Java Arraylist indexOf() Method

The indexOf() method of the ArrayList class is used to find the index of the first occurrence of a specified element in the list. If the element appears multiple times, it returns the index of its first occurrence. If the element is not present, it returns -1.

• Returns the index of the first matching element.
• Returns -1 if the specified element is not found.

import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {

        // Create an ArrayList
        ArrayList<String> fruits = new ArrayList<>();

        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Mango");

        // Find the index of "Banana"
        int index = fruits.indexOf("Banana");

        System.out.println("Index of Banana: " + index);
    }
}

Index of Banana: 1

Explanation: In the above example, the indexOf() method searches for "Banana" and returns its index, which is 1.

Syntax

public int indexOf(Object o)

Parameter: o-> The element whose first occurrence is to be searched.

Return Value

• Returns the index of the first occurrence of the specified element.
• Returns -1 if the element is not found.

Example: Finding the First and Last Occurrence

import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {

        // Create an ArrayList
        ArrayList<Integer> numbers = new ArrayList<>();

        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.add(20);
        numbers.add(40);

        // Find first and last occurrence
        System.out.println("First occurrence of 20: "
                + numbers.indexOf(20));

        System.out.println("Last occurrence of 20: "
                + numbers.lastIndexOf(20));
    }
}

First occurrence of 20: 1
Last occurrence of 20: 3

Explanation: The element 20 appears twice in the list. indexOf() returns the first occurrence (index 1), while lastIndexOf() returns the last occurrence (index 3).

Example: Searching for a Non-Existing Element

import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {

        // Create an ArrayList
        ArrayList<String> cities = new ArrayList<>();

        cities.add("Delhi");
        cities.add("Mumbai");
        cities.add("Chennai");

        // Search for an element
        int index = cities.indexOf("Pune");

        System.out.println("Index of Pune: " + index);
    }
}

Index of Pune: -1

Explanation: Since "Pune" is not present in the ArrayList, the indexOf() method returns -1, indicating that the element was not found.

indexOf() Vs lastIndexOf()

Advantages of indexOf() Method

• Quickly locates an element in an ArrayList.
• Returns the first occurrence when duplicate elements exist.
• Returns -1 if the element is absent, making existence checks easy.
• Works with all object types, including custom objects (using equals()).
• Useful for searching, validation, and conditional processing.