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POgeneric super argument
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  1. POgeneric super argument
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    copied!<p>Can anybody explain why the following code doesn't compile, and if there is anyway to write the doubleRequestExecute with the desired behavior (being able to pass in a <code>Callback&lt;Pair&lt;? super A, ? super B&gt;&gt;)</code> function that will actually work?</p> <pre><code>public class Example { public interface Callback&lt;T&gt; { void onSuccess(T result); void onFailure(Throwable caught); } public interface Entity {} public static class Request&lt;T extends Entity&gt; { public void execute(Callback&lt;? super T&gt; callback) { /* In real code, get a T somewhere and pass it to onSuccess */ callback.onSuccess(null); } } public static class Holder&lt;T&gt; { public T value; } public static class Pair&lt;A, B&gt; { public Pair(A first, B second) { this.first = first; this.second = second; } public final A first; public final B second; } public static &lt;A extends Entity, B extends Entity, C super A, D super B&gt; void doubleRequestExecute(Request&lt;A&gt; request1, Request&lt;B&gt; request2, final Callback&lt;Pair&lt;C, D&gt;&gt; callback) { final Holder&lt;A&gt; result1 = new Holder&lt;&gt;(); final Holder&lt;B&gt; result2 = new Holder&lt;&gt;(); request1.execute(new Callback&lt;A&gt;() { @Override public void onSuccess(A result) { if (result2.value != null) { callback.onSuccess(new Pair&lt;C, D&gt;(result, result2.value)); } else { result1.value = result; } } @Override public void onFailure(Throwable caught) { callback.onFailure(caught); } }); request2.execute(new Callback&lt;B&gt;() { @Override public void onSuccess(B result) { if (result1.value != null) { callback.onSuccess(new Pair&lt;C, D&gt;(result1.value, result)); } else { result2.value = result; } } @Override public void onFailure(Throwable caught) { callback.onFailure(caught); } }); } } </code></pre> <p>If I simply switch the callback argument to <code>Callback&lt;Pair&lt;A, B&gt;&gt;</code> it works fine, but it is overly restrictive on the callback types that can be used.</p> <p>Here is the compile error I get when I try to build the above code:</p> <pre><code>Main.java:27: error: &gt; expected public static &lt;A extends Entity, B extends Entity, C super A, D super B&gt; ^ Main.java:27: error: illegal start of type public static &lt;A extends Entity, B extends Entity, C super A, D super B&gt; ^ Main.java:27: error: '(' expected public static &lt;A extends Entity, B extends Entity, C super A, D super B&gt; ^ Main.java:27: error: &lt;identifier&gt; expected public static &lt;A extends Entity, B extends Entity, C super A, D super B&gt; ^ Main.java:27: error: &lt;identifier&gt; expected public static &lt;A extends Entity, B extends Entity, C super A, D super B&gt; ^ 5 errors </code></pre> <p>Example desired use:</p> <pre><code>doubleRequestExecute(new Request&lt;Entity&gt;(), new Request&lt;Entity&gt;(), new Callback&lt;Pair&lt;Object, Object&gt;&gt;() { @Override onSuccess(Pair&lt;Object, Object&gt; result) {} @Oberride onFailure(Throwable caught) {} }); </code></pre> <p>Also note that in the actual code, all the handlers are executed on the same thread, so the obvious ToCToU race condition isn't an issue (for a more general approach, one could create an atomic boolean and simply use compareAndSet to check if the other request has completed).</p>
 

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