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PO
primarykey
Id
4388173
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
1
CommunityOwnedDate
CreationDate
2010-12-08T13:47:47.360
FavoriteCount
0
LastActivityDate
2017-02-24T10:19:14.870
LastEditDate
2017-05-23T12:32:08.853
LastEditorUserId
-1
OwnerUserId
485343
ParentId
4388054
PostTypeId
2
Score
4
ViewCount
0
LastEditorDisplayName
text
Body
<code>extends</code> in generics doesn't really work that way. <code>T</code> != <code>? extends Component</code> even though <code>T extends Component</code>. What you have is in fact a <a href="https://stackoverflow.com/questions/19219635/java-hashmap-nested-generics-with-wildcards">wildcard capture</a>, it has a different purpose. And yes your solution is not type-safe - there is no relation between the two <code>?</code> marks in: <pre><code> private HashMap<Class<?>, HashMap<Entity, ? extends Component>> m_componentStores; </code></pre> So it becomes legal to put an instance of some subclass of <code>Component</code> in this structure using some other class (not even a subclass of <code>Component</code>) as the key. Remember that generic types are resolved at compile time only, so at run time <code>m_componentStores</code> has no way of knowing what exact type of the value you have in there, other than that it <code>extends</code> <code>Component</code>. So the type you get from <code>store.get(e)</code> is ... <code>Component</code>: <pre><code> Component result = store.get(e); </code></pre> When you cast <code>Component</code> to <code>T</code>, the compiler issues a warning because the cast cannot be checked statically. But if you're sure of the semantics of your data structure, you can simply suppress the warning. <pre><code> @SuppressWarnings("unchecked") T resultT = (T)result; </code></pre> <hr> PS: You don't need a wildcard capture, the following will work exactly the same in your case: <pre><code> private HashMap<Class<?>, HashMap<Entity, Component>> m_componentStores; </code></pre>
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1. POjava: How to fix the Unchecked cast warning
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1. PTAnswer
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1. USCommunity
UserOwnerUserId
1. USrustyx
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2. VO
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