ArrayList in Java:

 

ArrayList in Java: Methods

• The ArrayList class is the dynamic array found in the java.util package.
• The size of the normal array can not be changed, but ArrayList provides us the ability to increase or decrease the size.
• ArrayList is slower than the standard array, but it helps us to manipulate the data easily.

How to declare an ArrayList :

ArrayList<Integer> l1 = new ArrayList<>(); /*Creates an ArrayList object of
                                           integer type */

Performing various operations on ArrayList :

ArrayList comes with a number of methods that can be used to manipulate the data of the ArrayList. Let’s take a look at some of the important methods of ArrayList :

1. Adding an element :

   • add() method is used to insert an element in the ArrayList.
   • add(Object): Inserts an element at the end of the ArrayList.
   • add(Index,Object) : Inserts an element at the given index.
     

     Example :

     import java.util.*;
     public class CWH extends Thread{
         public static void main(String[] args) {
     
             ArrayList<Integer> l1 = new ArrayList<>();
             l1.add(1);
             l1.add(2);
             l1.add(3);
             l1.add(4);
             l1.add(6);
             l1.add(5,5);  // inserts 5 at the 5th index in l1
     
             System.out.println(l1);
         }
     }
     

Output :

[1, 2, 3, 4, 6, 5]

 Removing an Element :

• remove() method is used to delete or remove an element at a given index from the ArrayList.
  

  Syntax :

  L1.remove(index number) 

Example :

import java.util.*;
public class CWH extends Thread{
    public static void main(String[] args) {

        ArrayList<Integer> l1 = new ArrayList<>();
        l1.add(1);
        l1.add(2);
        l1.add(3);
        l1.add(4);
        l1.add(6);
        l1.add(5,5);  // inserts 5 at the 5th index in l1

        System.out.println("Array list before : "+ l1);

        l1.remove(0);
        System.out.println("ArrayList after removing the value at index 0 :" + l1);

    }
}

Output :

Array list before : [1, 2, 3, 4, 6, 5]
ArrayList after removing the value at index 0 : [2, 3, 4, 6, 5]

 Checking if an ArrayList contains a specific value or not : 

• contains() method is used to check if an ArrayList contains a specified element or not. This method returns the boolean value.

  Example :

  import java.util.*;
  public class CWH extends Thread{
      public static void main(String[] args) {
  
          ArrayList<Integer> l1 = new ArrayList<>();
          l1.add(1);
          l1.add(2);
          l1.add(3);
          l1.add(4);
          l1.add(5);
          l1.add(6);
  
          System.out.println("Array list : "+ l1);
          System.out.println("L1 list contains 7 : " + l1.contains(7));
          System.out.println("L1 list contains 4 : " + l1.contains(4));
  
  
      }
  }
  

Output :

Array list before : [1, 2, 3, 4, 5, 6]
L1 list contains 7 : false
L1 list contains 4 : true

Merging two ArrayLists :

• The elements of an ArrayList can be merged into another Arraylist with the help of the addAll() method.

  Example :

  import java.util.*;
  public class CWH extends Thread{
      public static void main(String[] args) {
  
          ArrayList<Integer> l1 = new ArrayList<>();
          ArrayList<Integer> l2 = new ArrayList<>();
  
          l1.add(1);
          l1.add(2);
          l1.add(3);
          l1.add(4);
          l1.add(5);
          l1.add(6);
  
          l2.add(11);
          l2.add(12);
          l2.add(13);
          l2.add(14);
  
          System.out.println("L1 Array list : "+ l1);
          System.out.println("L2 Array list : " +l2);
          l1.addAll(l2);
          System.out.println("L1 Array list after merging: "+ l1);
          System.out.println("L2 Array list : " +l2);
  
          
      }
  }
  

Output :

L1 Array list : [1, 2, 3, 4, 5, 6]
L2 Array list : [11, 12, 13, 14]
L1 Array list after merging: [1, 2, 3, 4, 5, 6, 11, 12, 13, 14]
L2 Array list : [11, 12, 13, 14]

You can add the elements of l2 at the starting of l1 by typing :

L1.addAll(0,l2);

Finding the first occurrence of a specified number in the ArrayList :

• Indexof() method prints the index of the first occurrence of a particular number. Returns -1 if the element is not present in the ArrayList.

  Example :

  import java.util.*;
  public class CWH extends Thread{
      public static void main(String[] args) {
  
          ArrayList<Integer> l1 = new ArrayList<>();
  
          l1.add(1);
          l1.add(2);
          l1.add(3);
          l1.add(4);
          l1.add(5);
          l1.add(6);
          l1.add(3);
  
          System.out.println("L1 Array list : "+ l1);
          System.out.println("The first occurrence of 3 in l1 is at index : " + l1.indexOf(3));
  
  
      }
  

Output :

L1 Array list : [1, 2, 3, 4, 5, 6, 3]
The first occurrence of 3 in l1 is at index : 2

Similarly, you can also find the index of the last occurrence of an element with the help of the lastIndexOf() method.

Example :

import java.util.*;
public class CWH extends Thread{
    public static void main(String[] args) {

        ArrayList<Integer> l1 = new ArrayList<>();

        l1.add(1);
        l1.add(2);
        l1.add(3);
        l1.add(4);
        l1.add(5);
        l1.add(3);
        l1.add(6);
        l1.add(3);

        System.out.println("L1 Array list : "+ l1);
        System.out.println("The last occurrence of 3 in l1 is at index : " + l1.lastIndexOf(3));


    }
}

Output :