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POWhy isn't the enclosing class included in the generic parameters for inner class constructors?
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  1. POWhy isn't the enclosing class included in the generic parameters for inner class constructors?
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    copied!<p>Check it out.</p> <pre><code>import java.lang.reflect.*; import java.util.Arrays; class Test { class Inner { public &lt;T&gt; Inner(T arg) { } } public static void main(String[ ] args) { Constructor&lt;?&gt; constructor = Test.Inner.class.getConstructors( )[0]; System.out.println ( "Regular parameters: " + Arrays.toString(constructor.getParameterTypes( )) + "\nGeneric parameters: " + Arrays.toString(constructor.getGenericParameterTypes( )) ); } } </code></pre> <p>Running this prints the following.</p> <blockquote> <p>Regular parameters: [class Test, class java.lang.Object]<br> Generic parameters: [T]</p> </blockquote> <p>My understanding of Java inner classes (i.e. non-static) is that their constructors implicitly have a parameter for their enclosing class. So, to get an instance of <code>Inner</code>, you call its constructor like so.</p> <pre><code>Test.Inner foo = new Test( ).new Inner(new Object( )); </code></pre> <p>And what actually happens is that the instance of <code>Test</code> is passed to the constructor for <code>Inner</code> (along with any other arguments), where it's assigned to an implicit instance field.</p> <p>Examining the nullary constructor for <code>Inner</code> bears this out, as calling <code>getParameterTypes</code> returns an array containing the <code>Class</code> objects representing <code>Test</code> and <code>Object</code> (the erasure of <code>T</code>).</p> <p>However, since the constructor is generic, calling <code>getGenericParameterTypes</code> instead returns an array containing only the type variable <code>T</code>. The implicit parameter for the enclosing class is missing.</p> <p>This only seems to happen when the inner class constructor declares a type variable. Why does it do that? Is it a bug?</p>
 

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