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    <p>So, you're trying to implement a workflow <strong>join</strong>. You want to take a pair continuation and turn it into two component continuations. I think there are race conditions in your implementation, but leaving that aside for the moment, let's talk about the types.</p> <p>If you're dead-set on allowing the component types to have names and be different from <code>A</code> and <code>B</code>, they need to be superclasses thereof (as you are going to get an A and a B back from your respective requests). Which is what you tried in the OP. Assuming <code>C</code> and <code>D</code> can only be <code>Entity</code>, turn the relationships around, and you can get it without using the <code>super</code> keyword where it isn't allowed:</p> <pre><code>public static &lt; C extends Entity, D extends Entity, A extends C, B extends D &gt; void doubleRequestExecute( Request&lt; A &gt; request1, Request&lt; B &gt; request2, Callback&lt; Pair&lt; C, D &gt; &gt; callback ) { ... } </code></pre> <p>For arbitrary <code>C</code> and <code>D</code> see my other answer. However, I think you can go as simply as</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, Callback&lt; ? super Pair&lt; A, B &gt; &gt; callback ) { // change all Cs to As and Ds to Bs in the original code } </code></pre> <p><em>if</em> you are willing to have calling code that looks like this (which I believe mimics @Flavio's answer):</p> <pre><code>doubleRequestExecute( new Request&lt;Entity&gt;(), new Request&lt;Entity&gt;(), new Callback&lt; Pair&lt; ?, ? &gt; &gt;() { @Override public void onSuccess(Pair&lt; ?, ? &gt; result) {} @Override public void onFailure(Throwable caught) {} } ); </code></pre> <p>You can also do</p> <pre><code>public static &lt; C, D &gt; void doubleRequestExecute( Request&lt; ? extends C &gt; reqA, Request&lt; ? extends D &gt; reqB, final Callback&lt; Pair&lt; C, D &gt; &gt; callback ) { // change all As to Cs and Bs to Ds in the original code } </code></pre> <p>(it seems Eclipse's Java is OK with the wildcards in the <code>Request</code> parameters not being expressly bounded by Entity) and call it exactly as you wished in the OP:</p> <pre><code>doubleRequestExecute( new Request&lt;Entity&gt;(), new Request&lt;Entity&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) {} } ); </code></pre>
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    1. COAs for the thread safety, the actual code is guaranteed that all handlers are executed on the same thread, so there is no race (should have mentioned that). Unfortunately, both solutions aren't adequate. The first, besides not really solving the problem at hand, isn't even possible since the object of type ? super Pair<A, B> can't possibly be instantiated (and it still requires A and B instead of anything super to them). The second solution is close, but C and D don't necessarily need to extend Entity (just the request type parameters), e.g. `Callback<Pair<Object, Object>>`.
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    2. COEdited to allow `C` and `D` to be any type (for the first suggestion).
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    3. COUnfortunately, you can't use type parameters in type arguments with multiple bounds. It fails to compile with the error `a type variable may not be followed by other bound`. If you switch Entity and C/D, it fails with a similar error.
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