What is the use of trimToSize() method of ArrayList ?

ArrayList.trimToSize() method trims the capacity of ArrayList to the current list size.

It used for memory optimization.

If ArrayList is having capacity of 10 but there are only 3 elements in it, calling trimToSize() method on this ArrayList would change the capacity from 10 to 3.

Example

ArrayList<Integer> arraylist = new ArrayList<Integer>(10);

arraylist.add(1);

arraylist.add(2);

arraylist.add(3);

// Debug the capacity of array list = 10

// [1, 2, 3, null, null, null, null, null, null, null]

System.out.println(arraylist.size());

arraylist.trimToSize();

// Debug the capacity of array list = 3

// [1, 2, 3]

System.out.println(arraylist.size());

// size() prints the actual number of elements, but not capacity

Output

3

3