Why do I get an OutOfMemoryError when inserting 50,000 objects into HashMap?

Question

I am trying to insert about 50,000 objects (and therefore 50,000 keys) into a java.util.HashMap. However, I keep getting an OutOfMemo

Answer 1

The implementations are backed by arrays usually. Arrays are fixed size blocks of memory. The hashmap implementation starts by storing data in one of these arrays at a given capacity, say 100 objects.

If it fills up the array and you keep adding objects the map needs to secretly increase its array size. Since arrays are fixed, it does this by creating an entirely new array, in memory, along with the current array, that is slightly larger. This is referred to as growing the array. Then all the items from the old array are copied into the new array and the old array is dereferenced with the hope it will be garbage collected and the memory freed at some point.

Usually the code that increases the capacity of the map by copying items into a larger array is the cause of such a problem. There are "dumb" implementations and smart ones that use a growth or load factor that determines the size of the new array based on the size of the old array. Some implementations hide these parameters and some do not so you cannot always set them. The problem is that when you cannot set it, it chooses some default load factor, like 2. So the new array is twice the size of the old. Now your supposedly 50k map has a backing array of 100k.

Look to see if you can reduce the load factor down to 0.25 or something. this causes more hash map collisions which hurts performance but you are hitting a memory bottleneck and need to do so.

Use this constructor:

(http://java.sun.com/javase/6/docs/api/java/util/HashMap.html#HashMap(int, float))