<p>由合理的类型注解使用以及插件式类型检查的存在，你可以写出更加健壮的代码，更加不易出错。</p>
<p>很多种情况下，你不需要自己实现类型检查模块。已有第三方为你实现了这些工作。例如，你可能会因为华盛顿大学 University of Washington创建的检查框架而受益.这个框架包含了 <code>NonNull</code>模块，一个正则表达式 regular expression 模块, 以及一个互斥锁 mutex lock 模块.参考 <a href="http://types.cs.washington.edu/checker-framework/" target="_blank" rel="external">Checker Framework</a>获取更多信息.</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;类型注解以及可插拔的类型系统&quot;&gt;&lt;a href=&quot;#类型注解以及可插拔的类型系统&quot; class=&quot;headerlink&quot; title=&quot;类型注解以及可插拔的类型系统&quot;&gt;&lt;/a&gt;类型注解以及可插拔的类型系统&lt;/h1&gt;&lt;p&gt;Java SE 8版本之前，注解只能被应用于
    
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      <category term="java" scheme="http://yoursite.com/categories/java/"/>
    
    
      <category term="java" scheme="http://yoursite.com/tags/java/"/>
    
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      <category term="注解" scheme="http://yoursite.com/tags/%E6%B3%A8%E8%A7%A3/"/>
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：可重复注解</title>
    <link href="http://yoursite.com/2018/03/23/java-tutorial-learning_annotation_repeating/"/>
    <id>http://yoursite.com/2018/03/23/java-tutorial-learning_annotation_repeating/</id>
    <published>2018-03-23T04:26:59.000Z</published>
    <updated>2018-03-26T01:56:11.771Z</updated>
    
    <content type="html"><![CDATA[<h1 id="Repeating-Annotations"><a href="#Repeating-Annotations" class="headerlink" title="Repeating Annotations"></a>Repeating Annotations</h1><p>#可重复注解</p>
<p>有时候你需要在定义或者类型使用时多次使用同一个注解。Java8中<em>repeating annotations</em> 可以实现.</p>
<p>例如，你在写一个程序，使用一个定时服务，是你的方法可以在特定的时间或者按着确切的调度计划执行。类似于UNIX系统的 <code>cron</code> 服务.现在你想要设置一个定时器来运行方法, <code>doPeriodicCleanup</code>, 在每个月的最后一天和每个周五的晚上11点。为了是计时器生效，创建<code>@Schedule</code>注解并在方法 <code>doPeriodicCleanup</code>方法上使用两次。第一次明确每个月的最后一天，第二个明确在每个周五晚上11点，一下是代码实例:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Schedule</span>(dayOfMonth=<span class="string">"last"</span>)</div><div class="line"><span class="meta">@Schedule</span>(dayOfWeek=<span class="string">"Fri"</span>, hour=<span class="string">"23"</span>)</div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">doPeriodicCleanup</span><span class="params">()</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>上一个例子在方法上使用了同一个注解两次。你可以在使用标准注解的任何地方重复注解。例如，你有一个处理访问权限的异常的类。使用一个 <code>@Alert</code> 注解作用于客户经理，另一个作用于管理员:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Alert</span>(role=<span class="string">"Manager"</span>)</div><div class="line"><span class="meta">@Alert</span>(role=<span class="string">"Administrator"</span>)</div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">UnauthorizedAccessException</span> <span class="keyword">extends</span> <span class="title">SecurityException</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>因为共存的原因，重复注解存储在有Java 编译器自动生成的的注解容器 <em>container annotation</em> 中。想要实现这种功能，需要在代码中做两处声明。</p>
<h2 id="Step-1-定义一个可重复的注解类型"><a href="#Step-1-定义一个可重复的注解类型" class="headerlink" title="Step 1: 定义一个可重复的注解类型"></a>Step 1: 定义一个可重复的注解类型</h2><p>注解类型必须由 <code>@Repeatable</code>元注解标注 meta-annotation. 下面的例子定义了一个自定义的 <code>@Schedule</code>可重负注解类型:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">import</span> java.lang.annotation.Repeatable;</div><div class="line"></div><div class="line"><span class="meta">@Repeatable</span>(Schedules.class)</div><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> Schedule &#123;</div><div class="line">  <span class="function">String <span class="title">dayOfMonth</span><span class="params">()</span> <span class="keyword">default</span> "first"</span>;</div><div class="line">  <span class="function">String <span class="title">dayOfWeek</span><span class="params">()</span> <span class="keyword">default</span> "Mon"</span>;</div><div class="line">  <span class="function"><span class="keyword">int</span> <span class="title">hour</span><span class="params">()</span> <span class="keyword">default</span> 12</span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>元注解 <code>@Repeatable</code> meta-annotation的值,包含在括号里, 是Java编译器自动生成的存储重复注解的注解容器的类型。本例中注解容器的类型是 <code>Schedules</code>,因此重复注解 <code>@Schedule</code> 存储在 <code>@Schedules</code> 注解中.</p>
<p>没有声明注解类型为可重复时而多次使用会导致编译时错误。</p>
<h2 id="Step-2-声明注解容器类型"><a href="#Step-2-声明注解容器类型" class="headerlink" title="Step 2: 声明注解容器类型"></a>Step 2: 声明注解容器类型</h2><p>注解容器类型必须有一个 <code>value</code>元素，元素为数组类型.数组的组件类型必须是可重复注解类型。注解容器 <code>Schedules</code> 类型定义如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="meta">@interface</span> Schedules &#123;</div><div class="line">    Schedule[] value();</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<h2 id="获取注解Retrieving-Annotations"><a href="#获取注解Retrieving-Annotations" class="headerlink" title="获取注解Retrieving Annotations"></a>获取注解Retrieving Annotations</h2><p>在反射接口 Reflection API中有几种可以取出注释的方法。方法的作用是返回一个单独的注解，例如<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html#getAnnotation-java.lang.Class-" target="_blank" rel="external">AnnotatedElement.getAnnotation(Class)</a>, 如果请求的注解类型只存在一个，那么结果不会改变，仍然只返回一个注解。如果某注解类型的注解存在多个，则要先获取他们的注解容器类型。这样，遗留代码仍然可以继续生效。在Java SE 8 中引入了其他的方法，可以通过注解容器一次性返回多个注解，例如<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html#getAnnotationsByType-java.lang.Class-" target="_blank" rel="external">AnnotatedElement.getAnnotationsByType(Class)</a>. 参考<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/AnnotatedElement.html" target="_blank" rel="external">AnnotatedElement</a> 类的详细介绍获取更多可用方法的信息。</p>
<h2 id="设计思路Design-Considerations"><a href="#设计思路Design-Considerations" class="headerlink" title="设计思路Design Considerations"></a>设计思路Design Considerations</h2><p>当设计一个注解类型时，你必须要考虑这种注解类型的注解基数<em>cardinality</em>。可以使用注解零次，一次，或者如果注解类型被标记为<code>@Repeatable</code>可以使用多次。同样可以通过@Target元注解可以显示注解类型使用的地方。例如，你可以创建一个只用于方法或者字段的可冲入注解。仔细的设计你的注解类型已保证开发者更加有弹性的使用注解，并发挥强大作用，这是非常重要的。</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;Repeating-Annotations&quot;&gt;&lt;a href=&quot;#Repeating-Annotations&quot; class=&quot;headerlink&quot; title=&quot;Repeating Annotations&quot;&gt;&lt;/a&gt;Repeating Annotations&lt;/
    
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      <category term="annotation" scheme="http://yoursite.com/tags/annotation/"/>
    
  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：预定义的注解类型</title>
    <link href="http://yoursite.com/2018/03/23/java-tutorial-learning_annotation_predefined/"/>
    <id>http://yoursite.com/2018/03/23/java-tutorial-learning_annotation_predefined/</id>
    <published>2018-03-23T03:26:59.000Z</published>
    <updated>2018-03-23T03:02:47.607Z</updated>
    
    <content type="html"><![CDATA[<h1 id="预定义的注解类型"><a href="#预定义的注解类型" class="headerlink" title="预定义的注解类型"></a>预定义的注解类型</h1><p>Java SE API 预定义了一系列的注解类型。一些是被Java编译器使用，另外一是应用于其他注解(注解的注解).</p>
<h2 id="Java语言使用的注解"><a href="#Java语言使用的注解" class="headerlink" title="Java语言使用的注解"></a>Java语言使用的注解</h2><p><code>java.lang</code> 包中所定义的注解类型有 <code>@Deprecated</code>, <code>@Override</code>, 以及 <code>@SuppressWarnings</code>.</p>
<p><strong>@Deprecated</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Deprecated.html" target="_blank" rel="external"><code>@Deprecated</code></a> 注解表明了标注的元素是不推荐的并且不应该再继续使用。无论什么时候程序使用了有该标记的类，方法，字段，编译器都会产生一个警告。如果元素标记了<code>@Deprecated</code>那么他的主时钟也必须是用Java文档 <code>@deprecated</code> 标签，正如下面的例子。在Java文档注释中使用符号<code>@</code>与注解中一样并不是巧合的coincidental：他们是关联的概念。不过，要注意，Java文档注释里的标签首字母为小写<code>d</code>注解中首字母为<code>D</code>.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">   <span class="comment">// Javadoc comment follows</span></div><div class="line">    <span class="comment">/**</span></div><div class="line">     * <span class="doctag">@deprecated</span></div><div class="line">     * explanation of why it was deprecated</div><div class="line">     */</div><div class="line">    <span class="meta">@Deprecated</span></div><div class="line">    <span class="function"><span class="keyword">static</span> <span class="keyword">void</span> <span class="title">deprecatedMethod</span><span class="params">()</span> </span>&#123; &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p><strong>@Override</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Override.html" target="_blank" rel="external"><code>@Override</code></a>  注解告知编译器元素是为了复写在父类中定义的元素。复写方法将会在<a href="https://docs.oracle.com/javase/tutorial/java/IandI/index.html" target="_blank" rel="external">接口与继承Interfaces and Inheritance</a>一篇中讨论到.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// mark method as a superclass method</span></div><div class="line"><span class="comment">// that has been overridden</span></div><div class="line"><span class="meta">@Override</span> </div><div class="line"><span class="function"><span class="keyword">int</span> <span class="title">overriddenMethod</span><span class="params">()</span> </span>&#123; &#125;</div></pre></td></tr></table></figure>
<p>当复写方法的时候并不是必须要使用这个注解，他可以帮助你预防错误。如果标记了<code>@Override</code>标记的方法没有正确的复写他父类中的一个方法，编译器就会产生错误。</p>
<p><strong>@SuppressWarnings</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/SuppressWarnings.html" target="_blank" rel="external"><code>@SuppressWarnings</code></a> 注解告诉编译器，阻止可能产生的特定的警告。下面的例子中，使用了一个不推荐的方法，编译器通常会生成一个警告。在这种情况下，使用这个注解就可以阻止产生警告.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// use a deprecated method and tell </span></div><div class="line"><span class="comment">// compiler not to generate a warning</span></div><div class="line"><span class="meta">@SuppressWarnings</span>(<span class="string">"deprecation"</span>)</div><div class="line"> <span class="function"><span class="keyword">void</span> <span class="title">useDeprecatedMethod</span><span class="params">()</span> </span>&#123;</div><div class="line">     <span class="comment">// deprecation warning</span></div><div class="line">     <span class="comment">// - suppressed</span></div><div class="line">     objectOne.deprecatedMethod();</div><div class="line"> &#125;</div></pre></td></tr></table></figure>
<p>每一个编译器警告属于一个分类。Java语言明确列出了两类警告： <code>deprecation</code> 以及 <code>unchecked</code>. </p>
<p> <code>unchecked</code>警告，当遇到泛型 <a href="https://docs.oracle.com/javase/tutorial/java/generics/index.html" target="_blank" rel="external">generics</a>出现之前的遗留代码时可能会出现。 如果要阻止多个类型的警告，参考以下语法:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">@SuppressWarnings(&#123;&quot;unchecked&quot;, &quot;deprecation&quot;&#125;)</div></pre></td></tr></table></figure>
<p><strong>@SafeVarargs</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/SafeVarargs.html" target="_blank" rel="external"><code>@SafeVarargs</code></a> 注解，当使用在一个构造器或者方法时，断定该方法在使用可变参数时没有潜在的不安全因素。使用了该注解，与 <code>varargs</code>可变参数使用相关的未检查警告可以被阻止。</p>
<p><strong>@FunctionalInterface</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html" target="_blank" rel="external"><code>@FunctionalInterface</code></a> 功能接口注解，在Java SE8中引入。表明是一个功能接口，与Java语言标准中定义的功能接口一样.</p>
<h2 id="用于其他注解的注解"><a href="#用于其他注解的注解" class="headerlink" title="用于其他注解的注解"></a>用于其他注解的注解</h2><p>用于其注解的注解叫做元注解<em>meta-annotations</em>. <code>java.lang.annotation</code>中定义了数个元注解.</p>
<p><strong>@Retention</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/annotation/Retention.html" target="_blank" rel="external"><code>@Retention</code></a> 保留注解表明了注解是如何被存储的:</p>
<ul>
<li><code>RetentionPolicy.SOURCE</code> – 该标记表明注解只在资源层面被保留，编译器会忽略.</li>
<li><code>RetentionPolicy.CLASS</code> – 该标记表明注解在编译时被保留，JVM会忽略.</li>
<li><code>RetentionPolicy.RUNTIME</code> – 该标记表明注解会被JVM保留，可以再运行时使用.</li>
</ul>
<p><strong>@Documented</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/annotation/Documented.html" target="_blank" rel="external"><code>@Documented</code></a> 文档注解，表明使用了对应注解的元素，会被Java文档工具Javadoc tool使用。(默认情况下，注解不会被包含在Java文档中)。参考Java文档工具页面 <a href="https://docs.oracle.com/javase/8/docs/technotes/guides/javadoc/index.html" target="_blank" rel="external">Javadoc tools page</a>，获取更多相关信息.</p>
<p><strong>@Target</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/annotation/Target.html" target="_blank" rel="external"><code>@Target</code></a> 注解，限制了限制了注解可以应用于那种Java元素类型。目标注解可以使用如下的元素类型作为它的值:</p>
<ul>
<li><code>ElementType.ANNOTATION_TYPE</code> 可以应用于注解类型.</li>
<li><code>ElementType.CONSTRUCTOR</code> 可以应用于构造器.</li>
<li><code>ElementType.FIELD</code> 可以应用于字段或者属性（字段、枚举的常量）.</li>
<li><code>ElementType.LOCAL_VARIABLE</code> 可以应用于局部变量.</li>
<li><code>ElementType.METHOD</code> 可以应用于方法级别的注解.</li>
<li><code>ElementType.PACKAGE</code> 可以应用于包的定义.</li>
<li><code>ElementType.PARAMETER</code> 可以应用于方法的参数。</li>
<li>ElementType.TYPE 可以应用于任何的类（接口、类、枚举、注解）。</li>
</ul>
<p><strong>@Inherited</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/annotation/Inherited.html" target="_blank" rel="external"><code>@Inherited</code></a> 继承注解表明，注解类型可以用从器的父类继承。(默认是不可以继承的)，当用户查询类的注解类型而类本身没有该注解时，就会查询其父类是否有该注解。这个注解只能使用与类的定义.</p>
<p><strong>@Repeatable</strong> <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/annotation/Repeatable.html" target="_blank" rel="external"><code>@Repeatable</code></a> 可重复的注解，在Java SE 8中引入，表明该注解在同一个声明或者类型处可以使用不止一次。参考可重复注解 <a href="https://docs.oracle.com/javase/tutorial/java/annotations/repeating.html" target="_blank" rel="external">Repeating Annotations</a>.</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;预定义的注解类型&quot;&gt;&lt;a href=&quot;#预定义的注解类型&quot; class=&quot;headerlink&quot; title=&quot;预定义的注解类型&quot;&gt;&lt;/a&gt;预定义的注解类型&lt;/h1&gt;&lt;p&gt;Java SE API 预定义了一系列的注解类型。一些是被Java编译器使用，另外一是应用于
    
