Hi,

I need something like WeakHashMap, but with the inverse function: I want
the values to be weakly referenced to, and the entry to be discarded
from the map once the value no longer exists.  Is there a standard
implementation for this?

But maybe, as happens often, I am saying: I want X, whereas actually I
want Y, but think I need X to achieve it.  So here is my real problem:

I have some small wrapper objects, whose identity is determined by the
array of some other objects they contain.  I want to avoid unnecessary
implementations of those wrappers, so I wanted to write a factory
method.  The first idea is to have a Map mapping lists to objects
(arrays do not give good hash codes, so I convert them with
Arrays.asList first).  Now, the whole idea is to save memory, so I don’t
want this map to get unnecessarily big.  The idea is: using weak
references I can have it clean up itself.  But WeakHashMap does just the
inverse: it weakly wraps the key, not the value.  And using the
suggestion in the javadoc will not work too, I think: I can wrap the
values myself, but the keys are lists made on the fly, so the weak hash
map will throw those entries away too soon!

One solution I see, is to use an ordinary HashMap, wrap the values in
weak references, and do some cleanup in the map from time to time.  But
I am not sure how I should implement ‘from time to time’, then.

Suggestions welcome!

TIA, H.
- --
Hendrik Maryns

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