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Javascript面向对象（一）- 对象属性
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<time title="Created: 2018-12-20 15:25:17 / Modified: 15:27:28" itemprop="dateCreated datePublished" datetime="2018-12-20T15:25:17+08:00">2018-12-20</time>
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<p>创建一个对象的的两种方法</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">第一种：通过对象方式创建</span><br><span class="line">var person = new Object();</span><br><span class="line">person.name = "Lee";</span><br><span class="line">person.age = 29;</span><br><span class="line">person.job = "someJob";</span><br><span class="line">person.sayName = function(){</span><br><span class="line"> console.log(this.name);</span><br><span class="line">} </span><br><span class="line"></span><br><span class="line">第二种：通过字面量创建</span><br><span class="line">var person = {</span><br><span class="line"> name: "Lee",</span><br><span class="line"> age: 29,</span><br><span class="line"> job: "someJob",</span><br><span class="line"> sayName: function(){</span><br><span class="line"> console.log(this.name);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>以上对象是一样的，包含三个属性和一个方法：name, age, job, sayName;</p>
<p>这些属性在创建的时候是有一些特征值的，Javascript可以通过这些特征来定义它们的行为；</p>
<p>###属性类型<br>ECMAScript中有两种属性：数据属性和访问器属性。</p>
<ul>
<li><ol>
<li>数据属性</li>
</ol>
</li>
</ul>
<p>数据属性包含一个数据值的位置。在这个位置可以读取与写入值。数据属性有4个描述其行为的特性。</p>
<p>（1) [[Configurable]]：描述属性是否可以被删除(delete)，特性是否可以被改变，或是否可以被修改为访问器属性;像上面定义的属性，默认值是ture;</p>
<p>（2）[[Enumerable]]：描述属性是否可以被遍历(for-in);像上面定义的属性，默认值是ture;</p>
<p>（3）[[Writable]]：描述属性是否是可写的;像上面定义的属性，默认值是ture;</p>
<p>（4）[[Value]]：属性值就保存在此位置，读取或者写入属性均操作此处;像上面定义的属性，默认值是undefined;</p>
<p>要修改属性的默认特性，需要用到ECMAScript 5的</p>
<p>Object.defineProperty(obj, prop, descriptor)</p>
<p>的方法，这个方法包含了三个参数，</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">obj</span><br><span class="line">属性所在的对象。</span><br><span class="line">prop</span><br><span class="line">属性的名称。</span><br><span class="line">descriptor</span><br><span class="line">一个属性描述符对象，其中描述符对象的属性必须是：configurable, enumerable, value, writable。设置其中一个或者多个值，并修改对应的特性值。</span><br></pre></td></tr></table></figure>
<p>举些例子</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">var person = {};</span><br><span class="line">Object.defineProperty(person, "name", {</span><br><span class="line"> writable: false,</span><br><span class="line"> value: "Lee"</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line">console.log(person.name);</span><br><span class="line">person.name = "hui";</span><br><span class="line">console.log(person.name);</span><br></pre></td></tr></table></figure>
<p>结果:</p>
<p><img src="/Users/lee/Desktop/04DB7385-2BFE-416E-8040-0813D228A6AF.png" alt=""></p>
<p>上面是非严格模式，如果是严格模式的话，将会报错<br><img src="/Users/lee/Library/Containers/com.tencent.qq/Data/Library/Application Support/QQ/Users/784197835/QQ/Temp.db/AF6C5100-6BAE-418B-87CD-B2180B75364B.png" alt=""></p>
<ul>
<li><ol start="2">
<li>访问器属性</li>
</ol>
</li>
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Java 判断两个变量是否相等
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<p>判断两个变量是否相等的方式有两种：</p>
<ul>
<li>利用运算符 ==</li>
<li>利用equals方法</li>
</ul>
<h3 id="1-比较基本Java基本数据类型"><a href="#1-比较基本Java基本数据类型" class="headerlink" title="(1)比较基本Java基本数据类型"></a>(1)比较基本Java基本数据类型</h3><p>比较基本数据类型，只能用“==”，不能用equals，这里比较的是两个变量的值；</p>
