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<div class="example" id="interfaces">
<h2><a href="./">Go by Example</a>: Interfaces</h2>
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<p><em>Interfaces</em> are named collections of method
signatures.</p>
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;math&quot;</span>
</pre></div>
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<p>Here&rsquo;s a basic interface for geometric shapes.</p>
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<div class="highlight"><pre><span class="kd">type</span> <span class="nx">geometry</span> <span class="kd">interface</span> <span class="p">{</span>
<span class="nx">area</span><span class="p">()</span> <span class="kt">float64</span>
<span class="nx">perim</span><span class="p">()</span> <span class="kt">float64</span>
<span class="p">}</span>
</pre></div>
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<p>For our example we&rsquo;ll implement this interface on
<code>square</code> and <code>circle</code> types.</p>
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<div class="highlight"><pre><span class="kd">type</span> <span class="nx">square</span> <span class="kd">struct</span> <span class="p">{</span>
<span class="nx">width</span><span class="p">,</span> <span class="nx">height</span> <span class="kt">float64</span>
<span class="p">}</span>
<span class="kd">type</span> <span class="nx">circle</span> <span class="kd">struct</span> <span class="p">{</span>
<span class="nx">radius</span> <span class="kt">float64</span>
<span class="p">}</span>
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<p>To implement an interface in Go, we just need to
implement all the methods in the interface. Here we
implement <code>geometry</code> on <code>square</code>s.</p>
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<div class="highlight"><pre><span class="kd">func</span> <span class="p">(</span><span class="nx">s</span> <span class="nx">square</span><span class="p">)</span> <span class="nx">area</span><span class="p">()</span> <span class="kt">float64</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">s</span><span class="p">.</span><span class="nx">width</span> <span class="o">*</span> <span class="nx">s</span><span class="p">.</span><span class="nx">height</span>
<span class="p">}</span>
<span class="kd">func</span> <span class="p">(</span><span class="nx">s</span> <span class="nx">square</span><span class="p">)</span> <span class="nx">perim</span><span class="p">()</span> <span class="kt">float64</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">2</span><span class="o">*</span><span class="nx">s</span><span class="p">.</span><span class="nx">width</span> <span class="o">+</span> <span class="mi">2</span><span class="o">*</span><span class="nx">s</span><span class="p">.</span><span class="nx">height</span>
<span class="p">}</span>
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<p>The implementation for <code>circle</code>s.</p>
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<div class="highlight"><pre><span class="kd">func</span> <span class="p">(</span><span class="nx">c</span> <span class="nx">circle</span><span class="p">)</span> <span class="nx">area</span><span class="p">()</span> <span class="kt">float64</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">math</span><span class="p">.</span><span class="nx">Pi</span> <span class="o">*</span> <span class="nx">c</span><span class="p">.</span><span class="nx">radius</span> <span class="o">*</span> <span class="nx">c</span><span class="p">.</span><span class="nx">radius</span>
<span class="p">}</span>
<span class="kd">func</span> <span class="p">(</span><span class="nx">c</span> <span class="nx">circle</span><span class="p">)</span> <span class="nx">perim</span><span class="p">()</span> <span class="kt">float64</span> <span class="p">{</span>
<span class="k">return</span> <span class="mi">2</span> <span class="o">*</span> <span class="nx">math</span><span class="p">.</span><span class="nx">Pi</span> <span class="o">*</span> <span class="nx">c</span><span class="p">.</span><span class="nx">radius</span>
<span class="p">}</span>
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<p>If a variable has an interface type, then we can call
methods that are in the named interface. Here&rsquo;s a
generic <code>measure</code> function taking advantage of this
to work on any <code>geometry</code>.</p>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">measure</span><span class="p">(</span><span class="nx">g</span> <span class="nx">geometry</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">g</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">g</span><span class="p">.</span><span class="nx">area</span><span class="p">())</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">g</span><span class="p">.</span><span class="nx">perim</span><span class="p">())</span>
<span class="p">}</span>
</pre></div>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">s</span> <span class="o">:=</span> <span class="nx">square</span><span class="p">{</span><span class="nx">width</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="nx">height</span><span class="p">:</span> <span class="mi">4</span><span class="p">}</span>
<span class="nx">c</span> <span class="o">:=</span> <span class="nx">circle</span><span class="p">{</span><span class="nx">radius</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
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<p>The <code>circle</code> and <code>square</code> struct types both
implement the <code>geometry</code> interface so we can use
instances of
these structs as arguments to measure.</p>
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<div class="highlight"><pre> <span class="nx">measure</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span>
<span class="nx">measure</span><span class="p">(</span><span class="nx">c</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<div class="highlight"><pre><span class="gp">$</span> go run interfaces.go
<span class="go">{3 4}</span>
<span class="go">12</span>
<span class="go">14</span>
<span class="go">{5}</span>
<span class="go">78.53981633974483</span>
<span class="go">31.41592653589793</span>
</pre></div>
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<p>To learn more about Go&rsquo;s interfaces, check out this
<a href="http://jordanorelli.tumblr.com/post/32665860244/how-to-use-interfaces-in-go">great blog post</a>.</p>
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<p class="next">
Next example: <a href="errors">Errors</a>.
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