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  <entry>
    <title>Java 学习指南_学习Java：基础-枚举类问题与练习</title>
    <link href="http://yoursite.com/2018/03/21/java-tutorial-learning_class&amp;object_enum_q&amp;e/"/>
    <id>http://yoursite.com/2018/03/21/java-tutorial-learning_class&amp;object_enum_q&amp;e/</id>
    <published>2018-03-21T10:51:59.000Z</published>
    <updated>2018-03-21T09:01:43.918Z</updated>
    
    <content type="html"><![CDATA[<h1 id="枚举类问题与练习"><a href="#枚举类问题与练习" class="headerlink" title="枚举类问题与练习"></a>枚举类问题与练习</h1><h2 id="问题"><a href="#问题" class="headerlink" title="问题"></a>问题</h2><ol>
<li>问题: True or false: 枚举类可以是 java.lang.String类的子类.</li>
</ol>
<h2 id="练习"><a href="#练习" class="headerlink" title="练习"></a>练习</h2><ol>
<li>练习: 重写 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/creating-questions.html" target="_blank" rel="external">Questions and Exercises: Classes</a> 中 <code>Card</code> 类，使用枚举类来表示扑克中的花色和点数。</li>
<li>练习: 重写 <code>Deck</code> 类.</li>
</ol>
<h1 id="答案"><a href="#答案" class="headerlink" title="答案"></a>答案</h1><a id="more"></a>
<h2 id="问题答案"><a href="#问题答案" class="headerlink" title="问题答案"></a>问题答案</h2><ol>
<li><p>问题: True or false: 枚举类可以是 java.lang.String类的子类.</p>
<p>答案: False. 所有的枚举类都继承自 <code>java.lang.Enum</code>.因为一个类只能继承字一个父类，Java语言不支持多重继承，因此枚举类不可能是任何其他类的子类.</p>
</li>
</ol>
<h2 id="练习答案"><a href="#练习答案" class="headerlink" title="练习答案"></a>练习答案</h2><ol>
<li><p>练习: 重写 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/creating-questions.html" target="_blank" rel="external">Questions and Exercises: Classes</a> 中 <code>Card</code> 类，使用枚举类来表示扑克中的花色和点数。</p>
<p>答案: 参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Card3.java" target="_blank" rel="external"><code>Card3.java</code></a>, <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Suit.java" target="_blank" rel="external"><code>Suit.java</code></a>, 和 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Rank.java" target="_blank" rel="external"><code>Rank.java</code></a>.</p>
</li>
<li><p>练习: 重写 <code>Deck</code> 类.</p>
<p>答案: 参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Deck3.java" target="_blank" rel="external"><code>Deck3.java</code></a>.</p>
</li>
</ol>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;枚举类问题与练习&quot;&gt;&lt;a href=&quot;#枚举类问题与练习&quot; class=&quot;headerlink&quot; title=&quot;枚举类问题与练习&quot;&gt;&lt;/a&gt;枚举类问题与练习&lt;/h1&gt;&lt;h2 id=&quot;问题&quot;&gt;&lt;a href=&quot;#问题&quot; class=&quot;headerlink&quot; title=&quot;问题&quot;&gt;&lt;/a&gt;问题&lt;/h2&gt;&lt;ol&gt;
&lt;li&gt;问题: True or false: 枚举类可以是 java.lang.String类的子类.&lt;/li&gt;
&lt;/ol&gt;
&lt;h2 id=&quot;练习&quot;&gt;&lt;a href=&quot;#练习&quot; class=&quot;headerlink&quot; title=&quot;练习&quot;&gt;&lt;/a&gt;练习&lt;/h2&gt;&lt;ol&gt;
&lt;li&gt;练习: 重写 &lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/creating-questions.html&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;Questions and Exercises: Classes&lt;/a&gt; 中 &lt;code&gt;Card&lt;/code&gt; 类，使用枚举类来表示扑克中的花色和点数。&lt;/li&gt;
&lt;li&gt;练习: 重写 &lt;code&gt;Deck&lt;/code&gt; 类.&lt;/li&gt;
&lt;/ol&gt;
&lt;h1 id=&quot;答案&quot;&gt;&lt;a href=&quot;#答案&quot; class=&quot;headerlink&quot; title=&quot;答案&quot;&gt;&lt;/a&gt;答案&lt;/h1&gt;
    
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  <entry>
    <title>Java 学习指南_学习Java：定义一个注解类型</title>
    <link href="http://yoursite.com/2018/03/21/java-tutorial-learning_annotation_declaring/"/>
    <id>http://yoursite.com/2018/03/21/java-tutorial-learning_annotation_declaring/</id>
    <published>2018-03-21T10:21:59.000Z</published>
    <updated>2018-03-21T10:22:06.572Z</updated>
    
    <content type="html"><![CDATA[<h1 id="定义一个注解类型"><a href="#定义一个注解类型" class="headerlink" title="定义一个注解类型"></a>定义一个注解类型</h1><p>许多注解代替代码中的注释。</p>
<p>假设一个软件组通常在类体的起始处填写注释已提供重要的信息:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Generation3List</span> <span class="keyword">extends</span> <span class="title">Generation2List</span> </span>&#123;</div><div class="line"></div><div class="line">   <span class="comment">// Author: John Doe</span></div><div class="line">   <span class="comment">// Date: 3/17/2002</span></div><div class="line">   <span class="comment">// Current revision: 6</span></div><div class="line">   <span class="comment">// Last modified: 4/12/2004</span></div><div class="line">   <span class="comment">// By: Jane Doe</span></div><div class="line">   <span class="comment">// Reviewers: Alice, Bill, Cindy</span></div><div class="line"></div><div class="line">   <span class="comment">// class code goes here</span></div><div class="line"></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>为了使用注解添加这些元数据，你必须先定义一个注解类型 <em>annotation type</em>. 其语法如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@interface</span> ClassPreamble &#123;</div><div class="line">   <span class="function">String <span class="title">author</span><span class="params">()</span></span>;</div><div class="line">   <span class="function">String <span class="title">date</span><span class="params">()</span></span>;</div><div class="line">   <span class="function"><span class="keyword">int</span> <span class="title">currentRevision</span><span class="params">()</span> <span class="keyword">default</span> 1</span>;</div><div class="line">   <span class="function">String <span class="title">lastModified</span><span class="params">()</span> <span class="keyword">default</span> "N/A"</span>;</div><div class="line">   <span class="function">String <span class="title">lastModifiedBy</span><span class="params">()</span> <span class="keyword">default</span> "N/A"</span>;</div><div class="line">   <span class="comment">// Note use of array</span></div><div class="line">   String[] reviewers();</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>注解类型的定义看起来像是接口的定义，只不过关键字<code>interface</code>前面多了一个符号 (<code>@</code>) (@ = AT, 与注解的符号一样).</p>
<p>注解类型是接口 <em>interface</em>的一种格式后面的课程会讲到,现在你不需要理解接口。</p>
<p>上面的注解类型定义，类体中包含了注解类型元素 <em>annotation type element</em> 的定义,看起来像是方法，不过注意，他们可以定义默认值。</p>
<p>定义注解类型之后，你可以使用这个注解类型，并填入元素值，像下面这样:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@ClassPreamble</span> (</div><div class="line">   author = <span class="string">"John Doe"</span>,</div><div class="line">   date = <span class="string">"3/17/2002"</span>,</div><div class="line">   currentRevision = <span class="number">6</span>,</div><div class="line">   lastModified = <span class="string">"4/12/2004"</span>,</div><div class="line">   lastModifiedBy = <span class="string">"Jane Doe"</span>,</div><div class="line">   <span class="comment">// Note array notation</span></div><div class="line">   reviewers = &#123;<span class="string">"Alice"</span>, <span class="string">"Bob"</span>, <span class="string">"Cindy"</span>&#125;</div><div class="line">)</div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Generation3List</span> <span class="keyword">extends</span> <span class="title">Generation2List</span> </span>&#123;</div><div class="line"></div><div class="line"><span class="comment">// class code goes here</span></div><div class="line"></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<hr>
<p><strong>注意:</strong> 为了是 <code>@ClassPreamble</code> 注解定义的元素出现在java文档生成器生成的文档中，你必须为 <code>@ClassPreamble</code> 注解的定义添加d <code>@Documented</code> 注解:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line">// import this to use @Documented</div><div class="line">import java.lang.annotation.*;</div><div class="line"></div><div class="line">@Documented</div><div class="line">@interface ClassPreamble &#123;</div><div class="line"></div><div class="line">   // Annotation element definitions</div><div class="line">   </div><div class="line">&#125;</div></pre></td></tr></table></figure>
<hr>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;定义一个注解类型&quot;&gt;&lt;a href=&quot;#定义一个注解类型&quot; class=&quot;headerlink&quot; title=&quot;定义一个注解类型&quot;&gt;&lt;/a&gt;定义一个注解类型&lt;/h1&gt;&lt;p&gt;许多注解代替代码中的注释。&lt;/p&gt;
&lt;p&gt;假设一个软件组通常在类体的起始处填写注释已提供重要
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：注解</title>
    <link href="http://yoursite.com/2018/03/21/java-tutorial-learning_annotation/"/>
    <id>http://yoursite.com/2018/03/21/java-tutorial-learning_annotation/</id>
    <published>2018-03-21T09:26:59.000Z</published>
    <updated>2018-03-21T09:27:12.734Z</updated>
    
    <content type="html"><![CDATA[<h1 id="课程-注解"><a href="#课程-注解" class="headerlink" title="课程: 注解"></a>课程: 注解</h1><p><em>注解Annotations</em>, 一种元数据的格式,提供程序相关的数据而不是程序本身. 注解并不会直接影响所标注的代码的影响。</p>
<p>注解有一系列的作用，分别如下:</p>
<ul>
<li><strong>编译所用的注解信息</strong> — 注解可以用来侦测错误或者忽略警告.</li>
<li><strong>编译时以及部署时的处理Compile-time and deployment-time processing</strong> — 软件工具可以处理注解信息来生成代码,XML文件等。</li>
<li><strong>运行时的处理Runtime processing</strong> — 某些注解可在运行时检测.</li>
</ul>
<p>本课程解释了哪里可以用到注解，如何应用注解，JavaSE api预定义了那些可用的注解类型，注解类型如何与插件式的系统想结果写出强壮的类型检查代码，以及如何实现可重复的注解。</p>
]]></content>
    
