Question

I use a large (millions) entries hashmap to cache values needed by an algorithm, the key is a combination of two objects as a long. Since it grows continuously (because keys in the map changes, so old ones are not needed anymore) it would be nice to be able to force wiping all the data contained in it and start again during the execution, is there a way to do effectively in Java?

I mean release the associated memory (about 1-1.5gb of hashmap) and restart from the empty hashmap..

Answer 1

You can call HashMap.clear(). That will remove all data. Note that this will only discard all entries, but keep the internal array used to store the entries at the same size (rather than shrinking to an initial capacity). If you also need to eliminate that, the easiest way would be to discard the whole HashMap and replace it with a new instance. That of course only works if you control who has a pointer to the map.

As for reclaiming the memory, you will have to let the garbage collector do its work.

Are your values also Long? In this case, you may want to look at a more (memory-) efficient
implementation than the generic HashMap, such as the TLongLongHashMap found in the GNU Trove library. That should save a lot of memory.