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POInferred wildcard generics in return type
 

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    <p>Java can often infer generics based on the arguments (and even on the return type, in contrast to e.g. C#).</p> <p>Case in point: I've got a generic class <code>Pair&lt;T1, T2&gt;</code> which just stores a pair of values and can be used in the following way:</p> <pre><code>Pair&lt;String, String&gt; pair = Pair.of("Hello", "World"); </code></pre> <p>The method <code>of</code> looks just like this:</p> <pre><code>public static &lt;T1, T2&gt; Pair&lt;T1, T2&gt; of(T1 first, T2 second) { return new Pair&lt;T1, T2&gt;(first, second); } </code></pre> <p>Very nice. However, this no longer works for the following use-case, which requires wildcards:</p> <pre><code>Pair&lt;Class&lt;?&gt;, String&gt; pair = Pair.of((Class&lt;?&gt;) List.class, "hello"); </code></pre> <p>(Notice the explicit cast to make <code>List.class</code> the correct type.)</p> <p>The code fails with the following error (provided by Eclipse):</p> <blockquote> <p>Type mismatch: cannot convert from <code>TestClass.Pair&lt;Class&lt;capture#1-of ?&gt;,String&gt;</code> to <code>TestClass.Pair&lt;Class&lt;?&gt;,String&gt;</code></p> </blockquote> <p>However, explicitly calling the constructor still works as expected:</p> <pre><code>Pair&lt;Class&lt;?&gt;, String&gt; pair = new Pair&lt;Class&lt;?&gt;, String&gt;((Class&lt;?&gt;) List.class, "hello"); </code></pre> <p>Can someone explain this behaviour? Is it by design? Is it <em>wanted</em>? Am I doing something wrong or did I stumble upon a flaw in the design / bug in the compiler?</p> <p>Wild guess: the “capture#1-of ?” somehow seems to imply that the wildcard is filled in by the compiler on the fly, making the type a <code>Class&lt;List&gt;</code>, and thus failing the conversion (from <code>Pair&lt;Class&lt;?&gt;, String&gt;</code> to <code>Pair&lt;Class&lt;List&gt;, String&gt;</code>). Is this right? Is there a way to work around this?</p> <hr> <p>For completeness’ sake, here is a simplified version of the <code>Pair</code> class:</p> <pre><code>public final class Pair&lt;T1, T2&gt; { public final T1 first; public final T2 second; public Pair(T1 first, T2 second) { this.first = first; this.second = second; } public static &lt;T1, T2&gt; Pair&lt;T1, T2&gt; of(T1 first, T2 second) { return new Pair&lt;T1, T2&gt;(first, second); } } </code></pre>
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    1. USKonrad Rudolph
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    1. COIt seems like the converter sees the signature of "of" as it returns a Pair<? extends Class<InputClass1>, ? extends Class<InputClass2>> type. For final classes it seems smart enough to reduce the extends part, that's why it does not complain on the String.
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    2. COHmmm, interesting. Thanks for linking me here.
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      1. USjjnguy
 

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