Arraylist indexOf() – Get index of element in arraylist

Learn how to get the index of first occurrence of a element in the ArrayList. We will be using ArrayList.indexOf() method to get the first occurrence.

1. ArrayList.indexOf() method

This method returns the index of the first occurrence of the specified element in this list. It will return '-1' if the list does not contain the element.

1.1. indexOf() method syntax

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

1.2. indexOf() method parameter

object – the object which needs to be searched in the list for it’s first index position.

1.3. indexOf() return value

Return value is of int type.

• index – first index position of element if element is found.
• -1 – if element is NOT found.

2. ArrayList get index of element

Java program for how to get first index of object in arraylist. In this example, we are looking for first occurrence of string “brian” in the given list.

We can use this method to find if an object is present in arraylist. If the object is present then return value will be greater than '-1‘.

Note – Please note that arraylist index starts from 0.

import java.util.ArrayList;
import java.util.Arrays;

public class ArrayListExample 
{
    public static void main(String[] args) 
    {
        ArrayList<String> list = new ArrayList<>(Arrays.asList("alex", "brian", "charles","alex","dough","gary","alex","harry"));
        
        int firstIndex = list.indexOf("brian");
        
        System.out.println(firstIndex);
        
        firstIndex = list.indexOf("hello");
        
        System.out.println(firstIndex);
    }
}

Program output.

1
-1

Happy Learning !!

Read More:

A Guide to Java ArrayList
ArrayList Java Docs