Java Arraylist indexOf() Method

Last Updated : 10 Dec, 2024

The indexOf() method in Java is used to find the index of the first occurrence of a specified element in an ArrayList.

Example 1: Here, we will use the indexOf() method to find the index of a specific element in an ArrayList.

// Java program to demonstrate the 
// use of indexOf() method
import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {
        
        // Creating an ArrayList of Integers
        ArrayList<Integer> arr = new ArrayList<Integer>(5);

        // Adding elements to the ArrayList
        arr.add(1);
        arr.add(2);
        arr.add(3);

        // Printing the initial values 
        // in the ArrayList
        System.out.print("");
        for (Integer v : arr) {
            System.out.print(v + " ");
        }  

        // Using indexOf() to 
        // find the index of 3
        int p = arr.indexOf(3);

        // Prints the index of the element
        System.out.println("\nThe element 3 is at index: " + p);
    }
}

Output

1 2 3 
The element 3 is at index: 2

Syntax of `ArrayList.indexOf()` Method

public int indexOf(Object o)

Parameter: The object o whose index is to find.
Return Type: An integer that represents the index of the first occurrence of the specified element in the list, or -1 if the element is not found.

Example 2: Here, we use the indexOf() method to find the first occurrence and lastIndexOf() to find the last occurrence of a specific element.

// Java program to demonstrate the use of 
// indexOf() and lastIndexOf() methods
import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {
        
        // Creating an ArrayList of Integers
        ArrayList<Integer> arr = new ArrayList<Integer>(6);

        // Adding elements to the ArrayList
        arr.add(1);
        arr.add(2);
        arr.add(4);
        arr.add(6);
        arr.add(5);
        arr.add(2);

        // Using indexOf() to find the 
        // first index of 4
        int p1 = arr.indexOf(4);
        
        // Using lastIndexOf() to find the 
        // last index of 4
        int p2 = arr.lastIndexOf(4);
        
        // Balancing the 0-based indexing
        p1 = p1 + 1;
        p2 = p2 + 1;
        
        // Printing first and last index of 4
        System.out.println("The first occurrence of 4 is: " + p1);
        System.out.println("The last occurrence of 4 is: " + p2);
    }
}

Output

The first occurrence of 4 is: 3
The last occurrence of 4 is: 3

Example 3: If the specified element is not found in the ArrayList, the indexOf() method will return -1.

// Java program to demonstrate the use of 
// indexOf() method with a non-existent element
import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {
        
        // Creating an ArrayList of Integers
        ArrayList<Integer> arr = new ArrayList<Integer>(5);

        // Adding elements to the ArrayList
        arr.add(10);
        arr.add(20);
        arr.add(30);

        // Trying to find the index of 
        // a non-existent element
        int p = arr.indexOf(50);

        System.out.println("The index of 50 is: " + p);
    }
}