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<title>Decompiling Scala code - Das Keyboard Shredder</title>
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<meta name="description" content="I was bored today and decided to decompile some Scala code for fun and profit. I’m using Scala 2.12.2 and Java 1.8.0_121. Scala object package javap object Test01 { def main(args: Array[String]): Unit = Unit } public final class javap.Test01$ { public static javap.Test01$ MODULE$; public static {}; public void main(java.lang.String[]); private javap.Test01$(); } public final class javap.Test01 { public static void main(java.lang.String[]); } As we can see a Scala object is compiled to 2 Java classes, Test01 with static methods for Java compatibility and a Test01$ with a static instance of itself as MODULE$, so that Test01 can be used as an instance value in Scala. Class constructors package javap class Test02(val x: Int, val y: Int, z: Int) { def this(x: Int, y: Int) = this(x, y, 0) } public class javap.Test02 { private final int x; private final int y; public int x(); public int y(); public javap.Test02(int, int, int); public javap.Test02(int, int); } Looks like the default constructor (val x: Int, val y: Int, z: Int) and the overloaded one (x: Int, y: Int) each generated a Java constructor. However only x and y …" />
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<meta property="og:description" content="I was bored today and decided to decompile some Scala code for fun and profit. I’m using Scala 2.12.2 and Java 1.8.0_121. Scala object package javap object Test01 { def main(args: Array[String]): Unit = Unit } public final class javap.Test01$ { public static javap.Test01$ MODULE$; public static {}; public void main(java.lang.String[]); private javap.Test01$(); } public final class javap.Test01 { public static void main(java.lang.String[]); } As we can see a Scala object is compiled to 2 Java classes, Test01 with static methods for Java compatibility and a Test01$ with a static instance of itself as MODULE$, so that Test01 can be used as an instance value in Scala. Class constructors package javap class Test02(val x: Int, val y: Int, z: Int) { def this(x: Int, y: Int) = this(x, y, 0) } public class javap.Test02 { private final int x; private final int y; public int x(); public int y(); public javap.Test02(int, int, int); public javap.Test02(int, int); } Looks like the default constructor (val x: Int, val y: Int, z: Int) and the overloaded one (x: Int, y: Int) each generated a Java constructor. However only x and y …"/>
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<p>I was bored today and decided to decompile some Scala code for fun and profit. I’m using Scala 2.12.2 and Java 1.8.0_121.</p>
<h2>Scala object</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">object</span> <span class="nc">Test01</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">main</span><span class="o">(</span><span class="n">args</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span><span class="k">:</span> <span class="kt">Unit</span> <span class="o">=</span> <span class="nc">Unit</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test01$</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test01$</span> <span class="n">MODULE$</span><span class="o">;</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="o">{};</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">String</span><span class="o">[]);</span>
<span class="kd">private</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test01$</span><span class="o">();</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test01</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">String</span><span class="o">[]);</span>
<span class="o">}</span>
</pre></div>
<p>As we can see a Scala object is compiled to 2 Java classes, <code>Test01</code> with static methods for Java compatibility and a <code>Test01$</code> with a static instance of itself as <code>MODULE$</code>, so that <code>Test01</code> can be used as an instance value in Scala.</p>
<h2>Class constructors</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">class</span> <span class="nc">Test02</span><span class="o">(</span><span class="k">val</span> <span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="k">val</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">z</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="o">{</span>
<span class="k">def</span> <span class="k">this</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=</span> <span class="k">this</span><span class="o">(</span><span class="n">x</span><span class="o">,</span> <span class="n">y</span><span class="o">,</span> <span class="mi">0</span><span class="o">)</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test02</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">x</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">y</span><span class="o">;</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">x</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">y</span><span class="o">();</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test02</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test02</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>Looks like the default constructor <code>(val x: Int, val y: Int, z: Int)</code> and the overloaded one <code>(x: Int, y: Int)</code> each generated a Java constructor. However only <code>x</code> and <code>y</code> are <code>val</code>s and became public member methods of the class, while <code>z</code> is used as a constructor argument only.</p>
<h2>Traits</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">trait</span> <span class="nc">Test03</span> <span class="o">{</span>
<span class="k">val</span> <span class="n">x</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">f</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">interface</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test03</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">abstract</span> <span class="kt">void</span> <span class="nf">javap$Test03$_setter_$x_$eq</span><span class="o">(</span><span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kd">abstract</span> <span class="kt">int</span> <span class="nf">x</span><span class="o">();</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kt">int</span> <span class="nf">f$</span><span class="o">(</span><span class="n">javap</span><span class="o">.</span><span class="na">Test03</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">f</span><span class="o">(</span><span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">$init$</span><span class="o">(</span><span class="n">javap</span><span class="o">.</span><span class="na">Test03</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>So <code>trait</code>s are not too different from <code>object</code>s. However method <code>f</code> generated two versions, one static with the <code>Test03</code> itself as an additional argument and one member method. There’s also a setter method, most likely to allow overriding <code>x</code> in sub-types.</p>
