Why does exist WeakHashMap, but absent WeakSet?

问题:

From J. Bloch

A ... source of memory leaks is listeners ... The best way to ensure that callbacks are garbage collected promptly is to store only weak references to them, for instance, by storing them only as keys in a WeakHashMap.

So, why there isn't any WeakSet in the Java Collections framework?

回答1:

It's simple: there are use cases for WeakHashMap (in particular, the case where you want to annotate objects with additional properties), but there are no use cases for WeakSets.

回答2:

Set<Object> weakHashSet = Collections.newSetFromMap(
        new WeakHashMap<Object, Boolean>());

As seen in Collections.newSetFromMap documentation, passing a WeakHashMap to get a Set.

回答3:

So, why there isn't any WeakSet in java collection framework?

The only really correct answer to that is that we can't tell you why because we are not the people who made the design decisions. Only the Java designers know why they made the decision¹.

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While there may be limited use-cases for WeakHashSet, part of the Java class library design philosophy was to avoid populating the class libraries with utility classes for all possible use-cases.

There are a number of other class libraries which include collection types; Apache Commons Collections and Google Collections (aka Guava) are good examples. However, WeakHashSet hasn't even "made the cut" for the Apache and Google libraries.

And, of course, you can use Collections.newSetFromMap to wrap a WeakHashMap instance.

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^{1 - Debating the correctness of that decision is out of scope for StackOverflow. This is a Q&A site, not discussion forum.}

回答4:

While you can indeed use Collections.newSetFromMap() to get a WeakSet, it's use cases are actually quite limited.

If you want to implement something like String.intern() you might want to have a look at Guava's Interners.newWeakInterner() functionality instead.