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      &lt;h1 id=&quot;课程-注解&quot;&gt;&lt;a href=&quot;#课程-注解&quot; class=&quot;headerlink&quot; title=&quot;课程: 注解&quot;&gt;&lt;/a&gt;课程: 注解&lt;/h1&gt;&lt;p&gt;&lt;em&gt;注解Annotations&lt;/em&gt;, 一种元数据的格式,提供程序相关的数据而不是程序本身. 注解并不会
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：注解基础</title>
    <link href="http://yoursite.com/2018/03/21/java-tutorial-learning_annotation_basic/"/>
    <id>http://yoursite.com/2018/03/21/java-tutorial-learning_annotation_basic/</id>
    <published>2018-03-21T09:26:59.000Z</published>
    <updated>2018-03-21T09:55:27.194Z</updated>
    
    <content type="html"><![CDATA[<h1 id="注解基础"><a href="#注解基础" class="headerlink" title="注解基础"></a>注解基础</h1><h2 id="注解的格式"><a href="#注解的格式" class="headerlink" title="注解的格式"></a>注解的格式</h2><p>最贱的注解格式实例如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Entity</span></div></pre></td></tr></table></figure>
<p>标志符号 (<code>@</code>) 向编译器表示紧跟着的信息是注解，下面的例子注解的名字为<code>Override</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">mySuperMethod</span><span class="params">()</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>注解可以包含元素 <em>elements</em>,可以是署名的，也可以是匿名的，元素有对应的值:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Author</span>(</div><div class="line">   name = <span class="string">"Benjamin Franklin"</span>,</div><div class="line">   date = <span class="string">"3/27/2003"</span></div><div class="line">)</div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyClass</span>() </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>或者</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@SuppressWarnings</span>(value = <span class="string">"unchecked"</span>)</div><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">myMethod</span><span class="params">()</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>如果只有一个名为<code>value</code>的元素，那么元素署名可以省略，如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@SuppressWarnings</span>(<span class="string">"unchecked"</span>)</div><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">myMethod</span><span class="params">()</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>如果注解不包含元素，那么小括号也可以移除，就像之前的<code>@Override</code>注解.</p>
<p>也可以在统一个声明或者定义处使用多个注解:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Author</span>(name = <span class="string">"Jane Doe"</span>)</div><div class="line"><span class="meta">@EBook</span></div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyClass</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>如果多个注解拥有同样的类型，叫做重复注解repeating annotation:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Author</span>(name = <span class="string">"Jane Doe"</span>)</div><div class="line"><span class="meta">@Author</span>(name = <span class="string">"John Smith"</span>)</div><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyClass</span> </span>&#123; ... &#125;</div></pre></td></tr></table></figure>
<p>重复注解在Java SE8中支持的，获取更多相关信息可以参看 <a href="https://docs.oracle.com/javase/tutorial/java/annotations/repeating.html" target="_blank" rel="external">Repeating Annotations</a>.</p>
<p>注解类型可以是<code>java.lang</code>或者<code>java.lang.annotation</code>包中的一种。之前的例子中<code>Override</code> and <code>SuppressWarnings</code> 是 <a href="https://docs.oracle.com/javase/tutorial/java/annotations/predefined.html" target="_blank" rel="external">预定义注解predefined Java annotations</a>.你也可以定义自己的注解类型， <code>Author</code> 和 <code>Ebook</code> 注解是自定义注解类型。</p>
<h2 id="注解可以用在哪里"><a href="#注解可以用在哪里" class="headerlink" title="注解可以用在哪里"></a>注解可以用在哪里</h2><p>注解可以用定义：类，字段，方法以及其他编程元素定义时可以使用。挡在定义时使用注解，按照惯例每一个注解占一行.</p>
<p>Java SE8 版本中，注解同样可以应用在类型使用时，以下是一些列子:</p>
<ul>
<li><p>类型实例创建表达式:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">new @Interned MyObject();</div></pre></td></tr></table></figure>
</li>
<li><p>类型转换:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">myString = (@NonNull String) str;</div></pre></td></tr></table></figure>
</li>
<li><p><code>implements</code> 引用时:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">class UnmodifiableList&lt;T&gt; implements</div><div class="line">    @Readonly List&lt;@Readonly T&gt; &#123; ... &#125;</div></pre></td></tr></table></figure>
</li>
<li><p>声明抛出异常时:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">void monitorTemperature() throws</div><div class="line">    @Critical TemperatureException &#123; ... &#125;</div></pre></td></tr></table></figure>
</li>
</ul>
<p>这种形式的注解叫做类型注解 <em>type annotation</em>.获取更多相关信息，参考 <a href="https://docs.oracle.com/javase/tutorial/java/annotations/type_annotations.html" target="_blank" rel="external">类型注解与插件式类型系统Type Annotations and Pluggable Type Systems</a>.</p>
]]></content>
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：基础-枚举类</title>
    <link href="http://yoursite.com/2018/03/19/java-tutorial-learning_class&amp;object_enum/"/>
    <id>http://yoursite.com/2018/03/19/java-tutorial-learning_class&amp;object_enum/</id>
    <published>2018-03-19T10:51:59.000Z</published>
    <updated>2018-03-19T10:27:32.406Z</updated>
    
    <content type="html"><![CDATA[<h1 id="枚举类"><a href="#枚举类" class="headerlink" title="枚举类"></a>枚举类</h1><p>枚举类型 <em>enum type</em> 是一种特殊的数据类型，可以使一个变量值为一组预定义的常量来。变量值必须等于预定义的常量值中的一个。常见的例子包括指南者中的方向(值 NORTH, SOUTH, EAST, and WEST) 以及一周中的每一天.</p>
<p>因为他们是常量，所以枚举类的成员都是用大写字母表示。</p>
<p>Java编程语言中，你可以使用<code>enum</code>关键字定义枚举类。例如，一周中的每一天可以定义如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="keyword">enum</span> Day &#123;</div><div class="line">    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,</div><div class="line">    THURSDAY, FRIDAY, SATURDAY </div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>你可以在任何你想表达一组固定的常量集合时使用枚举类。包含了自然界中的列举类型例如太阳系中的行星以及任何你可能在编译时就知道的任何可能值—例如，菜单中的选项，命令行中的标记等等.</p>
<p>以下代码是如何使用上面定义的<code>Day</code>枚举类型:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">EnumTest</span> </span>&#123;</div><div class="line">    Day day;</div><div class="line">    </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="title">EnumTest</span><span class="params">(Day day)</span> </span>&#123;</div><div class="line">        <span class="keyword">this</span>.day = day;</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">tellItLikeItIs</span><span class="params">()</span> </span>&#123;</div><div class="line">        <span class="keyword">switch</span> (day) &#123;</div><div class="line">            <span class="keyword">case</span> MONDAY:</div><div class="line">                System.out.println(<span class="string">"Mondays are bad."</span>);</div><div class="line">                <span class="keyword">break</span>;</div><div class="line">                    </div><div class="line">            <span class="keyword">case</span> FRIDAY:</div><div class="line">                System.out.println(<span class="string">"Fridays are better."</span>);</div><div class="line">                <span class="keyword">break</span>;</div><div class="line">                         </div><div class="line">            <span class="keyword">case</span> SATURDAY: <span class="keyword">case</span> SUNDAY:</div><div class="line">                System.out.println(<span class="string">"Weekends are best."</span>);</div><div class="line">                <span class="keyword">break</span>;</div><div class="line">                        </div><div class="line">            <span class="keyword">default</span>:</div><div class="line">                System.out.println(<span class="string">"Midweek days are so-so."</span>);</div><div class="line">                <span class="keyword">break</span>;</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">    </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</div><div class="line">        EnumTest firstDay = <span class="keyword">new</span> EnumTest(Day.MONDAY);</div><div class="line">        firstDay.tellItLikeItIs();</div><div class="line">        EnumTest thirdDay = <span class="keyword">new</span> EnumTest(Day.WEDNESDAY);</div><div class="line">        thirdDay.tellItLikeItIs();</div><div class="line">        EnumTest fifthDay = <span class="keyword">new</span> EnumTest(Day.FRIDAY);</div><div class="line">        fifthDay.tellItLikeItIs();</div><div class="line">        EnumTest sixthDay = <span class="keyword">new</span> EnumTest(Day.SATURDAY);</div><div class="line">        sixthDay.tellItLikeItIs();</div><div class="line">        EnumTest seventhDay = <span class="keyword">new</span> EnumTest(Day.SUNDAY);</div><div class="line">        seventhDay.tellItLikeItIs();</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>输出如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line">Mondays are bad.</div><div class="line">Midweek days are so-so.</div><div class="line">Fridays are better.</div><div class="line">Weekends are best.</div><div class="line">Weekends are best.</div></pre></td></tr></table></figure>
<p>Java编程语言中的枚举类型比其他语言中的要更加欠打。 <code>enum</code> 声明定义了一个类(交过枚举类型 <em>enum type</em>).枚举类型，类体也可以包含方法以及其他成员。编译器在创建枚举类型时会自动的增加一些特殊的代码。例如，他们有已个静态的<code>values</code>方法可以返回包含所有定义的枚举值的数据。这个方法通常与<code>for-each</code>结构结合使用，来迭代枚举类中的所有类型。例如，下面的代码 <code>Planet</code> 类例子遍历了太阳系中的所有行星.</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span> (Planet p : Planet.values()) &#123;</div><div class="line">    System.out.printf(<span class="string">"Your weight on %s is %f%n"</span>,</div><div class="line">                      p, p.surfaceWeight(mass));</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<hr>
<p><strong>注意</strong>:所有的枚举类型简单的继承了类 <code>java.lang.Enum</code>。因为Java只能继承自一个类（参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/classdecl.html" target="_blank" rel="external">Declaring Classes</a>).Java不支持多重继承 (参考 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/multipleinheritance.html" target="_blank" rel="external">Multiple Inheritance of State, Implementation, and Type</a>)，因此所有的枚举类都不能再继承任何类。</p>
<hr>
<p>下面的例子中，<code>Planet</code>是一个枚举类，代表了太阳系中的所有行星.他们通过的常量属性质量和半径来定义。</p>
<p>每一个枚举常量定义的同时包含了质量与半径参数。这些参数在常量创建的时候传递给构造器。Java要求常量必须首先声明，优先与任何的成员和方法。因此，当枚举类有额外的字段和方法时，枚举常量结束时必须以分号semicolon结束.</p>
<hr>
<p><strong>Note</strong>:枚举类的构造器必须是私有级别或者是包级的访问权限。它将在自动创建定义在枚举类体开始的常量。你不能自己调用枚举类的构造方法。</p>
<hr>
<p>另外，对于他的属性和构造器，<code>Planet</code>提供方法允许你获取每一个行星的表面重力以及重量。下面是一个简单的程序，获取你在地球的体重（任一单位）并计算出你在其他所有星球的体重（同样单位下）:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="keyword">enum</span> Planet &#123;</div><div class="line">    MERCURY (<span class="number">3.303e+23</span>, <span class="number">2.4397e6</span>),</div><div class="line">    VENUS   (<span class="number">4.869e+24</span>, <span class="number">6.0518e6</span>),</div><div class="line">    EARTH   (<span class="number">5.976e+24</span>, <span class="number">6.37814e6</span>),</div><div class="line">    MARS    (<span class="number">6.421e+23</span>, <span class="number">3.3972e6</span>),</div><div class="line">    JUPITER (<span class="number">1.9e+27</span>,   <span class="number">7.1492e7</span>),</div><div class="line">    SATURN  (<span class="number">5.688e+26</span>, <span class="number">6.0268e7</span>),</div><div class="line">    URANUS  (<span class="number">8.686e+25</span>, <span class="number">2.5559e7</span>),</div><div class="line">    NEPTUNE (<span class="number">1.024e+26</span>, <span class="number">2.4746e7</span>);</div><div class="line"></div><div class="line">    <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">double</span> mass;   <span class="comment">// in kilograms</span></div><div class="line">    <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">double</span> radius; <span class="comment">// in meters</span></div><div class="line">    Planet(<span class="keyword">double</span> mass, <span class="keyword">double</span> radius) &#123;</div><div class="line">        <span class="keyword">this</span>.mass = mass;</div><div class="line">        <span class="keyword">this</span>.radius = radius;</div><div class="line">    &#125;</div><div class="line">    <span class="function"><span class="keyword">private</span> <span class="keyword">double</span> <span class="title">mass</span><span class="params">()</span> </span>&#123; <span class="keyword">return</span> mass; &#125;</div><div class="line">    <span class="function"><span class="keyword">private</span> <span class="keyword">double</span> <span class="title">radius</span><span class="params">()</span> </span>&#123; <span class="keyword">return</span> radius; &#125;</div><div class="line"></div><div class="line">    <span class="comment">// universal gravitational constant  (m3 kg-1 s-2)</span></div><div class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">double</span> G = <span class="number">6.67300E-11</span>;</div><div class="line"></div><div class="line">    <span class="function"><span class="keyword">double</span> <span class="title">surfaceGravity</span><span class="params">()</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> G * mass / (radius * radius);</div><div class="line">    &#125;</div><div class="line">    <span class="function"><span class="keyword">double</span> <span class="title">surfaceWeight</span><span class="params">(<span class="keyword">double</span> otherMass)</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> otherMass * surfaceGravity();</div><div class="line">    &#125;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</div><div class="line">        <span class="keyword">if</span> (args.length != <span class="number">1</span>) &#123;</div><div class="line">            System.err.println(<span class="string">"Usage: java Planet &lt;earth_weight&gt;"</span>);</div><div class="line">            System.exit(-<span class="number">1</span>);</div><div class="line">        &#125;</div><div class="line">        <span class="keyword">double</span> earthWeight = Double.parseDouble(args[<span class="number">0</span>]);</div><div class="line">        <span class="keyword">double</span> mass = earthWeight/EARTH.surfaceGravity();</div><div class="line">        <span class="keyword">for</span> (Planet p : Planet.values())</div><div class="line">           System.out.printf(<span class="string">"Your weight on %s is %f%n"</span>,</div><div class="line">                             p, p.surfaceWeight(mass));</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>如果你运行这个类 <code>Planet.class</code> 并且输入参数175, 可以说的如下输出:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line">$ java Planet 175</div><div class="line">Your weight on MERCURY is 66.107583</div><div class="line">Your weight on VENUS is 158.374842</div><div class="line">Your weight on EARTH is 175.000000</div><div class="line">Your weight on MARS is 66.279007</div><div class="line">Your weight on JUPITER is 442.847567</div><div class="line">Your weight on SATURN is 186.552719</div><div class="line">Your weight on URANUS is 158.397260</div><div class="line">Your weight on NEPTUNE is 199.207413</div></pre></td></tr></table></figure>]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;枚举类&quot;&gt;&lt;a href=&quot;#枚举类&quot; class=&quot;headerlink&quot; title=&quot;枚举类&quot;&gt;&lt;/a&gt;枚举类&lt;/h1&gt;&lt;p&gt;枚举类型 &lt;em&gt;enum type&lt;/em&gt; 是一种特殊的数据类型，可以使一个变量值为一组预定义的常量来。变量值必须等于预定义的常
    