<h2>Member <code>val</code>s</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">class</span> <span class="nc">Test04</span> <span class="o">{</span>
<span class="k">val</span> <span class="n">a</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">lazy</span> <span class="k">val</span> <span class="n">b</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="nd">@transient</span> <span class="k">lazy</span> <span class="k">val</span> <span class="n">c</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">d</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test04</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kt">int</span> <span class="n">b</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">transient</span> <span class="kt">int</span> <span class="n">c</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">a</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">d</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">volatile</span> <span class="kt">byte</span> <span class="n">bitmap$0</span><span class="o">;</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">a</span><span class="o">();</span>
<span class="kd">private</span> <span class="kt">int</span> <span class="nf">b$lzycompute</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">b</span><span class="o">();</span>
<span class="kd">private</span> <span class="kt">int</span> <span class="nf">c$lzycompute</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">c</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">d</span><span class="o">();</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test04</span><span class="o">();</span>
<span class="o">}</span>
</pre></div>
<p>Here we can see clearly that Scala <code>val</code>s are compiled as accessor methods and private field. <code>b</code> and <code>c</code> are lazy and generated two <code>lzycompute</code> methods. In addition <code>c</code> is transient and the underlying field is marked so as well. There’s also a <code>bitmap$0</code> to keep track of the states of <code>lazy val</code>s. See this StackOverflow <a href="http://stackoverflow.com/questions/6877040/in-compiled-scala-what-is-the-bitmap0-field0">thread</a>.</p>
<h2>Primitives</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">class</span> <span class="nc">Test05</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">plus1</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">plus2</span><span class="o">[</span><span class="kt">T</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">plus3</span><span class="o">[</span><span class="kt">@specialized</span><span class="o">(</span><span class="kt">Int</span>, <span class="kt">Long</span>, <span class="kt">Float</span>, <span class="kt">Double</span><span class="o">)</span> <span class="kt">T</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="o">???</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test05</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">plus1</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="n">T</span> <span class="nf">plus2</span><span class="o">(</span><span class="n">T</span><span class="o">,</span> <span class="n">T</span><span class="o">);</span>
<span class="kd">public</span> <span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="n">T</span> <span class="nf">plus3</span><span class="o">(</span><span class="n">T</span><span class="o">,</span> <span class="n">T</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">double</span> <span class="nf">plus3$mDc$sp</span><span class="o">(</span><span class="kt">double</span><span class="o">,</span> <span class="kt">double</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">float</span> <span class="nf">plus3$mFc$sp</span><span class="o">(</span><span class="kt">float</span><span class="o">,</span> <span class="kt">float</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">plus3$mIc$sp</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">long</span> <span class="nf">plus3$mJc$sp</span><span class="o">(</span><span class="kt">long</span><span class="o">,</span> <span class="kt">long</span><span class="o">);</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test05</span><span class="o">();</span>
<span class="o">}</span>
</pre></div>
<p>Unlike Java, which has primitive (<code>int</code>, <code>long</code>, etc.) and boxed (<code>Integer</code>, <code>Long</code>, etc.) types, Scala has unified primitive types and will choose the appropriate type in generated code, like primitives in most cases and boxed types in generic classes. We can see here that <code>plus1</code> and <code>plus2</code> are identical to how one would write then in Java. However <code>plus3</code> generates one generic version and 4 specialized versions.</p>
<p>See these <a href="http://www.lyh.me/slides/primitives.html">slides</a> for a more in depth discussion of Scala primitives.</p>
<h2>Functions</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">class</span> <span class="nc">Test06</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">map</span><span class="o">(</span><span class="n">xs</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">Int</span><span class="o">],</span> <span class="n">f</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=></span> <span class="nc">Int</span><span class="o">)</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">Int</span><span class="o">]</span> <span class="k">=</span> <span class="n">xs</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">f</span><span class="o">)</span>
<span class="k">def</span> <span class="n">f</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">val</span> <span class="n">g</span> <span class="k">=</span> <span class="n">f</span> <span class="k">_</span>
<span class="k">val</span> <span class="n">h</span> <span class="k">=</span> <span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=></span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span><span class="k">:</span> <span class="kt">Int</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test06</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">scala</span><span class="o">.</span><span class="na">Function1</span><span class="o"><</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">,</span> <span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">></span> <span class="n">g</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">scala</span><span class="o">.</span><span class="na">Function1</span><span class="o"><</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">,</span> <span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">></span> <span class="n">h</span><span class="o">;</span>