<h3 id="2-比较包装类"><a href="#2-比较包装类" class="headerlink" title="(2)比较包装类"></a>(2)比较包装类</h3><p>比较包装类，举个例子<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">Integer i1 = new Integer(10);</span><br><span class="line">Integer i2 = new Integer(10);</span><br><span class="line"></span><br><span class="line">System.out.println(i1 == i2);</span><br><span class="line">System.out.println(i1.equals(i2));</span><br><span class="line"></span><br><span class="line">执行结果：</span><br><span class="line">false</span><br><span class="line">true</span><br></pre></td></tr></table></figure></p>
<p>如上例子可知，“==”比较的是两个变量的内存地址值，equals比较的是两个具体要看当前这个类的equals方法</p>
<p>查看Integer的equals的源码</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">public boolean equals(Object obj) {</span><br><span class="line">if (obj instanceof Integer) {</span><br><span class="line">return value == ((Integer)obj).intValue();</span><br><span class="line">}</span><br><span class="line">return false;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>可知比较的是两个变量的intValue；</p>
<h3 id="3-比较String"><a href="#3-比较String" class="headerlink" title="(3)比较String"></a>(3)比较String</h3><p>“==”比较的是内存地址，equals比较的是字符串的值<br>举个例子<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">String str1 = "abcd";</span><br><span class="line">String str2 = "abcd";</span><br><span class="line"></span><br><span class="line">String str3 = new String("abcd");</span><br><span class="line">String str4 = new String("abcd");</span><br><span class="line"></span><br><span class="line">System.out.println(str1 == str2);</span><br><span class="line">System.out.println(str1.equals(str2));</span><br><span class="line"></span><br><span class="line">System.out.println(str3 == str4);</span><br><span class="line">System.out.println(str3.equals(str4));</span><br><span class="line"></span><br><span class="line">执行结果：</span><br><span class="line">true</span><br><span class="line">true</span><br><span class="line">false</span><br><span class="line">true</span><br></pre></td></tr></table></figure></p>
<p>会不会有一个疑问，为什么前面两个比较的结果都是true！！！ 不是说 == 比较的是内存地址吗？<br>栈区存放的是基本数据和引用地址；堆区存放的是对象；其实还有一个是String池（独立于栈区和堆区)。<br>首先 String str1 = “abcd”； 在栈区创建str1的引用，然后在String池中寻找其指向的对象（”abcd”）,如果没有就创建一个，然后让str1<br>指向String池中的这个对象，如果有，就直接指向这个对象；</p>
<h3 id="4-比较对象"><a href="#4-比较对象" class="headerlink" title="(4)比较对象"></a>(4)比较对象</h3><p>“==”比较的是内存地址，equals比较的也是内存地址，如果没有重写equals方法的类都是调用的基类Object的equals的方法。</p>
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Java基础第一课 - Java 基本数据类型
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<time title="Created: 2018-09-10 10:56:40 / Modified: 15:21:46" itemprop="dateCreated datePublished" datetime="2018-09-10T10:56:40+08:00">2018-09-10</time>
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<h3 id="1-基本数据类型有八种，可分为四类"><a href="#1-基本数据类型有八种，可分为四类" class="headerlink" title="(1)基本数据类型有八种，可分为四类"></a>(1)基本数据类型有八种，可分为四类</h3><ul>
<li>byte (1个字节) – 整形</li>
<li>short （2个字节）- 整形</li>
<li>int （4个字节）- 整形</li>
<li>long （8个字节）- 整形</li>
<li>float （4个字节）- 浮点型</li>
<li>double （8个字节）- 浮点型</li>
<li>boolean （1位）- 逻辑型</li>
<li>char （2个字节）- 字符型<br>注意：1个字节（B） = 8位（bit）</li>
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<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">//基本数据类型</span><br><span class="line">byte bt = 0;</span><br><span class="line">short s = 0;</span><br><span class="line">int i = 0;</span><br><span class="line">long l = 0L;</span><br><span class="line">float f = 0.0f;</span><br><span class="line">double d = 0.0;</span><br><span class="line">boolean bool = false;</span><br><span class="line">char c = ' ';</span><br></pre></td></tr></table></figure>
