Java ArrayList ensureCapacity() Method example

ArrayList internally implements growable dynamic array which means it can increase and decrease its size automatically. If we try to add an element to a already full ArrayList then it automatically re-sized internally to accommodate the new element however sometimes its not a good approach.

Consider a scenario when there is a need to add huge number of elements to an already full ArrayList, in such case ArrayList has to be resized several number of times which would result in a poor performance. For such scenarios ensureCapacity() method of Java.util.ArrayList class is very useful as it increases the size of the ArrayList by a specified capacity.

public void ensureCapacity(int minCapacity)

Example

package beginnersbook.com;
import java.util.ArrayList;
public class EnsureCapacityExample {
  public static void main(String args[]) {
      // ArrayList with Capacity 4
      ArrayList<String> al = new ArrayList<String>(4);
      //Added 4 elements
      al.add("Hi");
      al.add("Hello");
      al.add("Bye");
      al.add("GM");

      //Increase capacity to 5
      al.ensureCapacity(5);

      al.add("GE");
      // let us print all the elements available in list
      for (String temp: al) {
            System.out.println(temp);
      }
   }
}

Output:

Hi
Hello
Bye
GM
GE