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Java ArrayList ensureCapacity() Method example

By Chaitanya Singh | Filed Under: Java Collections

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

Comments

  1. Ray says

    January 11, 2016 at 8:46 PM

    Thank you for those easy understanding examples. Is there a typo in al.ensureCapacity(55), (where 55 should be 5 according to the comment)?

    Reply
  2. Abir Nandy says

    August 22, 2018 at 6:22 PM

    It’s not working in my code where I am checking the size of the arraylist after ensuring the capacity.
    The code is as follows:-
    import java.io.*;
    import java.util.*;

    class Exp
    {
    public static void main(String args[])
    {
    int n,i;
    Scanner sc=new Scanner(System.in);
    n=sc.nextInt();
    ArrayList in=new ArrayList();
    in.ensureCapacity(n);
    System.out.println(in.size());
    }
    }

    Reply
  3. Abir Nandy says

    August 22, 2018 at 6:25 PM

    When an arraylist is having an element repeated than what will get returned if indexOf() that element has been used?

    Reply
    • Chaitanya Singh says

      September 9, 2018 at 1:32 PM

      It will return the index of the first occurrence of the element in the list

      Reply

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ArrayList Tutorial

  • Java ArrayList

Java ArrayList Methods

  • add(Object obj)
  • add(int index, Object element)
  • addAll(Collection c)
  • addAll(int index, Collection c)
  • contains()
  • get()
  • indexOf()
  • ensureCapacity()
  • isEmpty()
  • lastIndexOf()
  • remove()
  • remove(Object obj)
  • trimToSize()
  • set()
  • clone()
  • clear()
  • size()

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