24d1a45e6a
The current example shows `json.Unmarshal` on an array of bytes, including `num:6.0`. In the output however this comes out as `6`. Even in the casting to `float64`, it comes out as `6`. This can be confusion because it looks like a simple `int`. By adding an arbitray `.13` you see it's a float.
235 lines
9.9 KiB
Plaintext
235 lines
9.9 KiB
Plaintext
<!DOCTYPE html>
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<html>
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<head>
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<meta http-eqiv="content-type" content="text/html;charset=utf-8">
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<title>Go by Example: Interfaces</title>
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<body>
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<div class="example" id="interfaces">
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<h2><a href="./">Go by Example</a>: Interfaces</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Interfaces</em> are named collections of method
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signatures.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/vx8bwC-Bmq"><img title="Run code" src="play.png" class="run" /></a>
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"fmt"</span>
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<span class="kn">import</span> <span class="s">"math"</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Here’s a basic interface for geometric shapes.</p>
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</td>
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<td class="code leading">
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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>
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<span class="nx">area</span><span class="p">()</span> <span class="kt">float64</span>
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<span class="nx">perim</span><span class="p">()</span> <span class="kt">float64</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>For our example we’ll implement this interface on
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<code>square</code> and <code>circle</code> types.</p>
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</td>
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<td class="code leading">
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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>
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<span class="nx">width</span><span class="p">,</span> <span class="nx">height</span> <span class="kt">float64</span>
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<span class="p">}</span>
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<span class="kd">type</span> <span class="nx">circle</span> <span class="kd">struct</span> <span class="p">{</span>
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<span class="nx">radius</span> <span class="kt">float64</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>To implement an interface in Go, we just need to
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implement all the methods in the interface. Here we
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implement <code>geometry</code> on <code>square</code>s.</p>
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</td>
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<td class="code leading">
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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>
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<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>
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<span class="p">}</span>
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<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>
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<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>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>The implementation for <code>circle</code>s.</p>
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</td>
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<td class="code leading">
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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>
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<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>
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<span class="p">}</span>
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<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>
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<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>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>If a variable has an interface type, then we can call
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methods that are in the named interface. Here’s a
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generic <code>measure</code> function taking advantage of this
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to work on any <code>geometry</code>.</p>
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</td>
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<td class="code leading">
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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>
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<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>
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<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>
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<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>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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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>
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<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>
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<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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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>The <code>circle</code> and <code>square</code> struct types both
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implement the <code>geometry</code> interface so we can use
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instances of
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these structs as arguments to `measure.</p>
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</td>
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<td class="code">
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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>
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<span class="nx">measure</span><span class="p">(</span><span class="nx">c</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> go run interfaces.go
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<span class="go">{3 4}</span>
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<span class="go">12</span>
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<span class="go">14</span>
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<span class="go">{5}</span>
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<span class="go">78.53981633974483</span>
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<span class="go">31.41592653589793</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>To learn more about Go’s interfaces, check out this
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<a href="http://jordanorelli.tumblr.com/post/32665860244/how-to-use-interfaces-in-go">great blog post</a>.</p>
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</td>
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<td class="code empty">
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="errors">Errors</a>.
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</p>
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<p class="footer">
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by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/interfaces">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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