ArrayList indexOf() method

In this tutorial, we will see the indexOf() method of the ArrayList class in Java. This method returns the index of the first occurrence of the specified element in this list.

Introduction

ArrayList is a widely used class to store and retrieve data in a collection framework.

Also, we have seen a few methods of the ArrayList class before like add(), addAll(), remove(), set(), iterate(), clear(), contains(), get(), removeRange(), retainAll() and trimToSize() method.

Today we will see the indexOf(Object o) method.

ArrayList indexOf(Object o) method

We know the important feature of the list is that it maintains the insertion order of the elements. Also, it allows duplicate elements in the list.

So, when we try to find any elements, there are chances that the same elements may present in the list multiple times at multiple indexes.

The indexOf(Object o) method returns the index of the first occurrences of the object specified. It returns -1 if the specified object o is not present in the list.

The method signature is given below.

1
public int indexOf(Object o)

The indexOf() method accepts only one parameter, the Object o whose index of the first occurrence will be returned.

The method has a return type as an int. It returns the index of the first occurrences of the object specified as an int . If the specified object o is not present in the list, it will return -1.

Let’s see the Java program for a better understanding.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
/**
 * A java program to explain the indexOf() method
 * of the ArrayList class in Java.
 * 
 * @author coderolls.com
 */
public class ArrayListIndexOfExample {

  public static void main(String[] args) {
    // create an empty ArrayList object 'states'
    List<String> states = new ArrayList<String>();
    
    // add state in the ArrayList, Florida multiple times
    states.add("California");
    states.add("Texas");
    states.add("Florida");
    states.add("Florida");
    states.add("New Jersey");
    states.add("Washington");
    states.add("Florida");
    
    int index = states.indexOf("Florida");
    
    //prints index of the first occurrences of Florida i.e. 2
    System.out.println("The index of Florida: "+ index);
    
    //trying to get the index of the element, which is not present
    int index2 = states.indexOf("Alaska");
    
    System.out.println("The index of Alaska: " + index2);
  }
}

Output:

1
2
The index of Florida: 2
The index of Alaska: -1

Conclusion

1
public int indexOf(Object o)

The indexOf(Object o) method returns the index of the first occurrences of the object specified. If the specified object o is not present in the list, it returns the -1.

The example Java program used in the above article can be found at this GitHub repository.

Please write your thoughts in the comment section below.