<h3 id="2-基本数据类型的拆装箱（JDK1-5-才有拆装箱）"><a href="#2-基本数据类型的拆装箱（JDK1-5-才有拆装箱）" class="headerlink" title="(2)基本数据类型的拆装箱（JDK1.5 才有拆装箱）"></a>(2)基本数据类型的拆装箱（JDK1.5 才有拆装箱）</h3><table>
<thead>
<tr>
<th style="text-align:center">基本类型</th>
<th style="text-align:center">包装类</th>
<th style="text-align:center">默认值</th>
</tr>
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<tbody>
<tr>
<td style="text-align:center">byte</td>
<td style="text-align:center">Byte</td>
<td style="text-align:center">0</td>
</tr>
<tr>
<td style="text-align:center">short</td>
<td style="text-align:center">Short</td>
<td style="text-align:center">0</td>
</tr>
<tr>
<td style="text-align:center">int</td>
<td style="text-align:center">Integer</td>
<td style="text-align:center">0</td>
</tr>
<tr>
<td style="text-align:center">long</td>
<td style="text-align:center">Long</td>
<td style="text-align:center">0</td>
</tr>
<tr>
<td style="text-align:center">float</td>
<td style="text-align:center">Float</td>
<td style="text-align:center">0.0</td>
</tr>
<tr>
<td style="text-align:center">double</td>
<td style="text-align:center">Double</td>
<td style="text-align:center">0.0</td>
</tr>
<tr>
<td style="text-align:center">boolean</td>
<td style="text-align:center">Boolean</td>
<td style="text-align:center">false</td>
</tr>
<tr>
<td style="text-align:center">char</td>
<td style="text-align:center">Character</td>
<td style="text-align:center">空</td>
</tr>
</tbody>
</table>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">//装箱</span><br><span class="line">Integer integer = new Integer(0);</span><br><span class="line">integer = 1024;</span><br><span class="line"></span><br><span class="line">//拆箱</span><br><span class="line">int i = integer;</span><br></pre></td></tr></table></figure>
<p>第一类：整数类型（byte、short、int、long）<br><img src="https://upload-images.jianshu.io/upload_images/871379-fafdcdc0c864933c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"><br>1、JAVA各整数类型有固定的表示范围和字段长度，其不受操作系统的影响，以保证Java的可移植性<br>2、JAVA整型常量有三种表示方法：<br>十进制：如：123，55等；<br>八进制：以0开头，如：022等；<br>十六进制：以0x或0X开头，如0X32等；<br>3、JAVA的整型变量默认为int类型，申明long类型的可以在数值后面加L，<br> 比如：int i=234;//正确 long j=888888888888888L;//不加L则错误。</p>
<p>第二类：浮点类型（float、double）<br><img src="https://upload-images.jianshu.io/upload_images/871379-db297b21b5213642.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"><br>1、JAVA各浮点类型有固定的表示范围和字段长度，其不受操作系统的影响，以保证Java的可移植性<br>2、JAVA浮点型常量有两种表示方法：<br>十进制数形式：比如：3.0，32.66等；<br>科学计数法形式：比如：3.15E2，5.23e2，5.23e-2；<br>3、JAVA浮点型常量默认为double型，申明float类型可以在数值后面加F<br> 比如：double a=2.333;//正确 float b=3.555F;//不加L则出错。<br>4.字面量为浮点型时，默认是double型；</p>
<p>第三类：文本型（char）<br>1、字符常量是用单引号引起来的单个字符。<br> 比如：char a=’B’ ; char name=‘中’;<br>2、JAVA采用unicode编码来表示，每个字符占两个字节（一个字节有八位），因此可以用十六进制编码表示：<br> 比如：char a=‘\u0056’;<br>3、JAVA语言中还支持转义字符，来将其后面的字符转为其他含义：<br> 比如：char a=‘\n’;</p>
<p>第四类：布尔型（boolean）<br>布尔值只可以取true或false,不可以用0或者非0来替代，这点和C、C++等不同</p>
<h3 id="3-基本类型转换"><a href="#3-基本类型转换" class="headerlink" title="(3)基本类型转换"></a>(3)基本类型转换</h3><p>(1)自动类型转换</p>
<p>Java所有的数值类型变量都可以互相转换，如果系统把某种基本类型的值直接赋给另一种基本类型的变量，则这种方式被称为自动类型转换。</p>
<p>int e = 1;</p>
<p>double d =e;</p>
<p>以上代码不会报错，系统会自动转换。</p>
<p>(2) 强制类型转换</p>
<p>当把一个范围大的数值或变量赋给另一个范围小的变量时，需要进行强制转换。</p>
<p> double d =1.5;</p>
<p> int e = d;</p>
<p> 以上代码会报错，把大范围的数值赋给小范围的变量，需要强制转换，这样才能通过编译，且大范围的数值会丢失精度。</p>
<p> int e = (int) d;</p>
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