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    <h2>First attempt</h2> <p>(Does not work, see comments)</p> <p>Try this way:</p> <pre><code>public static &lt;A extends Entity, B extends Entity&gt; void doubleRequestExecute(Request&lt;A&gt; request1, Request&lt;B&gt; request2, final Callback&lt;Pair&lt;? super A, ? super B&gt;&gt; callback) ... callback.onSuccess(new Pair&lt;A, B&gt;(result, result2.value)); ... </code></pre> <h2>Second attempt</h2> <p>I think I got it now. Here:</p> <pre><code>public static &lt;A extends Entity, B extends Entity&gt; void doubleRequestExecute(Request&lt;A&gt; request1, Request&lt;B&gt; request2, final Callback&lt;? super Pair&lt;? extends A, ? extends B&gt;&gt; callback) { ... } </code></pre> <p>Example usage:</p> <pre><code>private static final class E1 extends Entity { } private static final class E2 extends Entity { } public static void main(String[] args) { Request&lt;E1&gt; r1 = null; Request&lt;E2&gt; r2 = null; // This is the callback you try to define in your comment Callback&lt;Pair&lt;? extends Entity, ? extends Entity&gt;&gt; cb = null; // This is another option Callback&lt;Object&gt; cb2 = null; // Further option mentioned in comments Callback&lt;Pair&lt;? extends Object, ? extends Object&gt;&gt; cb3 = null; doubleRequestExecute(r1, r2, cb); doubleRequestExecute(r1, r2, cb2); doubleRequestExecute(r1, r2, cb3); } </code></pre> <h2>Why can't I simply use <code>Callback&lt;Pair&lt;Object, Object&gt;&gt;</code>?</h2> <p>This is the same problem as why <code>List&lt;Integer&gt;</code> is not assignable to <code>List&lt;Number&gt;</code>. Consider this code:</p> <pre><code>Pair&lt;Integer, Integer&gt; pII = new Pair&lt;Integer, Integer&gt;(); Pair&lt;Object, Object&gt; pOO = pII; // Compile error pOO.setFirst("Whoa"); Integer i = pII.getFirst(); </code></pre> <p>I understand your <code>Pair</code>s have no setter method, but the definition of Java generics imply that a <code>Pair&lt;A, B&gt;</code> is not assignable to a <code>Pair&lt;Object, Object&gt;</code>, but only to a <code>Pair&lt;? extends Object, ? extends Object&gt;</code>.</p> <h2>I want <code>Callback&lt;Pair&lt;? super A, ? super B&gt;&gt;</code>!</h2> <p>Unfortunately, you can't instantiate a type with that signature in any useful (to you) way. Consider again <code>List</code>s.</p> <pre><code>List&lt;List&lt;? super Integer&gt;&gt; l = null; // TODO l.add(new ArrayList&lt;Integer&gt;()); l.add(new ArrayList&lt;Number&gt;()); l.add(new ArrayList&lt;Objec&gt;()); </code></pre> <p>How do you instantiate <code>l</code>? It is a list of lists composed of something bounded by <code>Integer</code>. </p> <pre><code>List&lt;List&lt;? super Integer&gt;&gt; l = new ArrayList&lt;List&lt;Integer&gt;&gt;(); // NO List&lt;List&lt;? super Integer&gt;&gt; l = new ArrayList&lt;List&lt;Number&gt;&gt;(); // NOPE List&lt;List&lt;? super Integer&gt;&gt; l = new ArrayList&lt;List&lt;Object&gt;&gt;(); // NEITHER List&lt;List&lt;? super Integer&gt;&gt; l = new ArrayList&lt;List&lt;? super Integer&gt;&gt;(); // OK </code></pre> <p>Same goes for your callback.</p> <pre><code>// Only possible declaration Callback&lt;Pair&lt;? super E1, ? super E2&gt;&gt; cb = new Callback&lt;Pair&lt;? super E1, ? super E2&gt;&gt;() { ... } </code></pre> <h2>Example use</h2> <pre><code>doubleRequestExecute(new Request&lt;E1&gt;(), new Request&lt;E2&gt;(), new Callback&lt;Pair&lt;? extends Object, ? extends Object&gt;&gt;() { @Override public void onSuccess(Pair&lt;? extends Object, ? extends Object&gt; result) {} @Override public void onFailure(Throwable caught) {} }); </code></pre>
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    1. COThat was the first thing I tried, but it falls over really quick. When called, compilation fails with the following error `no instance(s) of type variable(s) A,B exist so that argument type <anonymous Callback<Pair<Entity,Entity>>> conforms to formal parameter type Callback<Pair<? super A,? super B>>`. If you explicitly do `Example.<Entity, Entity>(doubleRequestExecute)`, it says `actual argument <anonymous Callback<Pair<Entity,Entity>>> cannot be converted to Callback<Pair<? super Entity,? super Entity>> by method invocation conversion`.
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    2. COFurthermore, even if it got past that, `Pair<A, B>` can't be converted to `Pair<? super A, ? super B>`. The pair need to be created as the actual type that the call back expects (or at lest `? extends Pair<A, B>`).
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    3. COYou're right, I made some corrections. Don't ask me to explain why it works though, I can't entirely wrap my head around this.
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