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    copied!<p>Eureka.</p> <p>The fact that there is no declaration time variance declarations in Java means that deeply-nested generic types like this cause no end of headaches. The solution is to use a stupid wrapper class to force the inheritance to work.</p> <pre><code>public static class PairCallback&lt; A, B &gt; implements Callback&lt; Pair&lt; A, B &gt; &gt; { private final Callback&lt; Pair&lt; A, B &gt; &gt; cbDelegate; public PairCallback( Callback&lt; Pair&lt; A, B &gt; &gt; cbDelegate ) { this.cbDelegate = cbDelegate; } public void onSuccess( A a, B b ) { onSuccess( new Pair&lt; A, B &gt;( a, b ) ); } @Override public void onSuccess( Pair&lt; A, B &gt; p ) { cbDelegate.onSuccess( p ); } @Override public void onFailure( Throwable caught ) { cbDelegate.onFailure( caught ); } } public static &lt; A extends Entity, B extends Entity &gt; void doubleRequestExecute( Request&lt; A &gt; reqA, Request&lt; B &gt; reqB, final PairCallback&lt; ? super A, ? super B &gt; callback ) { final Holder&lt; A &gt; result1 = new Holder&lt; A &gt;(); final Holder&lt; B &gt; result2 = new Holder&lt; B &gt;(); reqA.execute(new Callback&lt;A&gt;() { @Override public void onSuccess(A result) { if (result2.value != null) { callback.onSuccess(result, result2.value); } else { result1.value = result; } } @Override public void onFailure(Throwable caught) { callback.onFailure(caught); } }); reqB.execute(new Callback&lt;B&gt;() { @Override public void onSuccess(B result) { if (result1.value != null) { callback.onSuccess(result1.value, result); } else { result2.value = result; } } @Override public void onFailure(Throwable caught) { callback.onFailure(caught); } }); } private static class Entity1 extends Entity {} private static class Entity2 extends Entity1 {} public static void main( String... args ) { doubleRequestExecute( new Request&lt; Entity &gt;(), new Request&lt; Entity &gt;(), new PairCallback&lt; Object, Object &gt;( new Callback&lt; Pair&lt; Object, Object &gt; &gt;() { @Override public void onSuccess( Pair&lt; Object, Object&gt; result ) {} @Override public void onFailure( Throwable caught ) {} } ) ); doubleRequestExecute( new Request&lt; Entity2 &gt;(), new Request&lt; Entity1 &gt;(), new PairCallback&lt; Entity1, Entity &gt;( new Callback&lt; Pair&lt; Entity1, Entity &gt; &gt;() { @Override public void onSuccess( Pair&lt; Entity1, Entity &gt; result ) {} @Override public void onFailure( Throwable caught ) {} } ) ); } </code></pre> <p>I'm going to up-vote your question because of how challenging and relevant it is.</p>
 

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