The use of weak references is something that I've never seen an implementation of so I'm trying to figure out what the use case for them is and how the implementation would work. When have you needed to use a WeakHashMap or WeakReference and how was it used?
Answer
One problem with strong references is
caching, particular with very large
structures like images. Suppose you
have an application which has to work
with user-supplied images, like the
web site design tool I work on.
Naturally you want to cache these
images, because loading them from disk
is very expensive and you want to
avoid the possibility of having two
copies of the (potentially gigantic)
image in memory at once.
Because an image cache is supposed to
prevent us from reloading images when
we don't absolutely need to, you will
quickly realize that the cache should
always contain a reference to any
image which is already in memory. With
ordinary strong references, though,
that reference itself will force the
image to remain in memory, which
requires you to somehow determine when
the image is no longer needed in
memory and remove it from the cache,
so that it becomes eligible for
garbage collection. You are forced to
duplicate the behavior of the garbage
collector and manually determine
whether or not an object should be in
memory.
Understanding Weak References, Ethan Nicholas
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