List indexOf() Method in Java with Examples

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.

Syntax:

public int indexOf(Object o)

Parameters: This function has a single parameter, i.e, the element to be searched in the list.

Returns: This method returns the index of first occurrence of the given element in the list and returns “-1” if element is not in the list.

Below programs show the implementation of this method.

Program 1:

// Java code to show the implementation of
// indexOf method in list interface
import java.util.*;
public class GfG {

    // Driver code
    public static void main(String[] args)
    {

        // Initializing a list of type Linkedlist
        List<Integer> l = new LinkedList<>();
        l.add(1);
        l.add(3);
        l.add(5);
        l.add(7);
        System.out.println(l);
        System.out.println(l.indexOf(5));
    }
}

Output:

[1, 3, 5, 7]
2

Program 2: Below is the code to show implementation of list.hashCode() using Linkedlist.

// Java code to show the implementation of
// indexOf method in list interface
import java.util.*;
public class GfG {

    // Driver code
    public static void main(String[] args)
    {

        // Initializing a list of type Linkedlist
        List<String> l = new LinkedList<>();
        l.add("10");
        l.add("15");
        l.add("20");
        System.out.println(l);
        System.out.println(l.indexOf("20"));
    }
}

Output:

[10, 15, 20]
2

Reference:
Oracle Docs