<span class="kd">public</span> <span class="kt">int</span><span class="o">[]</span> <span class="nf">map</span><span class="o">(</span><span class="kt">int</span><span class="o">[],</span> <span class="n">scala</span><span class="o">.</span><span class="na">Function1</span><span class="o"><</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">,</span> <span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">>);</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">f</span><span class="o">(</span><span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="n">scala</span><span class="o">.</span><span class="na">Function1</span><span class="o"><</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">,</span> <span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">></span> <span class="nf">g</span><span class="o">();</span>
<span class="kd">public</span> <span class="n">scala</span><span class="o">.</span><span class="na">Function1</span><span class="o"><</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">,</span> <span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span><span class="o">></span> <span class="nf">h</span><span class="o">();</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="nf">$anonfun$g$1</span><span class="o">(</span><span class="n">javap</span><span class="o">.</span><span class="na">Test06</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kt">int</span> <span class="nf">$anonfun$h$1</span><span class="o">(</span><span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test06</span><span class="o">();</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">Object</span> <span class="nf">$deserializeLambda$</span><span class="o">(</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">invoke</span><span class="o">.</span><span class="na">SerializedLambda</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>These Scala anonymous functions are realy just instances of the trait <code>Function1[T, R]</code>. Scala generates both static methods (<code>$anonfun$g$1</code>, <code>$anonfun$h$1</code>) and function instances (<code>g()</code>, <code>h()</code>) for <code>g</code> and <code>f</code> since they are declared as <code>val</code>s. <code>f</code> is declared as a method and has no instances. This will probably happen at the call site.</p>
<h2>Other features</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">class</span> <span class="nc">Test07</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">plus1</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">z</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="mi">0</span><span class="o">)</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">plus2</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)(</span><span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)(</span><span class="n">z</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">plus3</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)(</span><span class="k">implicit</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">plus4</span><span class="o">[</span><span class="kt">T</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">T</span><span class="o">)(</span><span class="k">implicit</span> <span class="n">num</span><span class="k">:</span> <span class="kt">Numeric</span><span class="o">[</span><span class="kt">T</span><span class="o">])</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="o">???</span>
<span class="k">def</span> <span class="n">plus5</span><span class="o">[</span><span class="kt">T:</span> <span class="kt">Numeric</span><span class="o">](</span><span class="n">x</span><span class="k">:</span> <span class="kt">T</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="o">???</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test07</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">plus1</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">plus1$default$3</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">plus2</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">plus3</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="n">T</span> <span class="nf">plus4</span><span class="o">(</span><span class="n">T</span><span class="o">,</span> <span class="n">T</span><span class="o">,</span> <span class="n">scala</span><span class="o">.</span><span class="na">math</span><span class="o">.</span><span class="na">Numeric</span><span class="o"><</span><span class="n">T</span><span class="o">>);</span>
<span class="kd">public</span> <span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="n">T</span> <span class="nf">plus5</span><span class="o">(</span><span class="n">T</span><span class="o">,</span> <span class="n">T</span><span class="o">,</span> <span class="n">scala</span><span class="o">.</span><span class="na">math</span><span class="o">.</span><span class="na">Numeric</span><span class="o"><</span><span class="n">T</span><span class="o">>);</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test07</span><span class="o">();</span>
<span class="o">}</span>
</pre></div>
<p>Here we can see that there’s not much special in the generated methods. Method <code>plus1</code> with default argument generated a special case. While curried <code>plus2</code> and <code>plus3</code> with implicit argument don’t look special at all. <code>plus4</code> and <code>plus5</code> look identical since an <code>implicit</code> type class argument is equivalent to a context bound <code>[T: Numeric]</code>.</p>
<h2>Conclusion</h2>
<p>In my experience as a library builder, digging into generated byte code allows me to better understand the following topics</p>
<ul>
<li>Java inter-op, e.g. Scala features that translate well or badly when called in Java</li>
<li>Performance considerations, e.g. primitive vs. boxed types, class method vs. function instances</li>
<li>Serialization implications, e.g. pulling in non-serializable trait or object module, or initialization sequence of <code>@transient lazy val</code>s</li>
</ul>
<p>The script that generated these can be found at <a href="https://github.com/nevillelyh/scala-playground/blob/master/javap.sh">https://github.com/nevillelyh/scala-playground/blob/master/javap.sh</a>.</p>
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