Is Java HashMap.clear() and remove() memory effective?

Question

Consider the follwing HashMap.clear() code:

 /**
 * Removes all of the mappings from this map.
 * The map will be empty after this call returns.
 */
public void clear() {
    modCount++;
    Entry[] tab = table;
    for (int i = 0; i < tab.length; i++)
        tab[i] = null;
    size = 0;
}

It seems, that the internal array (table) of Entry objects is never shrinked. So, when I add 10000 elements to a map, and after that call map.clear(), it will keep 10000 nulls in it's internal array. So, my question is, how does JVM handle this array of nothing, and thus, is HashMap memory effective?

Answer 1

The idea is that clear() is only called when you want to re-use the HashMap. Reusing an object should only be done for the same reason it was used before, so chances are that you'll have roughly the same number of entries. To avoid useless shrinking and resizing of the Map the capacity is held the same when clear() is called.

If all you want to do is discard the data in the Map, then you need not (and in fact should not) call clear() on it, but simply clear all references to the Map itself, in which case it will be garbage collected eventually.

Answer 2

You are right, but considering that increasing the array is a much more expensive operation, it's not unreasonable for the HashMap to think "once the user has increased the array, chances are he'll need the array this size again later" and just leave the array instead of decreasing it and risking to have to expensively expand it later again. It's a heuristic I guess - you could advocate the other way around too.

Answer 3

Another thing to consider is that each element in table is simply a reference. Setting these entries to null will remove the references from the items in your Map, which will then be free for garbage collection. So it isn't as if you are not freeing any memory at all.

However, if you need to free even the memory being used by the Map itself, then you should release it as per Joachim Sauer's suggestion.

Answer 4

Looking at the source code, it does look like HashMap never shrinks. The resize method is called to double the size whenever required, but doesn't have anything ala ArrayList.trimToSize().

If you're using a HashMap in such a way that it grows and shrinks dramatically often, you may want to just create a new HashMap instead of calling clear().