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      <category term="java" scheme="http://yoursite.com/categories/java/"/>
    
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：基础-嵌套类问题与联系</title>
    <link href="http://yoursite.com/2018/03/19/java-tutorial-learning_class&amp;object_nested_class_q&amp;e/"/>
    <id>http://yoursite.com/2018/03/19/java-tutorial-learning_class&amp;object_nested_class_q&amp;e/</id>
    <published>2018-03-19T09:51:59.000Z</published>
    <updated>2018-03-19T09:41:58.533Z</updated>
    
    <content type="html"><![CDATA[<h1 id="嵌套类：问题与练习"><a href="#嵌套类：问题与练习" class="headerlink" title="嵌套类：问题与练习"></a>嵌套类：问题与练习</h1><h2 id="问题"><a href="#问题" class="headerlink" title="问题"></a>问题</h2><ol>
<li><p><strong>问题</strong>: 程序 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/Problem.java" target="_blank" rel="external"><code>Problem.java</code></a> 不能编译，你需要怎么做才能让它成功编译?为什么?</p>
</li>
<li><p>使用 <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html" target="_blank" rel="external"><code>Box</code></a> 类(在 <code>javax.swing</code> 包中)的Java API文档 帮助你问答如下问题 .</p>
<ol>
<li><p><strong>问题</strong>: <code>Box</code> 定义了什么静态嵌套类?</p>
</li>
<li><p><strong>问题</strong>:  <code>Box</code> 定义了什么内部类?</p>
</li>
<li><p><strong>问题</strong>:  <code>Box</code>的内部类的父类是什么?</p>
</li>
<li><p><strong>问题</strong>:  <code>Box</code>的哪一个嵌套类你可以在任何一个类中使用?</p>
</li>
<li><p><strong>问题</strong>: 如何创建一个 <code>Box</code>的`Filler嵌套类的实例?</p>
</li>
</ol>
</li>
</ol>
<h2 id="练习"><a href="#练习" class="headerlink" title="练习"></a>练习</h2><ol>
<li><p><strong>练习</strong>: 获取文件 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/Class1.java" target="_blank" rel="external"><code>Class1.java</code></a>. 编译并运行 <code>Class1</code>. 输出是什么?</p>
</li>
<li><p><strong>练习</strong>: 下面的联系涉及到修改 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/DataStructure.java" target="_blank" rel="external"><code>DataStructure.java</code></a>类, 该类在 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html" target="_blank" rel="external">Inner Class Example</a> 中有讨论.</p>
<ol>
<li>定义一个名为 <code>print(DataStructureIterator iterator)</code>的方法.用一个类<code>EvenIterator</code>类的实例调用这个方法实现<code>printEven</code>方法的功能。</li>
</ol>
</li>
</ol>
<ol>
<li>调用方法 <code>print(DataStructureIterator iterator)</code> 来打印奇数索引的元素.使用一个匿名类作为方法的参数，代替接口 <code>DataStructureIterator</code>的实例.</li>
</ol>
<ol>
<li>定义一个方法 <code>print(java.util.Function&lt;Integer, Boolean&gt; iterator)</code> 实现 <code>print(DataStructureIterator iterator)</code>相同的功能.通过一个lambda表达式参数调用该方法实现打印出偶数下标的元素，重新用lambda表达式参数调用并实现打印出奇数下标的元素。</li>
</ol>
<ol>
<li><p>定义两个方法可以是如下语句分别打印偶数下标对应的值和技术下标对应的值:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">DataStructure ds = <span class="keyword">new</span> DataStructure()</div><div class="line"><span class="comment">// ...</span></div><div class="line">ds.print(DataStructure::isEvenIndex);</div><div class="line">ds.print(DataStructure::isOddIndex);</div></pre></td></tr></table></figure>
</li>
</ol>
<h1 id="嵌套类：问题与练习答案"><a href="#嵌套类：问题与练习答案" class="headerlink" title="嵌套类：问题与练习答案"></a>嵌套类：问题与练习答案</h1><a id="more"></a>
<h2 id="问题答案"><a href="#问题答案" class="headerlink" title="问题答案"></a>问题答案</h2><ol>
<li><p><strong>问题</strong>: 程序 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/Problem.java" target="_blank" rel="external"><code>Problem.java</code></a> 不能编译，你需要怎么做才能让它成功编译?为什么?</p>
<p><strong>答案</strong>: 删除<code>Inner</code>类的<code>static</code>关键字修饰。静态嵌套类不能访问外部类的实例成员参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/ProblemSolved.java" target="_blank" rel="external"><code>ProblemSolved.java</code></a>.</p>
</li>
<li><p>使用 <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html" target="_blank" rel="external"><code>Box</code></a> 类(在 <code>javax.swing</code> 包中)的Java API文档 帮助你问答如下问题 .</p>
<ol>
<li><p><strong>问题</strong>: <code>Box</code> 定义了什么静态嵌套类?</p>
<p><strong>答案</strong>: <code>Box.Filler</code></p>
</li>
<li><p><strong>问题</strong>:  <code>Box</code> 定义了什么内部类?</p>
<p><strong>答案</strong>: <code>Box.AccessibleBox</code></p>
</li>
<li><p><strong>问题</strong>:  <code>Box</code>的内部类的父类是什么?</p>
<p><strong>答案</strong>: <code>[java.awt.]Container.AccessibleAWTContainer</code></p>
</li>
<li><p><strong>问题</strong>:  <code>Box</code>的哪一个嵌套类你可以在任何一个类中使用?</p>
<p><strong>答案</strong>: <code>Box.Filler</code></p>
</li>
<li><p><strong>问题</strong>: 如何创建一个 <code>Box</code>的`Filler嵌套类的实例?</p>
<p><strong>答案</strong>: <code>new Box.Filler(minDimension, prefDimension, maxDimension)</code></p>
</li>
</ol>
</li>
</ol>
<h2 id="练习答案"><a href="#练习答案" class="headerlink" title="练习答案"></a>练习答案</h2><ol>
<li><p><strong>练习</strong>: 获取文件 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/Class1.java" target="_blank" rel="external"><code>Class1.java</code></a>. 编译并运行 <code>Class1</code>. 输出是什么?</p>
<p><strong>答案</strong>: <code>InnerClass1: getString invoked.InnerClass1: getAnotherString invoked.</code></p>
</li>
<li><p><strong>练习</strong>: 下面的联系涉及到修改 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/DataStructure.java" target="_blank" rel="external"><code>DataStructure.java</code></a>类, 该类在 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html" target="_blank" rel="external">Inner Class Example</a> 中有讨论.</p>
<ol>
<li><p>定义一个名为 <code>print(DataStructureIterator iterator)</code>的方法.用一个类<code>EvenIterator</code>类的实例调用这个方法实现<code>printEven</code>方法的功能。</p>
<p><strong>提示</strong>: 一下语句在<code>main</code>方法中奖不能通过编译:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">DataStructure ds = <span class="keyword">new</span> DataStructure();</div><div class="line">ds.print(<span class="keyword">new</span> EvenIterator());</div></pre></td></tr></table></figure>
<p>编译器产生错误信息 “non-static variable this cannot be referenced from a static context—非京台的变量不能再静态上下文中引用”，表达式 <code>new EvenIterator()</code>会产生这个错误. 类 <code>EvenIterator</code> 是一个内部类，非静态.意味着你必须在外部类的一个实例中创建它的实例, <code>DataStructure</code>数据结构.</p>
<p>你尅定义一个方法 <code>DataStructure</code> 来创建和返回<code>EvenIterator</code>的实例.</p>
</li>
<li><p>调用方法 <code>print(DataStructureIterator iterator)</code> 来打印奇数索引的元素.使用一个匿名类作为方法的参数，代替接口 <code>DataStructureIterator</code>的实例.</p>
<p><strong>提示</strong>:你无法访问 <code>DataStructure</code>之外的私有成员 <code>SIZE</code> 和 <code>arrayOfInts</code> ，意味着你不能从匿名类访问定义在其之外的私用成员.</p>
</li>
<li><p>定义一个方法 <code>print(java.util.Function&lt;Integer, Boolean&gt; iterator)</code> 实现 <code>print(DataStructureIterator iterator)</code>相同的功能.通过一个lambda表达式参数调用该方法实现打印出偶数下标的元素，重新用lambda表达式参数调用并实现打印出奇数下标的元素。</p>
<p><strong>提示</strong>: 在 <code>print</code> 方法中,你可以通过<code>for</code>语句遍历<code>arrayOfInts</code>.对于每一个下标志，调用<code>function.apply</code>方法。如果方法返回<code>True</code>，打印出下表对应的元素值。</p>
<p>为了实现打印偶数下标的值,你可以定义特定的Lambda表达式实现 <code>Boolean Function.apply(Integer t)</code>方法.表达式接受一个 <code>Integer</code> 参数 (下标)并且返回一个布尔值 (<code>Boolean.TRUE</code> 如果下标是偶数, 否则返回<code>Boolean.FALSE</code> ).</p>
</li>
<li><p>定义两个方法可以是如下语句分别打印偶数下标对应的值和技术下标对应的值:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">DataStructure ds = <span class="keyword">new</span> DataStructure()</div><div class="line"><span class="comment">// ...</span></div><div class="line">ds.print(DataStructure::isEvenIndex);</div><div class="line">ds.print(DataStructure::isOddIndex);</div></pre></td></tr></table></figure>
<p><strong>提示</strong>: 在类中 <code>DataStructure</code> 创建两个名为 <code>isEvenIndex</code> 和 <code>isOddIndex</code> 的方法，两个方法拥有同样的参数列和且返回类型，同抽象方法 <code>Boolean Function&lt;Integer, Boolean&gt;.apply(Integer t)</code>一致. 这两个方法同样接受 <code>Integer</code> 参数 (下标志) 并且返回 <code>Boolean</code> 值.</p>
</li>
</ol>
<p><strong>答案</strong>: 参考文件<a href="https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/DataStructure.java" target="_blank" rel="external"><code>DataStructure.java</code></a>.</p>
</li>
</ol>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;嵌套类：问题与练习&quot;&gt;&lt;a href=&quot;#嵌套类：问题与练习&quot; class=&quot;headerlink&quot; title=&quot;嵌套类：问题与练习&quot;&gt;&lt;/a&gt;嵌套类：问题与练习&lt;/h1&gt;&lt;h2 id=&quot;问题&quot;&gt;&lt;a href=&quot;#问题&quot; class=&quot;headerlink&quot; title=&quot;问题&quot;&gt;&lt;/a&gt;问题&lt;/h2&gt;&lt;ol&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;: 程序 &lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/Problem.java&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;&lt;code&gt;Problem.java&lt;/code&gt;&lt;/a&gt; 不能编译，你需要怎么做才能让它成功编译?为什么?&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;使用 &lt;a href=&quot;https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;&lt;code&gt;Box&lt;/code&gt;&lt;/a&gt; 类(在 &lt;code&gt;javax.swing&lt;/code&gt; 包中)的Java API文档 帮助你问答如下问题 .&lt;/p&gt;
&lt;ol&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;: &lt;code&gt;Box&lt;/code&gt; 定义了什么静态嵌套类?&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;:  &lt;code&gt;Box&lt;/code&gt; 定义了什么内部类?&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;:  &lt;code&gt;Box&lt;/code&gt;的内部类的父类是什么?&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;:  &lt;code&gt;Box&lt;/code&gt;的哪一个嵌套类你可以在任何一个类中使用?&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;问题&lt;/strong&gt;: 如何创建一个 &lt;code&gt;Box&lt;/code&gt;的`Filler嵌套类的实例?&lt;/p&gt;
&lt;/li&gt;
&lt;/ol&gt;
&lt;/li&gt;
&lt;/ol&gt;
&lt;h2 id=&quot;练习&quot;&gt;&lt;a href=&quot;#练习&quot; class=&quot;headerlink&quot; title=&quot;练习&quot;&gt;&lt;/a&gt;练习&lt;/h2&gt;&lt;ol&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;练习&lt;/strong&gt;: 获取文件 &lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/Class1.java&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;&lt;code&gt;Class1.java&lt;/code&gt;&lt;/a&gt;. 编译并运行 &lt;code&gt;Class1&lt;/code&gt;. 输出是什么?&lt;/p&gt;
&lt;/li&gt;
&lt;li&gt;&lt;p&gt;&lt;strong&gt;练习&lt;/strong&gt;: 下面的联系涉及到修改 &lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/examples/DataStructure.java&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;&lt;code&gt;DataStructure.java&lt;/code&gt;&lt;/a&gt;类, 该类在 &lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;Inner Class Example&lt;/a&gt; 中有讨论.&lt;/p&gt;
&lt;ol&gt;
&lt;li&gt;定义一个名为 &lt;code&gt;print(DataStructureIterator iterator)&lt;/code&gt;的方法.用一个类&lt;code&gt;EvenIterator&lt;/code&gt;类的实例调用这个方法实现&lt;code&gt;printEven&lt;/code&gt;方法的功能。&lt;/li&gt;
&lt;/ol&gt;
&lt;/li&gt;
&lt;/ol&gt;
&lt;ol&gt;
&lt;li&gt;调用方法 &lt;code&gt;print(DataStructureIterator iterator)&lt;/code&gt; 来打印奇数索引的元素.使用一个匿名类作为方法的参数，代替接口 &lt;code&gt;DataStructureIterator&lt;/code&gt;的实例.&lt;/li&gt;
&lt;/ol&gt;
&lt;ol&gt;
&lt;li&gt;定义一个方法 &lt;code&gt;print(java.util.Function&amp;lt;Integer, Boolean&amp;gt; iterator)&lt;/code&gt; 实现 &lt;code&gt;print(DataStructureIterator iterator)&lt;/code&gt;相同的功能.通过一个lambda表达式参数调用该方法实现打印出偶数下标的元素，重新用lambda表达式参数调用并实现打印出奇数下标的元素。&lt;/li&gt;
&lt;/ol&gt;
&lt;ol&gt;
&lt;li&gt;&lt;p&gt;定义两个方法可以是如下语句分别打印偶数下标对应的值和技术下标对应的值:&lt;/p&gt;
&lt;figure class=&quot;highlight java&quot;&gt;&lt;table&gt;&lt;tr&gt;&lt;td class=&quot;gutter&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;1&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;2&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;3&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;4&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;td class=&quot;code&quot;&gt;&lt;pre&gt;&lt;div class=&quot;line&quot;&gt;DataStructure ds = &lt;span class=&quot;keyword&quot;&gt;new&lt;/span&gt; DataStructure()&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;&lt;span class=&quot;comment&quot;&gt;// ...&lt;/span&gt;&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;ds.print(DataStructure::isEvenIndex);&lt;/div&gt;&lt;div class=&quot;line&quot;&gt;ds.print(DataStructure::isOddIndex);&lt;/div&gt;&lt;/pre&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/figure&gt;
&lt;/li&gt;
&lt;/ol&gt;
&lt;h1 id=&quot;嵌套类：问题与练习答案&quot;&gt;&lt;a href=&quot;#嵌套类：问题与练习答案&quot; class=&quot;headerlink&quot; title=&quot;嵌套类：问题与练习答案&quot;&gt;&lt;/a&gt;嵌套类：问题与练习答案&lt;/h1&gt;
    
