AbstractList indexOf() method in Java with Examples

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 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 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 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