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
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