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：基础-何时使用嵌套类，局部类，匿名类，Lambda</title>
    <link href="http://yoursite.com/2018/03/19/java-tutorial-learning_class&amp;object_when_to_use/"/>
    <id>http://yoursite.com/2018/03/19/java-tutorial-learning_class&amp;object_when_to_use/</id>
    <published>2018-03-19T06:51:59.000Z</published>
    <updated>2018-03-19T09:02:44.203Z</updated>
    
    <content type="html"><![CDATA[<h1 id="何时使用嵌套类，局部类，匿名类，Lambda表达式"><a href="#何时使用嵌套类，局部类，匿名类，Lambda表达式" class="headerlink" title="何时使用嵌套类，局部类，匿名类，Lambda表达式"></a>何时使用嵌套类，局部类，匿名类，Lambda表达式</h1><p>正如在嵌套类 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html" target="_blank" rel="external">Nested Classes</a>一章节中提到的,嵌套类可以使你将只是用一次的代码符合逻辑的整理到一块，增加代码的封装性，创建更可读和易于管理的代码。局部类，匿名类以及Lambda表达式同样适用于这些优势；并且他们可以用于更加具体的场景:</p>
<ul>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html" target="_blank" rel="external">局部类Local class</a>: 如果你需要创建类的不止一个实例，访问其构造函数，或者引入新的命名类型（因为，譬如你需要在后续条用额外的方法).</li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html" target="_blank" rel="external">匿名类Anonymous class</a>: 如果你需要定义额外的字段或者方法就使用匿名类.</li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html" target="_blank" rel="external">Lambda表达式Lambda expression</a>:<ul>
<li>如果你想要封装一个单独的行为床底给其他的代码就是用Lambda表达式。例如，你你需要创建一个具体的作用于集合中每个元素的操作，当一个程序完成时的处理，或者程序遇到错误的处理。</li>
<li>当你需要你功能接口的你个简单实例并且不需要预先的条件限制。（例如，你不需要使用构造器，不需要明明类型，字段，或者额外的方法）</li>
</ul>
</li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html" target="_blank" rel="external">嵌套类Nested class</a>: 当你的需求与局部类相似，但是又想让类有更广的可用范围，并且你不需要访问局部变量和更多的方法参数就使用嵌套类。<ul>
<li>使用非静态的嵌套类（或者内部类）如果你需要访问闭合空间内的非公有字段或者方法。是用静态的嵌套类如果你不需要这些访问接入。</li>
</ul>
</li>
</ul>
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  </entry>
  
  <entry>
    <title>Java 学习指南_学习Java：基础-Lambda表达式</title>
    <link href="http://yoursite.com/2018/03/15/java-tutorial-learning_the_lang_basic_lambda/"/>
    <id>http://yoursite.com/2018/03/15/java-tutorial-learning_the_lang_basic_lambda/</id>
    <published>2018-03-15T07:51:59.000Z</published>
    <updated>2018-03-19T08:11:25.046Z</updated>
    
