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I have a class which extends LinkedHashMap which I'm using as a cache. This cache has a property which defines the maximum size in bytes that the cache is allowed to store. I use a rough estimate for the size of the objects that I store.

I override put, so that I update the total size that the cache is currently storing.

I also override remove to subtract the size of the object I've removed from the total.

At the moment I'm checking whether or not I need to remove stale entries from the cache to free up space on adding a new value in the put method, however I would like to move this functionality to an implementation of the removeEldestEntry method.

The problem I've encountered is that it seems if my overridden removeEldestEntry returns true, my overridden remove method is not called. This leads to a problem where my current stored size variable is not updated when the removeEldestEntry removes stale entries.

Does anyone know what code path is taken to remove the eldest entry when removeEldestEntry returns true. Is it possible for me to override the method that this uses to remove entries?

Is it possible for me to do my update calculation when LinkedHashMap triggers a removal on removeEldestEntry.

I know the api allows you to do the actual removal in the removeEldestEntry method as long as you return false afterwards, however I want to explore other options before doing this. I view this as a last case scenario.

Kialandei
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1 Answers1

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Looking at the source code of LinkedHashMap where removeEldestEntry is called, it doesn't call remove, but instead calls removeNode - which is also called in the default implementation of remove.

Note that this is sort of an example of the Fragile Base Class problem, wherein you are attempting to rely upon an implementation detail in the base class (i.e. if removeEldestEntry returns true, remove is called) that isn't actually true - the difference is that it is already not true, rather than it becoming not true at some point in the future.

You might want to consider implementing your own wrapper class around the LinkedHashMap which delegates a lot of the behavior to LHM, but in which you implement the specific eviction logic that you want.

Andy Turner
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  • Thanks Andy. RemoveNode is final, so there's no way for me to override that. I think the best solution for me is either going to be implement a wrapper as you suggested, or call my remove method manually in removeEldestEntry and return false from it as the API suggests. – Kialandei Feb 03 '16 at 16:35