Question

I need to associate some data with a key for its lifetime, so I am using a WeakHashMap. However, in addition I need to get a key by its corresponding value. The easy way to do it is to hold on to the reference when creating a value:

public class Key {}

public class Value { 
    final public Key key;
    public Value(Key k) {
        key = k;
    }
}

Of course, while I use Value in my program, its key won’t go away. However, if there are no more references to either key or its value outside the map, will it be garbage collected? Or does the surviving strong reference in the value prevent it?

Answer 1

No it won’t be garbage collected, see the Javadoc:

Implementation note: The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded.

As mentioned by @biziclop one solution would be to store a weak reference to the key in your value object.

public class Value { 
  final public WeakReference<Key> key;
  public Value(Key k) {
    this.key = new WeakReference<Key>(k);
  }
}