    <content type="html"><![CDATA[<h1 id="Lambda-表达式-Lambda-Expressions"><a href="#Lambda-表达式-Lambda-Expressions" class="headerlink" title="Lambda 表达式 Lambda Expressions"></a>Lambda 表达式 Lambda Expressions</h1><p>使用匿名类的一个问题在于，如果你要实现的匿名类非常的简单，例如只包含一个方法的借口，那么匿名列的语法看起来就不够简洁和轻便。这种情况下，你通常会尝试将某个功能作为参数传递给一其他方法，例如当点击按钮的时候会采取什么措施。Lambda表达式可以提供这样的功能，将功能作为方法的参数，或者代码作为数据。</p>
<p>上一节中， <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html" target="_blank" rel="external">Anonymous Classes</a>, 向你展示了实现一个基本的类而不需要命名，譬如只有一个方法的类，不过匿名类看起来有点复杂和笨重。Lambda表达式可以使你更加紧凑的表达一个单方法类的实例。</p>
<p>本节将包含以下的内容:</p>
<ul>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#use-case" target="_blank" rel="external">Lambda表达式的经典应用Ideal Use Case for Lambda Expressions</a><ul>
<li><a href="#approach1">方法1：创建搜索某一特点的成员的方法Approach 1: Create Methods That Search for Members That Match One Characteristic</a></li>
<li><a href="#approach2">方法2：创建更加通用的搜索方法Approach 2: Create More Generalized Search Methods</a></li>
<li><a href="#approach3">方法3：用局部类实现特定搜索规则相关的的代码Approach 3: Specify Search Criteria Code in a Local Class</a></li>
<li><a href="#approach4">方法4：用匿名类实现特定搜索规则相关的代码Approach 4: Specify Search Criteria Code in an Anonymous Class</a></li>
<li><a href="#approach5">方法5：用Lambda表达式实现特定搜索规则的代码Approach 5: Specify Search Criteria Code with a Lambda Expression</a></li>
<li><a href="#approach6">方法6：通过标准的功能接口使用lambda表达式Approach 6: Use Standard Functional Interfaces with Lambda Expressions</a></li>
<li><a href="#approach7">方法7：在你的应用中使用Lambda表达式Approach 7: Use Lambda Expressions Throughout Your Application</a></li>
<li><a href="#approach8">方法8：使用反应提高可扩展性Approach 8: Use Generics More Extensively</a></li>
<li><a href="#approach9">方法9：使用已Lambda表达式作为参数的聚合操作Approach 9: Use Aggregate Operations That Accept Lambda Expressions as Parameters</a></li>
</ul>
</li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#lambda-expressions-in-gui-applications" target="_blank" rel="external">在GUI应用中使用Lambda表达式Lambda Expressions in GUI Applications</a></li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#syntax" target="_blank" rel="external">Lambda表达式语法Syntax of Lambda Expressions</a></li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#accessing-local-variables" target="_blank" rel="external">访问闭合区间的局部变量Accessing Local Variables of the Enclosing Scope</a></li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#target-typing" target="_blank" rel="external">目标类型Target Typing</a><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#target-types-and-method-arguments" target="_blank" rel="external">目标类型以及方法参数Target Types and Method Arguments</a></li>
<li><a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#serialization" target="_blank" rel="external">序列化Serialization</a></li>
</ul>
<a id="more"></a>
<h2 id="Lambda表达式最佳实践Ideal-Use-Case-for-Lambda-Expressions"><a href="#Lambda表达式最佳实践Ideal-Use-Case-for-Lambda-Expressions" class="headerlink" title="Lambda表达式最佳实践Ideal Use Case for Lambda Expressions"></a><a href="">Lambda表达式最佳实践Ideal Use Case for Lambda Expressions</a></h2><p>假设你正在创建一个社交网络应用。你想要创建一个功能来满足管理员的各种操作，例如，向社交网络应用中中满足特定规则的成员发送消息。下面的表格描述了这个案例的详细信息。</p>
<table>
<thead>
<tr>
<th>字段</th>
<th>描述</th>
</tr>
</thead>
<tbody>
<tr>
<td>名称</td>
<td>对选择的成员进行操作</td>
</tr>
<tr>
<td>主要角色</td>
<td>管理员Administrator</td>
</tr>
<tr>
<td>前置条件</td>
<td>管理员已登录到系统</td>
</tr>
<tr>
<td>完成条件</td>
<td>操作只对符合特定规则的成员生效</td>
</tr>
<tr>
<td>内容梗概</td>
<td>管理员确定要执行操作的成员。管理员明确要执行的操作。管理员选择Submit按钮。系统找到所有满足条件的成员。系统对所有符合条件成员执行操作</td>
</tr>
<tr>
<td>扩展</td>
<td>1a. 管理员可以在提交操作之前预览那些符合条件的成员。</td>
</tr>
<tr>
<td>发生频率Frequency</td>
<td>每天数次</td>
</tr>
</tbody>
</table>
<p>假设社交网络应用中的成员可以用如下的 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Person.java" target="_blank" rel="external"><code>Person</code></a> 类表示:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Person</span> </span>&#123;</div><div class="line"></div><div class="line">    <span class="keyword">public</span> <span class="keyword">enum</span> Sex &#123;</div><div class="line">        MALE, FEMALE</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    String name;</div><div class="line">    LocalDate birthday;</div><div class="line">    Sex gender;</div><div class="line">    String emailAddress;</div><div class="line"></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getAge</span><span class="params">()</span> </span>&#123;</div><div class="line">        <span class="comment">// ...</span></div><div class="line">    &#125;</div><div class="line"></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">printPerson</span><span class="params">()</span> </span>&#123;</div><div class="line">        <span class="comment">// ...</span></div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>假设你社交网络应用中的成员是存储在一个 <code>List&lt;Person&gt;</code> 实例中.</p>
<p>本章开始用一种稚拙的方法实现这个用例。然后使用局部类类和匿名类来改进，最后使用lambda表达式通过一种高效和简洁的方式完成。本章节中 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/RosterTest.java" target="_blank" rel="external"><code>RosterTest</code></a>例子中可以找到相关的代码片段。.</p>
<h3 id="方法1：创建搜索符合某些特征的成员的方法"><a href="#方法1：创建搜索符合某些特征的成员的方法" class="headerlink" title="方法1：创建搜索符合某些特征的成员的方法"></a><a href="">方法1：创建搜索符合某些特征的成员的方法</a></h3><p>一个简单的途径就是创建数个方法methods;每一个方法搜索符合一个特征的成员，譬如性别或者年龄。下面的代码打印出了年龄大于某特定年龄的成员:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">printPersonsOlderThan</span><span class="params">(List&lt;Person&gt; roster, <span class="keyword">int</span> age)</span> </span>&#123;</div><div class="line">    <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">        <span class="keyword">if</span> (p.getAge() &gt;= age) &#123;</div><div class="line">            p.printPerson();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p><strong>注意</strong>:  <a href="https://docs.oracle.com/javase/8/docs/api/java/util/List.html" target="_blank" rel="external"><code>List</code></a> 是一个有序的集合 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html" target="_blank" rel="external"><code>Collection</code></a>. 集合是在一个单独单眼里包含多个元素的对象。集合是用来存储，获取，操作以及表达集合数据。参考 <a href="https://docs.oracle.com/javase/tutorial/collections/index.html" target="_blank" rel="external">Collections</a> 获取更多集合相关的信息.</p>
<p>这种方法可能使你的应用非常的脆弱，譬如可能英文介绍里提到的扩展(例如新的数据类型)而导致程序无法工作。假设你应用的升级改变了<code>Person</code>类的数据结构，该数据结构包含了不同的成员变量；或许类使用了不同的数据类型来存储年龄，或者用不同的算法来计量年龄。你就不得不重写你的API来满足这些改变。另外，这个方法还有不必要的限制，如果你想要打印出小于特定年龄的成员时改怎么办呢?</p>
<h3 id="方案2：创建更加通用的方法Approach-2-Create-More-Generalized-Search-Methods"><a href="#方案2：创建更加通用的方法Approach-2-Create-More-Generalized-Search-Methods" class="headerlink" title="方案2：创建更加通用的方法Approach 2: Create More Generalized Search Methods"></a><a href="">方案2：创建更加通用的方法Approach 2: Create More Generalized Search Methods</a></h3><p>下面的方法比上一个方法<code>printPersonOlderThan</code>更通用一些；可以打印出特定年龄范围的成员：:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">printPersonsWithinAgeRange</span><span class="params">(</span></span></div><div class="line">    List&lt;Person&gt; roster, <span class="keyword">int</span> low, <span class="keyword">int</span> high) &#123;</div><div class="line">    <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">        <span class="keyword">if</span> (low &lt;= p.getAge() &amp;&amp; p.getAge() &lt; high) &#123;</div><div class="line">            p.printPerson();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>那么如果你想要打印出某一性别，或者是某一性别和特定年龄范围组合的相关成员？如果你决定改变<code>Person</code>类型的属性增加诸如情感状态或者地理位置？尽管这个方法比之前的方法更通用，但是尝试为每一种可能的搜索都创建单独的方法同样导致脆弱的代码。你可以将说明具体搜索范围的代码分割出来，创建另外一个类。</p>
<h3 id="方案3：在局部类中说明搜索标准Approach-3-Specify-Search-Criteria-Code-in-a-Local-Class"><a href="#方案3：在局部类中说明搜索标准Approach-3-Specify-Search-Criteria-Code-in-a-Local-Class" class="headerlink" title="方案3：在局部类中说明搜索标准Approach 3: Specify Search Criteria Code in a Local Class"></a><a href="">方案3：在局部类中说明搜索标准Approach 3: Specify Search Criteria Code in a Local Class</a></h3><p>下面的方法打印出符合你搜索标准的成员:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">printPersons</span><span class="params">(</span></span></div><div class="line">    List&lt;Person&gt; roster, CheckPerson tester) &#123;</div><div class="line">    <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">        <span class="keyword">if</span> (tester.test(p)) &#123;</div><div class="line">            p.printPerson();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>这个方法检查<code>List</code>参数<code>roster</code>中的每一个<code>Person</code>实例是否满搜索标准，通过调用<code>CheckPerson</code>参数<code>tester</code>的<code>test</code>方法来实现。如果<code>tester.test</code>返回一个<code>True</code>，呢么<code>Person</code>实例中的<code>printPerson</code>方法就回被调用.</p>
<p>针对不同的搜索标准，你需要实现<code>CheckPerson</code>接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="class"><span class="keyword">interface</span> <span class="title">CheckPerson</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">test</span><span class="params">(Person p)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>下面的类实现了<code>CheckPerson</code>接口，明确了<code>test</code>方法的一个具体实现。这个方法过滤了符合美国服兵役条件的成员:如果<code>Person</code>参数是男性并且年龄在18到25之间则返回<code>True</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="class"><span class="keyword">class</span> <span class="title">CheckPersonEligibleForSelectiveService</span> <span class="keyword">implements</span> <span class="title">CheckPerson</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">test</span><span class="params">(Person p)</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> p.gender == Person.Sex.MALE &amp;&amp;</div><div class="line">            p.getAge() &gt;= <span class="number">18</span> &amp;&amp;</div><div class="line">            p.getAge() &lt;= <span class="number">25</span>;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>通过这个类，你创建一个新的<code>Person</code>实例并且调用<code>printPersons</code>方法:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">printPersons(</div><div class="line">    roster, <span class="keyword">new</span> CheckPersonEligibleForSelectiveService());</div></pre></td></tr></table></figure>
<p>尽管这个方案没有那么脆弱了——当你改变Person的数据结构时，无需再重写你的方法——当你仍然需要额外的代码：一个新的结构以及一个本地类来查询你计划进行操作的成员。因为<code>CheckPersonEligibleForSelectiveService</code>实现了一个接口，所以你可以通过匿名类来代替局部类，并且可以避免对每一中搜索都创建一个新的类。</p>
<h3 id="方案4：Approach-4-Specify-Search-Criteria-Code-in-an-Anonymous-Class"><a href="#方案4：Approach-4-Specify-Search-Criteria-Code-in-an-Anonymous-Class" class="headerlink" title="方案4：Approach 4: Specify Search Criteria Code in an Anonymous Class"></a><a href="">方案4：Approach 4: Specify Search Criteria Code in an Anonymous Class</a></h3><p>下列代码中<code>printPersons</code>方法调用中的一个参数是一个匿名类，这个匿名类筛选出美国需要服兵役的成员:年龄介于18到25岁的男性成员。:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div></pre></td><td class="code"><pre><div class="line">printPersons(</div><div class="line">    roster,</div><div class="line">    <span class="keyword">new</span> CheckPerson() &#123;</div><div class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">test</span><span class="params">(Person p)</span> </span>&#123;</div><div class="line">            <span class="keyword">return</span> p.getGender() == Person.Sex.MALE</div><div class="line">                &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">                &amp;&amp; p.getAge() &lt;= <span class="number">25</span>;</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">);</div></pre></td></tr></table></figure>
<p>这个方案减少了必须的代码，因为你不需要为每一个查询都创建一个新的类。然而，匿名类的语法非常的笨重，考虑到<code>CheckPerson</code>接口中实际只包含了一个方法。在这种情况下，你可以使用你个lambda表达式来代替匿名类，也就是下一节所讲的内容.</p>
<h3 id="方案5：使用lambda表达式表明搜索标准Approach-5-Specify-Search-Criteria-Code-with-a-Lambda-Expression"><a href="#方案5：使用lambda表达式表明搜索标准Approach-5-Specify-Search-Criteria-Code-with-a-Lambda-Expression" class="headerlink" title="方案5：使用lambda表达式表明搜索标准Approach 5: Specify Search Criteria Code with a Lambda Expression"></a><a href="">方案5：使用lambda表达式表明搜索标准Approach 5: Specify Search Criteria Code with a Lambda Expression</a></h3><p><code>CheckPerson</code>接口是一个<em>functional interface</em>功能接口.功能接口是指只包含一个抽象方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html" target="_blank" rel="external">abstract method</a>的接口. (当然功能接口可能包含一个或者多个默认方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html" target="_blank" rel="external">default methods</a>或者静态方法 <a href="https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html#static" target="_blank" rel="external">static methods</a>.) 因为一个功能接口只包含了一个抽象方法，当你实现它的时候你可以移除方法名。这么做就替代了匿名类的表达式，使用了lambda表达式 <em>lambda expression</em>,正如如下代码方法调用中说所用的样子:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line">printPersons(</div><div class="line">    roster,</div><div class="line">    (Person p) -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">        &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">        &amp;&amp; p.getAge() &lt;= <span class="number">25</span></div><div class="line">);</div></pre></td></tr></table></figure>
<p>参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#syntax" target="_blank" rel="external">Lambda 表达式语法Syntax of Lambda Expressions</a>获取如何定义Lambda表达式的更多信息.</p>
<p>你可以使用标准的功能接口来代替<code>CheckPerson</code>接口，可以进一步减少所需的代码量.</p>
<h3 id="方法6：使用标准的功能接口实现Lambda表达式Approach-6-Use-Standard-Functional-Interfaces-with-Lambda-Expressions"><a href="#方法6：使用标准的功能接口实现Lambda表达式Approach-6-Use-Standard-Functional-Interfaces-with-Lambda-Expressions" class="headerlink" title="方法6：使用标准的功能接口实现Lambda表达式Approach 6: Use Standard Functional Interfaces with Lambda Expressions"></a><a href="">方法6：使用标准的功能接口实现Lambda表达式Approach 6: Use Standard Functional Interfaces with Lambda Expressions</a></h3><p>重现审视CheckPerson 接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="class"><span class="keyword">interface</span> <span class="title">CheckPerson</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">test</span><span class="params">(Person p)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>这是一个非常简单的是接口。因为他只包含一个抽象方法所以他是一个功能接口。这个方法接口一个参数并返回布尔值。这个方法如此简单所以你没有必要再应用中定义它。因此，JDK定义了数个标准的功能接口，你可以在<code>java.util.function</code>包中找到这些接口。</p>
<p>例如，你个使用 <code>Predicate&lt;T&gt;</code>接口来代替 <code>CheckPerson</code>.这个接口包含了方法 <code>boolean test(T t)</code>:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="class"><span class="keyword">interface</span> <span class="title">Predicate</span>&lt;<span class="title">T</span>&gt; </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">test</span><span class="params">(T t)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>接口 <code>Predicate&lt;T&gt;</code> 是泛型接口的一个例子. (参考泛型（更新） <a href="https://docs.oracle.com/javase/tutorial/java/generics/index.html" target="_blank" rel="external">Generics (Updated)</a> 一颗来获取更多关于泛型的信息.) 泛型类（例如泛型接口）在尖括号中表明一个或多个类型参数(<code>&lt;&gt;</code>). 这个接口只包含了一个参数，<code>T</code>,当你定义实际的类型参数时，你有一个参数类型，例如 <code>Predicate&lt;Person&gt;</code> :</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line"><span class="class"><span class="keyword">interface</span> <span class="title">Predicate</span>&lt;<span class="title">Person</span>&gt; </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">test</span><span class="params">(Person t)</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>参数类型，包含了一个拥有同样参数类型的方法 <code>CheckPerson.boolean test(Person p)</code>. 因此，你可以使用 <code>Predicate&lt;T&gt;</code> 替换 <code>CheckPerson</code>类，如下所示:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">printPersonsWithPredicate</span><span class="params">(</span></span></div><div class="line">    List&lt;Person&gt; roster, Predicate&lt;Person&gt; tester) &#123;</div><div class="line">    <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">        <span class="keyword">if</span> (tester.test(p)) &#123;</div><div class="line">            p.printPerson();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>结果，下面的方法调用就如同你在方法3中 <code>printPersons</code> <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#approach3" target="_blank" rel="external">Approach 3: Specify Search Criteria Code in a Local Class</a> 一样，可以获取符合服兵役条件的成员:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line">printPersonsWithPredicate(</div><div class="line">    roster,</div><div class="line">    p -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">        &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">        &amp;&amp; p.getAge() &lt;= <span class="number">25</span></div><div class="line">);</div></pre></td></tr></table></figure>
<p>这里并不是唯一可以使用lambda表达式的地方。下面的方案推荐了其他使用lambda表达式的方法.</p>
<h3 id="方案7-在你的整个应用中使用lambda表达式Approach-7-Use-Lambda-Expressions-Throughout-Your-Application"><a href="#方案7-在你的整个应用中使用lambda表达式Approach-7-Use-Lambda-Expressions-Throughout-Your-Application" class="headerlink" title="方案7:在你的整个应用中使用lambda表达式Approach 7: Use Lambda Expressions Throughout Your Application"></a><a href="">方案7:在你的整个应用中使用lambda表达式Approach 7: Use Lambda Expressions Throughout Your Application</a></h3><p>重新考虑方法 <code>printPersonsWithPredicate</code> ，看看你可以在哪里使用lambda表达式:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">printPersonsWithPredicate</span><span class="params">(</span></span></div><div class="line">    List&lt;Person&gt; roster, Predicate&lt;Person&gt; tester) &#123;</div><div class="line">    <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">        <span class="keyword">if</span> (tester.test(p)) &#123;</div><div class="line">            p.printPerson();</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>这个犯法检查了<code>List</code>参数<code>roster</code>中每一个<code>Person</code>实例是否符合具体的标准<code>Predicate</code>参数<code>tester</code>。如果Person实例确实满足<code>tester</code>的条件，<code>Person</code>实例就调用<code>printPerson</code>方法.</p>
<p>取代了调用<code>printPerson</code>方法，你可以对满足搜索条件的成员制定不同的操作。你同样可以通过lambda表达式来实现操作。假如你希望有一个类似printPerson方法的lambda表达式，获取一个Person参数并返回void。记住，使用lambda表达式，你需要实现一个功能接口。本例中，你需要实现一个接受Person参数并且返回为void的接口。<code>Consumer&lt;T&gt;</code>接口，包含了<code>void accept(T t)</code>方法，刚好符合这一特征.下面的代码使用你个<code>Consumer&lt;Person&gt;</code>的实例的<code>accept</code>方法调用，替换了<code>p.printPerson()</code>方法的调用:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">processPersons</span><span class="params">(</span></span></div><div class="line">    List&lt;Person&gt; roster,</div><div class="line">    Predicate&lt;Person&gt; tester,</div><div class="line">    Consumer&lt;Person&gt; block) &#123;</div><div class="line">        <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">            <span class="keyword">if</span> (tester.test(p)) &#123;</div><div class="line">                block.accept(p);</div><div class="line">            &#125;</div><div class="line">        &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>这样，如下的方法调用同样实现了方案3中的功能，lambda表达式使用的代码如下所示:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line">processPersons(</div><div class="line">     roster,</div><div class="line">     p -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">         &amp;&amp; p.getAge() &gt;= 18</div><div class="line">         &amp;&amp; p.getAge() &lt;= 25,</div><div class="line">     p -&gt; p.printPerson()</div><div class="line">);</div></pre></td></tr></table></figure>
<p>如果你想要对成员的信息做更多的操作而不只是打印出来。假设你需要验证成员的简介并且获取他们的联系信息。在本例中，你需要一个方能接口把汗了一个返回值的抽象方法。 <code>Function&lt;T,R&gt;</code>接口包含一个方法 <code>R apply(T t)</code>.下面的代码通过参数<code>mapper</code>，获取了数据并通过<code>block</code>参数进行的具体的操作：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">processPersonsWithFunction</span><span class="params">(</span></span></div><div class="line">    List&lt;Person&gt; roster,</div><div class="line">    Predicate&lt;Person&gt; tester,</div><div class="line">    Function&lt;Person, String&gt; mapper,</div><div class="line">    Consumer&lt;String&gt; block) &#123;</div><div class="line">    <span class="keyword">for</span> (Person p : roster) &#123;</div><div class="line">        <span class="keyword">if</span> (tester.test(p)) &#123;</div><div class="line">            String data = mapper.apply(p);</div><div class="line">            block.accept(data);</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>下面的方法取得了过滤后的成员的邮箱地址并且打印出了他们:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">processPersonsWithFunction(</div><div class="line">    roster,</div><div class="line">    p -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">        &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">        &amp;&amp; p.getAge() &lt;= <span class="number">25</span>,</div><div class="line">    p -&gt; p.getEmailAddress(),</div><div class="line">    email -&gt; System.out.println(email)</div><div class="line">);</div></pre></td></tr></table></figure>
