Question

I am holding an array of hashmaps, I want to gain maximum performance and memory usage so I would like to resue the hashmaps inside an array.

So when there is a hashmap in the array that is not needed any more and I want to add new hashmap to the array I just clear the hashmap and use put() to add new values.

I also need to copy back values when I retireve hashmap from array.

I am not sure if this is better than creating new HashMap() every time.
What is better?

UPDATE

need to cycle about 50 milions of hashmaps, each hash map has about 10 key-value pairs. If size of the array 20,000 I need just 20,000 hashmaps instead of 50 milions new hashmaps()

Answer 1

Be very careful with this approach. Although it may be better performance-wise to recycle objects, you may get into trouble by modifying the same reference several times, as illustrated in the following example:

public class A {
    public int counter = 0;

    public static void main(String[] args) {

         A a = new A();
         a.counter = 5;
         A b = a; // I want to save a into b and then recycle a for other purposes
         a.counter = 10; // now b.counter is also 10
    }
}

I’m sure you got the point, however if you are not copying around references to HashMaps from the array, then it should be ok.