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PO
primarykey
Id
16206568
data
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0
AnswerCount
0
ClosedDate
CommentCount
5
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CreationDate
2013-04-25T04:46:30.553
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2013-04-25T15:44:33.373
LastEditDate
2017-05-23T12:28:08.250
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16205915
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2
Score
5
ViewCount
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LastEditorDisplayName
text
Body
You're getting an error because a <code>Pair<String, Future<String>></code> is not a <code>Pair<String, Future<?>></code>. Generic types are not covariant. You can however type <code>futureTasks</code> as <code>List<Pair<String, ? extends Future<?>>></code>. Then you'll be able to add a <code>Pair<String, Future<String>></code> to it. EDIT: To explain this better, think of trying to assign an object to a variable instead of adding it to a list. If we have a <code>List<T></code>, we can only add an object if it is a <code>T</code>. So this is really the same situation: <pre><code>Pair<String, Future<String>> pairOfStringAndFutureOfString; Pair<String, Future<?>> pairOfStringAndFutureOfSomething; // incompatible types pairOfStringAndFutureOfSomething = pairOfStringAndFutureOfString; </code></pre> Again, this isn't allowed because generics aren't covariant. Why? Consider what could happen if they were: <pre><code>Future<Integer> futureOfInt; Future<?> futureOfSomething; futureOfSomething = futureOfInt; // a future of int is a future of something Pair<String, Future<String>> pairOfStringAndFutureOfString; Pair<String, Future<?>> pairOfStringAndFutureOfSomething; // pretend this is legal pairOfStringAndFutureOfSomething = pairOfStringAndFutureOfString; // danger! pairOfStringAndFutureOfSomething.setRight(futureOfSomething); Future<String> futureOfString = pairOfStringAndFutureOfString.getRight(); //sometime later... String string = futureOfString.get(); //ClassCastException </code></pre> We got <code>futureOfString</code> to point to something that was actually a <code>Future<Integer></code>. But this didn't even cause a <code>ClassCastException</code> right away because thanks to type erasure, the JVM only saw <code>Future</code>s. This situation is called "heap pollution" - where a variable points to an object it shouldn't have been allowed to. The actual runtime error only happened once we tried to unwrap <code>futureOfString</code>. So that explains why generics aren't covariant. Let's look at the workaround: <pre><code>Pair<String, ? extends Future<?>> pairOfStringAndSomethingThatIsAFutureOfSomething; </code></pre> That variable name is a mouthful but I wanted it to describe exactly what's going on here. Before, the second type argument was exactly <code>Future<?></code>. Now, we're saying that it's "some unknown type, which is or extends <code>Future<?></code>. For example, it could be <code>Future<?></code>, it could be <code>Future<String></code>, it could even be <code>MySpecialFuture<Integer></code>. We're voluntarily giving up some information about the type of this variable in order to relax what can be assigned to it. Now, this is legal: <pre><code>pairOfStringAndSomethingThatIsAFutureOfSomething = pairOfStringAndFutureOfString; </code></pre> The compiler protects against the earlier "heap pollution" scenario by preventing this unknown type from being "consumed": <pre><code>//compiler error - nothing can be legally passed into setRight pairOfStringAndSomethingThatIsAFutureOfSomething.setRight(futureOfSomething); </code></pre> To learn more about these retrictions, see this post: <a href="https://stackoverflow.com/questions/2723397/java-generics-what-is-pecs">What is PECS (Producer Extends Consumer Super)?</a>
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1. POAdding an element into wildchar list cause compile error
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1. USPaul Bellora
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