<h3 id="方案8：使用泛型提高扩展性Approach-8-Use-Generics-More-Extensively"><a href="#方案8：使用泛型提高扩展性Approach-8-Use-Generics-More-Extensively" class="headerlink" title="方案8：使用泛型提高扩展性Approach 8: Use Generics More Extensively"></a><a href="">方案8：使用泛型提高扩展性Approach 8: Use Generics More Extensively</a></h3><p>重新考虑方法 <code>processPersonsWithFunction</code>. 如下代码是一个泛型的版本，可以接受任何类型元素集合作为参数:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="keyword">static</span> &lt;X, Y&gt; <span class="function"><span class="keyword">void</span> <span class="title">processElements</span><span class="params">(</span></span></div><div class="line">    Iterable&lt;X&gt; source,</div><div class="line">    Predicate&lt;X&gt; tester,</div><div class="line">    Function &lt;X, Y&gt; mapper,</div><div class="line">    Consumer&lt;Y&gt; block) &#123;</div><div class="line">    <span class="keyword">for</span> (X p : source) &#123;</div><div class="line">        <span class="keyword">if</span> (tester.test(p)) &#123;</div><div class="line">            Y data = mapper.apply(p);</div><div class="line">            block.accept(data);</div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>如果需要打印符合条件的成员的e-mail地址，调用<code>processElements</code>方法的代码如下所示:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">processElements(</div><div class="line">    roster,</div><div class="line">    p -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">        &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">        &amp;&amp; p.getAge() &lt;= <span class="number">25</span>,</div><div class="line">    p -&gt; p.getEmailAddress(),</div><div class="line">    email -&gt; System.out.println(email)</div><div class="line">);</div></pre></td></tr></table></figure>
<p>方法调用执行了一下的操作:</p>
<ol>
<li>获取对象集合 <code>source</code>. 本例中，获取了<code>Person</code>对象的集合<code>roster</code> .<code>roster</code>集合是一个<code>List</code>同样是个 <code>Iterable</code>可迭代的对象.</li>
<li>过滤对象中匹配 <code>Predicate</code> 对象 <code>tester</code>条件的 成员.  <code>Predicate</code>对象在本例中满足了服兵役条件的成员。</li>
<li>通过 <code>Function</code> 对象 <code>mapper</code>映射每一个成员.本例中获取了每个成员的e-mail地址.</li>
<li>通过 <code>Consumer</code> 对象 <code>block</code>方法对映射后的结果做具体的操做，本例中将应设计过也就是e-mail地址打印了出来.</li>
</ol>
<p>你同样可以使用聚合操作（连贯操作）来替代以上的操作步骤。.</p>
<h3 id="方案9：使用接受Labmda表达式为参数的集合操作Approach-9-Use-Aggregate-Operations-That-Accept-Lambda-Expressions-as-Parameters"><a href="#方案9：使用接受Labmda表达式为参数的集合操作Approach-9-Use-Aggregate-Operations-That-Accept-Lambda-Expressions-as-Parameters" class="headerlink" title="方案9：使用接受Labmda表达式为参数的集合操作Approach 9: Use Aggregate Operations That Accept Lambda Expressions as Parameters"></a><a href="">方案9：使用接受Labmda表达式为参数的集合操作Approach 9: Use Aggregate Operations That Accept Lambda Expressions as Parameters</a></h3><p>下面的例子使用连贯操作打印出了符合条件的承欢的e-mail地址:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">roster</div><div class="line">    .stream()</div><div class="line">    .filter(</div><div class="line">        p -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">            &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">            &amp;&amp; p.getAge() &lt;= <span class="number">25</span>)</div><div class="line">    .map(p -&gt; p.getEmailAddress())</div><div class="line">    .forEach(email -&gt; System.out.println(email));</div></pre></td></tr></table></figure>
<p>如下表格列出了processElements功能中各个步骤以及对应的连贯操作方法:</p>
<table>
<thead>
<tr>
<th><code>processElements</code> Action（方法步骤）</th>
<th>Aggregate Operation(集合操作)</th>
</tr>
</thead>
<tbody>
<tr>
<td>Obtain a source of objects获取资源，集合对象</td>
<td><code>Stream&lt;E&gt; **stream**()</code></td>
</tr>
<tr>
<td>Filter objects that match a <code>Predicate</code> object过滤对象</td>
<td><code>Stream&lt;T&gt; **filter**(Predicate&lt;? super T&gt; predicate)</code></td>
</tr>
<tr>
<td>Map objects to another value as specified by a <code>Function</code> object将集合中对象映射为其他的值</td>
<td><code>&lt;R&gt; Stream&lt;R&gt; **map**(Function&lt;? super T,? extends R&gt; mapper)</code></td>
</tr>
<tr>
<td>Perform an action as specified by a <code>Consumer</code> object通过<code>Consumer</code>对象执行具体的操作</td>
<td><code>void **forEach**(Consumer&lt;? super T&gt; action)</code></td>
</tr>
</tbody>
</table>
<p>操作 <code>filter</code>, <code>map</code>, 以及 <code>forEach</code> 是 <em>aggregate operations</em>聚合操作. 局和操作将元素作为一个流来处理，而不是直接从集合中（也就是为什么要首先调用<code>stream</code>方法的原因）.<em>stream</em>流是一个元素集合的序列。区别与集合的是，它并不是存储元素的数据结构，流负责从集合中取出资源，通过一个管道。管道pipeline是一系列的流式操作，在本例中的<code>filter-map-forEach</code>。另外，局和操作通常接受lambda表达式作为参数，是你可以自定义操作的行为。</p>
<p>获取更多关于聚合操作的信息，你可以参考 <a href="https://docs.oracle.com/javase/tutorial/collections/streams/index.html" target="_blank" rel="external">Aggregate Operations</a> 一课.</p>
<h2 id="Lambda表达式在GUI应用的使用Lambda-Expressions-in-GUI-Applications"><a href="#Lambda表达式在GUI应用的使用Lambda-Expressions-in-GUI-Applications" class="headerlink" title="Lambda表达式在GUI应用的使用Lambda Expressions in GUI Applications"></a><a href="">Lambda表达式在GUI应用的使用Lambda Expressions in GUI Applications</a></h2><p>处理GUI应用的事件，例如按键事件，鼠标事件，以及滚动事件。通常你都会创建事件处理器，一般都是通过实现特定的接口。一般来说，事件处理器接口是功能接口；他们通常只有一个方法。</p>
<p>在JAVAFX例子中 <a href="https://docs.oracle.com/javase/8/javafx/get-started-tutorial/hello_world.htm" target="_blank" rel="external"><code>HelloWorld.java</code></a> (前一章节中 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html" target="_blank" rel="external">Anonymous Classes</a>), 中提到的你可以将如下所示的匿名类代码替换为Lambda表达式:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line">btn.setOnAction(<span class="keyword">new</span> EventHandler&lt;ActionEvent&gt;() &#123;</div><div class="line"></div><div class="line">    <span class="meta">@Override</span></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">handle</span><span class="params">(ActionEvent event)</span> </span>&#123;</div><div class="line">        System.out.println(<span class="string">"Hello World!"</span>);</div><div class="line">    &#125;</div><div class="line">&#125;);</div></pre></td></tr></table></figure>
<p><code>btn.setOnAction</code>方法调用定义当你鼠标选择了<code>btn</code>对象代表的按钮式要放生的事情。这个方法要求一个<code>EventHandler&lt;ActionEvent&gt;</code>类型的对象作为参数。<code>EventHandler&lt;ActionEvent&gt;</code> 接口只包含了一个方法<code>void handle&lt;T event&gt;</code> 。这个接口是一个功能接口，因此你可以使用如下的<code>Lambda</code>来替换他:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">btn.setOnAction(</div><div class="line">  event -&gt; System.out.println(<span class="string">"Hello World!"</span>)</div><div class="line">);</div></pre></td></tr></table></figure>
<h2 id="Lambda表达式的语法Syntax-of-Lambda-Expressions"><a href="#Lambda表达式的语法Syntax-of-Lambda-Expressions" class="headerlink" title="Lambda表达式的语法Syntax of Lambda Expressions"></a><a href="">Lambda表达式的语法Syntax of Lambda Expressions</a></h2><p>一个Lambda表示由如下几部分构成:</p>
<ul>
<li><p>一个圆括号包围、逗号分隔的参数形式的列表.<code>CheckPerson.test</code>方法包含了一个参数<code>p</code>,代表了<code>Person</code>类的实例。</p>
<p><strong>注意</strong>:你可以省略参数类型。同样当只有一个参数时，你也可以省略圆括号。例如下面的Lambda表达式依然是有效的:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">p -&gt; p.getGender() == Person.Sex.MALE </div><div class="line">    &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">    &amp;&amp; p.getAge() &lt;= <span class="number">25</span></div></pre></td></tr></table></figure>
</li>
<li><p>箭头符号, <code>-&gt;</code></p>
</li>
<li><p>一个方法体, 包含一个单独的表达式或者语句块:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">p.getGender() == Person.Sex.MALE </div><div class="line">    &amp;&amp; p.getAge() &gt;= 18</div><div class="line">    &amp;&amp; p.getAge() &lt;= 25</div></pre></td></tr></table></figure>
<p>如果只有一个表达式，Java运行时会返回表达式的值，同样你可以使用返回语句，如下:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line">p -&gt; &#123;</div><div class="line">    return p.getGender() == Person.Sex.MALE</div><div class="line">        &amp;&amp; p.getAge() &gt;= 18</div><div class="line">        &amp;&amp; p.getAge() &lt;= 25;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>返回语句不是一个表达式；在lambda表达式中如果你使用的语句块则必须用<code>{}</code>来闭合，但是你只调用了没有返回值的方法时时可以省略<code>{}</code>,如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">email -&gt; System.out.println(email)</div></pre></td></tr></table></figure>
</li>
</ul>
<p>注意lambda表达式的定义看起来像是方法的定义;你可以认为lambda表达式是一个匿名方法——没有命名的方法.</p>
<p>接下来的例子, <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/Calculator.java" target="_blank" rel="external"><code>Calculator</code></a>, 使用了不止一个参数:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Calculator</span> </span>&#123;</div><div class="line">  </div><div class="line">    <span class="class"><span class="keyword">interface</span> <span class="title">IntegerMath</span> </span>&#123;</div><div class="line">        <span class="function"><span class="keyword">int</span> <span class="title">operation</span><span class="params">(<span class="keyword">int</span> a, <span class="keyword">int</span> b)</span></span>;   </div><div class="line">    &#125;</div><div class="line">  </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">operateBinary</span><span class="params">(<span class="keyword">int</span> a, <span class="keyword">int</span> b, IntegerMath op)</span> </span>&#123;</div><div class="line">        <span class="keyword">return</span> op.operation(a, b);</div><div class="line">    &#125;</div><div class="line"> </div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String... args)</span> </span>&#123;</div><div class="line">    </div><div class="line">        Calculator myApp = <span class="keyword">new</span> Calculator();</div><div class="line">        IntegerMath addition = (a, b) -&gt; a + b;</div><div class="line">        IntegerMath subtraction = (a, b) -&gt; a - b;</div><div class="line">        System.out.println(<span class="string">"40 + 2 = "</span> +</div><div class="line">            myApp.operateBinary(<span class="number">40</span>, <span class="number">2</span>, addition));</div><div class="line">        System.out.println(<span class="string">"20 - 10 = "</span> +</div><div class="line">            myApp.operateBinary(<span class="number">20</span>, <span class="number">10</span>, subtraction));    </div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>方法<code>operationBinary</code>实现类两个整形的数学运算。运算有每个实例自身区分实现。本例中定义了加法和减法两种运算，代码输出结果如下:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">40 + 2 = 42</div><div class="line">20 - 10 = 10</div></pre></td></tr></table></figure>
<h2 id="访问局部变量Accessing-Local-Variables-of-the-Enclosing-Scope"><a href="#访问局部变量Accessing-Local-Variables-of-the-Enclosing-Scope" class="headerlink" title="访问局部变量Accessing Local Variables of the Enclosing Scope"></a><a href="">访问局部变量Accessing Local Variables of the Enclosing Scope</a></h2><p>就像局部类和匿名类一样，lambda表达式可以获取变量 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html#accessing-members-of-an-enclosing-class" target="_blank" rel="external">capture variables</a>; 他们同局部类和匿名类具有相同的访问权限，lambda表达式么有任何的遮蔽问题（参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html#shadowing" target="_blank" rel="external">Shadowing</a> 获取更多信息).Lambda表达式是此法范围的。意思是他不从父类继承命名也不引入新的命名空间。定义Lambda表达式只解释为它是在闭合的环境内，下面的例子 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/examples/LambdaScopeTest.java" target="_blank" rel="external"><code>LambdaScopeTest</code></a>, 进行了演示:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">import</span> java.util.function.Consumer;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">LambdaScopeTest</span> </span>&#123;</div><div class="line"></div><div class="line">    <span class="keyword">public</span> <span class="keyword">int</span> x = <span class="number">0</span>;</div><div class="line"></div><div class="line">    <span class="class"><span class="keyword">class</span> <span class="title">FirstLevel</span> </span>&#123;</div><div class="line"></div><div class="line">        <span class="keyword">public</span> <span class="keyword">int</span> x = <span class="number">1</span>;</div><div class="line"></div><div class="line">        <span class="function"><span class="keyword">void</span> <span class="title">methodInFirstLevel</span><span class="params">(<span class="keyword">int</span> x)</span> </span>&#123;</div><div class="line">            </div><div class="line">            <span class="comment">// The following statement causes the compiler to generate</span></div><div class="line">            <span class="comment">// the error "local variables referenced from a lambda expression</span></div><div class="line">            <span class="comment">// must be final or effectively final" in statement A:</span></div><div class="line">            <span class="comment">//</span></div><div class="line">            <span class="comment">// x = 99;</span></div><div class="line">            </div><div class="line">            Consumer&lt;Integer&gt; myConsumer = (y) -&gt; </div><div class="line">            &#123;</div><div class="line">                System.out.println(<span class="string">"x = "</span> + x); <span class="comment">// Statement A</span></div><div class="line">                System.out.println(<span class="string">"y = "</span> + y);</div><div class="line">                System.out.println(<span class="string">"this.x = "</span> + <span class="keyword">this</span>.x);</div><div class="line">                System.out.println(<span class="string">"LambdaScopeTest.this.x = "</span> +</div><div class="line">                    LambdaScopeTest.<span class="keyword">this</span>.x);</div><div class="line">            &#125;;</div><div class="line"></div><div class="line">            myConsumer.accept(x);</div><div class="line"></div><div class="line">        &#125;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String... args)</span> </span>&#123;</div><div class="line">        LambdaScopeTest st = <span class="keyword">new</span> LambdaScopeTest();</div><div class="line">        LambdaScopeTest.FirstLevel fl = st.new FirstLevel();</div><div class="line">        fl.methodInFirstLevel(<span class="number">23</span>);</div><div class="line">    &#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>上述例子输出如下:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line">x = <span class="number">23</span></div><div class="line">y = <span class="number">23</span></div><div class="line"><span class="keyword">this</span>.x = <span class="number">1</span></div><div class="line">LambdaScopeTest.<span class="keyword">this</span>.x = <span class="number">0</span></div></pre></td></tr></table></figure>
<p>如果你在定义lambda表达式时将y替换为x，编译器将会产生错误:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">Consumer&lt;Integer&gt; myConsumer = (x) -&gt; &#123;</div><div class="line">    <span class="comment">// ...</span></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>编译器产生错误”变量x已经在方法<code>methodInFirstLevel(int)</code>中声明”因为lambda表达式并不引入新的命名空间，因此，你可以直接访问闭合范围内的成员变量，方法，以及局部变量。例如，lambda比到时直接访问方法<code>methodInFirstLevel</code>的参数<code>x</code>.要方位闭合类中的变量，则需要使用关键字<code>this</code>。本例中，<code>this.x</code>指向成员变量<code>FirstLevel.x</code>.</p>
<p>然而，就像局部类和匿名类一样，你个lambda表达式只能访问闭合块中定义为final或者final作用的变量或者参数，假设你在代码中增加一行:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">methodInFirstLevel</span><span class="params">(<span class="keyword">int</span> x)</span> </span>&#123;</div><div class="line">    x = <span class="number">99</span>;</div><div class="line">    <span class="comment">// ...</span></div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>应为代码片段中，变量FirstLevel.x 不再是有效的最终变量.将导致编译器差生类似”lambda表达式必须引用final或者等同final的变量”:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">System.out.println(<span class="string">"x = "</span> + x);</div></pre></td></tr></table></figure>
<h2 id="目标类型Target-Typing"><a href="#目标类型Target-Typing" class="headerlink" title="目标类型Target Typing"></a><a href="">目标类型Target Typing</a></h2><p>怎么确定lambda表达式的类型呢？回想一下筛选成员是所用的lambda表达式:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">p -&gt; p.getGender() == Person.Sex.MALE</div><div class="line">    &amp;&amp; p.getAge() &gt;= <span class="number">18</span></div><div class="line">    &amp;&amp; p.getAge() &lt;= <span class="number">25</span></div></pre></td></tr></table></figure>
<p>这个lambda表达式在以下两个方法中用到:</p>
<ul>
<li><code>public static void printPersons(List&lt;Person&gt; roster, CheckPerson tester)</code> in <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#approach3" target="_blank" rel="external">Approach 3: Specify Search Criteria Code in a Local Class</a></li>
<li><code>public void printPersonsWithPredicate(List&lt;Person&gt; roster, Predicate&lt;Person&gt; tester)</code> in <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#approach6" target="_blank" rel="external">Approach 6: Use Standard Functional Interfaces with Lambda Expressions</a></li>
</ul>
<p>当Java运行时调用方法<code>printPersons</code>是，他期望的数据类型是<code>CheckPerson</code>,因此lambda是就是这种类型，但是当java运行时调用方法<code>printPersonWithPredicate</code>时，他期望的类型是<code>Predicate&lt;Person&gt;</code>,因此lambda表达式就是这种类型。这种方法期望的数据类型叫做目标类型 <em>target type</em>.为了确认lambda表达式的类型，java编译器根据其出现的上下文或者情景中的目标类型来决定。下面列出了java编译器可以确定目标类型的情形:</p>
<ul>
<li>变量声明Variable declarations</li>
<li>赋值Assignments</li>
<li>返回语句Return statements</li>
<li>数组初始化Array initializers</li>
<li>方法或者狗高企参数Method or constructor arguments</li>
<li>Lambda表达式体Lambda expression bodies</li>
<li>表达式<code>?:</code>情形Conditional expressions, <code>?:</code></li>
<li>类型转换表达式Cast expressions</li>
</ul>
<h3 id="目标类型和方法参数Target-Types-and-Method-Arguments"><a href="#目标类型和方法参数Target-Types-and-Method-Arguments" class="headerlink" title="目标类型和方法参数Target Types and Method Arguments"></a><a href="">目标类型和方法参数Target Types and Method Arguments</a></h3><p>作为方法参数，java编译器根据另外两个语言特性确认目标累心：重载以及类型参数引用。</p>
<p>考虑下面的两个功能接口 ( <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Runnable.html" target="_blank" rel="external"><code>java.lang.Runnable</code></a> 以及 <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Callable.html" target="_blank" rel="external"><code>java.util.concurrent.Callable</code></a>):</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Runnable</span> </span>&#123;</div><div class="line">    <span class="function"><span class="keyword">void</span> <span class="title">run</span><span class="params">()</span></span>;</div><div class="line">&#125;</div><div class="line"></div><div class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Callable</span>&lt;<span class="title">V</span>&gt; </span>&#123;</div><div class="line">    <span class="function">V <span class="title">call</span><span class="params">()</span></span>;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>方法 <code>Runnable.run</code> 不返回值 ，而 <code>Callable&lt;V&gt;.call</code> 返回.</p>
<p>假设你重载了方法<code>invoke</code>如下所示 (参考 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/methods.html" target="_blank" rel="external">Defining Methods</a> 获取更多重载的信息):</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">void</span> <span class="title">invoke</span><span class="params">(Runnable r)</span> </span>&#123;</div><div class="line">    r.run();</div><div class="line">&#125;</div><div class="line"></div><div class="line">&lt;T&gt; <span class="function">T <span class="title">invoke</span><span class="params">(Callable&lt;T&gt; c)</span> </span>&#123;</div><div class="line">    <span class="keyword">return</span> c.call();</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>而方法的调用如下?</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">String s = invoke(() -&gt; <span class="string">"done"</span>);</div></pre></td></tr></table></figure>
<p>方法调用 <code>invoke(Callable&lt;T&gt;)</code> 会被调用因为他有一个返回值;而方法<code>invoke(Runnable)</code>不会因为他没有返回值，所以lambda表达式的<code>() -&gt; &quot;done&quot;</code> 的类型是 <code>Callable&lt;T&gt;</code>.</p>
<h2 id="序列化Serialization"><a href="#序列化Serialization" class="headerlink" title="序列化Serialization"></a><a href="">序列化Serialization</a></h2><p>如果你个lambda表达式的目标类型以及获取的参数是可以序列化的，那么你可以序列化 <a href="https://docs.oracle.com/javase/tutorial/jndi/objects/serial.html" target="_blank" rel="external">serialize</a> 一个lambda表达式。不过，就像内部类 <a href="https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html#serialization" target="_blank" rel="external">inner classes</a>一样，强烈不建议序列化.</p>
]]></content>
    
