Java.util.Arraylist.indexOf() in Java

Last Updated : 10 Oct, 2019

The indexOf() method of ArrayList returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.

Syntax :

public int IndexOf(Object o)
obj : The element to search for.

// Java code to demonstrate the working of 
// indexOf in ArrayList 
  
// for ArrayList functions 
import java.util.ArrayList; 
  
public class IndexOfEx { 
  public static void main(String[] args) { 
       
  // creating an Empty Integer ArrayList 
  ArrayList<Integer> arr = new ArrayList<Integer>(5); 
  
  // using add() to initialize values 
  arr.add(1); 
  arr.add(2); 
  arr.add(3); 
  arr.add(4); 
  
  // printing initial value 
  System.out.print("The initial values in ArrayList are : "); 
  for (Integer value : arr) { 
  System.out.print(value); 
  System.out.print(" "); 
  }   
  
  // using indexOf() to find index of 3 
  int pos =arr.indexOf(3); 
    
  // prints 2 
  System.out.println("\nThe element 3 is at index : " + pos); 
  } 
    
}    

Output:

The initial values in ArrayList are : 1 2 3 4 
The element 3 is at index : 2

Practical Application : The index functions are mostly useful to determine last or first occurrence of events, for example last occurrence of 6 in a throw of a die, or 1st occurrence of any letter in a name etc.

One more example:

// Java code to demonstrate the application of 
// index functions in ArrayList 
  
// for ArrayList functions 
import java.util.ArrayList; 
  
public class AppliIndex { 
  public static void main(String[] args) { 
       
  // creating an Empty Integer ArrayList 
  ArrayList<Integer> arr = new ArrayList<Integer>(10); 
  
  // using add() to initialize dice values 
  arr.add(1); 
  arr.add(2); 
  arr.add(4); 
  arr.add(6); 
  arr.add(5); 
  arr.add(2); 
  arr.add(6); 
  arr.add(1); 
  arr.add(6); 
  arr.add(4); 
  
  // using IndexOf() to find first index of 6 
  int pos1 =arr.indexOf(6); 
    
  // using lastIndexOf() to find last index of 6 
  int pos2 =arr.lastIndexOf(6); 
    
  // to balance 0 based indexing 
  pos1 = pos1+1; 
  pos2 = pos2+1; 
    
  // printing first index of 6 
  System.out.println("The first occurrence of 6 is  : " + pos1); 
    
  // printing last index of 6 
  System.out.println("The last occurrence of 6 is  : " + pos2); 
    
  } 
    
}    

Output:

The first occurrence of 6 is  : 4
The last occurrence of 6 is  : 9

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Java.util.Arraylist.indexOf() in Java

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