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Map.clear() vs new Map : Which one will be better?

Asked 2011-07-20T06:31:43.420
115

I have a Map as syntax as Map<String, String> testMap = new HashMap<String, String>();. In this map there can be 1000 data.

When my application requires to new list of data, then I must clear the Map. But when I saw the code of Map.clear() as 

/**
     * 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;
    }

I realize that clear method goes in loop for n times (Where n is number of data in Map). So I thought there can be a way to redefine that Map as testMap = new HashMap<String, String>(); and previously used Map will be Garbage collected.

But I am not sure this will be a good way. I am working on mobile application.

Can you please guide me?

java android performance collections
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2 Answers

6

The idea of having clear() method is to remove references to other objects from the map, so that the key/values are not held up from gcing if the "map is referenced somewhere else".

But if your map is a local map only used by your specific code( i.e. "map is 'not' referenced somewhere else") then go ahead and use a new map instead, but setting a 1000 references to null wont be a big performance hit anyway.

answered 2011-07-20T06:41:51.520
1

The map.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.

answered 2011-07-20T06:39:05.113

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