    <summary type="html">
    
      &lt;h1 id=&quot;Lambda-表达式-Lambda-Expressions&quot;&gt;&lt;a href=&quot;#Lambda-表达式-Lambda-Expressions&quot; class=&quot;headerlink&quot; title=&quot;Lambda 表达式 Lambda Expressions&quot;&gt;&lt;/a&gt;Lambda 表达式 Lambda Expressions&lt;/h1&gt;&lt;p&gt;使用匿名类的一个问题在于，如果你要实现的匿名类非常的简单，例如只包含一个方法的借口，那么匿名列的语法看起来就不够简洁和轻便。这种情况下，你通常会尝试将某个功能作为参数传递给一其他方法，例如当点击按钮的时候会采取什么措施。Lambda表达式可以提供这样的功能，将功能作为方法的参数，或者代码作为数据。&lt;/p&gt;
&lt;p&gt;上一节中， &lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;Anonymous Classes&lt;/a&gt;, 向你展示了实现一个基本的类而不需要命名，譬如只有一个方法的类，不过匿名类看起来有点复杂和笨重。Lambda表达式可以使你更加紧凑的表达一个单方法类的实例。&lt;/p&gt;
&lt;p&gt;本节将包含以下的内容:&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#use-case&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;Lambda表达式的经典应用Ideal Use Case for Lambda Expressions&lt;/a&gt;&lt;ul&gt;
&lt;li&gt;&lt;a href=&quot;#approach1&quot;&gt;方法1：创建搜索某一特点的成员的方法Approach 1: Create Methods That Search for Members That Match One Characteristic&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach2&quot;&gt;方法2：创建更加通用的搜索方法Approach 2: Create More Generalized Search Methods&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach3&quot;&gt;方法3：用局部类实现特定搜索规则相关的的代码Approach 3: Specify Search Criteria Code in a Local Class&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach4&quot;&gt;方法4：用匿名类实现特定搜索规则相关的代码Approach 4: Specify Search Criteria Code in an Anonymous Class&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach5&quot;&gt;方法5：用Lambda表达式实现特定搜索规则的代码Approach 5: Specify Search Criteria Code with a Lambda Expression&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach6&quot;&gt;方法6：通过标准的功能接口使用lambda表达式Approach 6: Use Standard Functional Interfaces with Lambda Expressions&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach7&quot;&gt;方法7：在你的应用中使用Lambda表达式Approach 7: Use Lambda Expressions Throughout Your Application&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach8&quot;&gt;方法8：使用反应提高可扩展性Approach 8: Use Generics More Extensively&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;#approach9&quot;&gt;方法9：使用已Lambda表达式作为参数的聚合操作Approach 9: Use Aggregate Operations That Accept Lambda Expressions as Parameters&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#lambda-expressions-in-gui-applications&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;在GUI应用中使用Lambda表达式Lambda Expressions in GUI Applications&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#syntax&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;Lambda表达式语法Syntax of Lambda Expressions&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#accessing-local-variables&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;访问闭合区间的局部变量Accessing Local Variables of the Enclosing Scope&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#target-typing&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;目标类型Target Typing&lt;/a&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#target-types-and-method-arguments&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;目标类型以及方法参数Target Types and Method Arguments&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href=&quot;https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#serialization&quot; target=&quot;_blank&quot; rel=&quot;external&quot;&gt;序列化Serialization&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;
    
    </summary>
    
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