Java ArrayList IndexOf() Method

June 22, 2023

ArrayLIst indexOf() method

What is an ArrayList ?

Arraylist is a part of collection framework in java. It contains several inbuilt method like IndexOf() method which can be found in the java.util.ArrayList Package. It implements the List interface of the collections framework.

Here, in the page we will discuss about the ArrayList’s indexOf() Method in java.

ArrayList indexOf() Function:

When an element is specified, the java.util.ArrayList.indexOf(Object) method returns the index of the element’s first occurrence in the list, or -1 if the element is absent from the list.

Syntax :

arraylist.indexOf(Object obj)

Return Type :

The index of the first time the specified element appears in this list, or -1 if the element doesn’t appear in this list.

Polymorphism in Java

Encapsulation in Java

Abstraction in Java

Multithreading in Java

ArrayList IndexOf() Method

Example code :

Run

//This is the Example code for ArraryList indexOf() Method
import java.util.*;

public class Main{
    public static void main(String[] args) {
		
        ArrayList list = new ArrayList<>();
		
        //Adding elements to the list
        list.add("A");       
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("E");
        list.add("F");
		
	//Printing the index of the elements
        System.out.println(list.indexOf("A")); 
        System.out.println(list.indexOf("B"));
        System.out.println(list.indexOf("C"));
        System.out.println(list.indexOf("D"));
        System.out.println(list.indexOf("E"));
        System.out.println(list.indexOf("F"));
        System.out.println(list.indexOf("G"));
    }
}

Output :

0
1
2
3
4
5
-1  //We get -1 here because G is not present in the list

Explanation : 

This code example demonstrates the use of the indexOf() method in Java’s ArrayList. The code creates an ArrayList of Strings and adds elements (“A”, “B”, “C”, “D”, “E”, and “F”) to the list. The indexOf() method is then used to find the index of each element in the list and print it. If the element is not found in the list, the indexOf() method returns -1, which is the case for the element “G”.

Run

import java.util.*;

public class Main {
    public static void main(String[] args) {
        ArrayList numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        int index = numbers.indexOf(3);
        System.out.println("Index of 3: " + index);

        index = numbers.indexOf(6);
        System.out.println("Index of 6: " + index);
    }
}

Output :

Index of 3: 2
Index of 6: -1

Explanation :  

In this example, an ArrayList of Integers is created and populated with 5 elements (1, 2, 3, 4, and 5). The indexOf() method is then used to find the index of the number 3 in the list and print it. If the number is not found in the list, the indexOf() method returns -1, which is the case for the number 6.

indexof

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