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Showing content from https://www.geeksforgeeks.org/java/abstractlist-indexof-method-in-java-with-examples/ below:

AbstractList indexOf() method in Java with Examples

Last Updated : 11 Jul, 2025

The

indexOf()

method of

java.util.AbstractList

class is used to return the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

Syntax:

public int indexOf(Object o)

Parameters:

This method takes

Object o

as parameter which is the element to search for.

Return Value:

This method returns the

index

of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. Below are the examples to illustrate the

indexOf()

method.

Example 1: Java


 // Java program to demonstrate
// indexOf() method
// for Integer value

import java.util.*;

public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {

        try {

            // Creating object of AbstractList<Integer>
            AbstractList<Integer>
                arrlist1 = new ArrayList<Integer>();

            // Populating arrlist1
            arrlist1.add(10);
            arrlist1.add(20);
            arrlist1.add(30);
            arrlist1.add(40);
            arrlist1.add(50);

            // print arrlist1
            System.out.println("ArrayListlist : "
                               + arrlist1);

            // getting the index of element 30
            // using indexOf() method
            int index = arrlist1.indexOf(30);

            // print the index
            System.out.println("index : " + index);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

ArrayListlist : [10, 20, 30, 40, 50]
index : 2

Example 2: Java


 // Java program to demonstrate
// indexOf() method
// for Integer value

import java.util.*;

public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {

        try {

            // Creating object of AbstractList<Integer>
            AbstractList<Integer>
                arrlist1 = new ArrayList<Integer>();

            // Populating arrlist1
            arrlist1.add(10);
            arrlist1.add(20);
            arrlist1.add(30);
            arrlist1.add(40);
            arrlist1.add(50);

            // print arrlist1
            System.out.println("ArrayListlist : "
                               + arrlist1);

            // getting the index of element 25
            // using indexOf() method
            int index = arrlist1.indexOf(25);

            // print the index
            System.out.println("Index of 25 : "
                               + index);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

ArrayListlist : [10, 20, 30, 40, 50]
Index of 25 : -1

Example 3:

For Null value

Java


 // Java program to demonstrate
// indexOf() method
// for null value

import java.util.*;

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
        try {

            // Creating object of AbstractList<Integer>
            AbstractList<Integer>
                arrlist1 = new ArrayList<Integer>();

            // Populating arrlist1
            arrlist1.add(10);
            arrlist1.add(20);
            arrlist1.add(30);
            arrlist1.add(40);
            arrlist1.add(50);

            // print arrlist1
            System.out.println("ArrayListlist : "
                               + arrlist1);

            // creating null object
            Object value = null;

            // getting the index of element null
            // using indexOf() method
            int index = arrlist1.indexOf(value);

            // print the index
            System.out.println("Index of null : "
                               + index);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

ArrayListlist : [10, 20, 30, 40, 50]
Index of null : -1

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