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interfaces
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Interfaces</title>
<link rel=stylesheet href="site.css">
</head>
<script>
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<body>
<div class="example" id="interfaces">
<h2><a href="./">Go by Example</a>: Interfaces</h2>
<table>
<tr>
<td class="docs">
<p><em>Interfaces</em> are named collections of method
signatures.</p>
</td>
<td class="code empty leading">
</td>
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<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/XJASG4MxBQr"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
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</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"fmt"</span>
<span class="s">"math"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Here’s a basic interface for geometric shapes.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">type</span> <span class="nx">geometry</span> <span class="kd">interface</span> <span class="p">{</span>
<span class="nf">area</span><span class="p">()</span> <span class="kt">float64</span>
<span class="nf">perim</span><span class="p">()</span> <span class="kt">float64</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>For our example we’ll implement this interface on
<code>rect</code> and <code>circle</code> types.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">type</span> <span class="nx">rect</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>
</pre>
</td>
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<tr>
<td class="docs">
<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>rect</code>s.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="p">(</span><span class="nx">r</span> <span class="nx">rect</span><span class="p">)</span> <span class="nf">area</span><span class="p">()</span> <span class="kt">float64</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">r</span><span class="p">.</span><span class="nx">width</span> <span class="o">*</span> <span class="nx">r</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">r</span> <span class="nx">rect</span><span class="p">)</span> <span class="nf">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">r</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">r</span><span class="p">.</span><span class="nx">height</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>The implementation for <code>circle</code>s.</p>
</td>
<td class="code leading">
<pre class="chroma">
<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="nf">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="nf">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>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>If a variable has an interface type, then we can call
methods that are in the named interface. Here’s a
generic <code>measure</code> function taking advantage of this
to work on any <code>geometry</code>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">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="nf">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="nf">Println</span><span class="p">(</span><span class="nx">g</span><span class="p">.</span><span class="nf">area</span><span class="p">())</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">g</span><span class="p">.</span><span class="nf">perim</span><span class="p">())</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">r</span> <span class="o">:=</span> <span class="nx">rect</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>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>The <code>circle</code> and <code>rect</code> struct types both
implement the <code>geometry</code> interface so we can use
instances of
these structs as arguments to <code>measure</code>.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="nf">measure</span><span class="p">(</span><span class="nx">r</span><span class="p">)</span>
<span class="nf">measure</span><span class="p">(</span><span class="nx">c</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><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>
</td>
</tr>
<tr>
<td class="docs">
<p>To learn more about Go’s interfaces, check out this
<a href="http://jordanorelli.tumblr.com/post/32665860244/how-to-use-interfaces-in-go">great blog post</a>.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="embedding">Embedding</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
<script>
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"math\"\u000A)\u000A');codeLines.push('type geometry interface {\u000A area() float64\u000A perim() float64\u000A}\u000A');codeLines.push('type rect struct {\u000A width, height float64\u000A}\u000Atype circle struct {\u000A radius float64\u000A}\u000A');codeLines.push('func (r rect) area() float64 {\u000A return r.width * r.height\u000A}\u000Afunc (r rect) perim() float64 {\u000A return 2*r.width + 2*r.height\u000A}\u000A');codeLines.push('func (c circle) area() float64 {\u000A return math.Pi * c.radius * c.radius\u000A}\u000Afunc (c circle) perim() float64 {\u000A return 2 * math.Pi * c.radius\u000A}\u000A');codeLines.push('func measure(g geometry) {\u000A fmt.Println(g)\u000A fmt.Println(g.area())\u000A fmt.Println(g.perim())\u000A}\u000A');codeLines.push('func main() {\u000A r :\u003D rect{width: 3, height: 4}\u000A c :\u003D circle{radius: 5}\u000A');codeLines.push(' measure(r)\u000A measure(c)\u000A}\u000A');codeLines.push('');codeLines.push('');
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