How to Find Index of Element in ArrayList in Java

To find the index of an element in a Java ArrayList, use the indexOf() method. It returns the first position of the element or -1 if the element is not found.

Syntax

The indexOf() method is called on an ArrayList object and takes one argument: the element you want to find. It returns an int representing the index of the element.

• arrayList.indexOf(element): Returns the index of the first occurrence of element.
• Returns -1 if the element is not found.

java

int index = arrayList.indexOf(element);

Example

This example shows how to create an ArrayList of strings and find the index of a specific element using indexOf(). It prints the index or a message if the element is not found.

java

import java.util.ArrayList;

public class FindIndexExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");
        fruits.add("Date");

        String searchFruit = "Cherry";
        int index = fruits.indexOf(searchFruit);

        if (index != -1) {
            System.out.println(searchFruit + " found at index: " + index);
        } else {
            System.out.println(searchFruit + " not found in the list.");
        }
    }
}

Output

Cherry found at index: 2

Common Pitfalls

Some common mistakes when using indexOf() include:

• Expecting indexOf() to find elements by content when the elements are custom objects without properly overridden equals() method.
• Not checking for -1 which means the element was not found.
• Using indexOf() on a list with duplicate elements returns only the first occurrence.

Always ensure your objects have equals() defined if you want to find them by value.

java

import java.util.ArrayList;

class Person {
    String name;
    Person(String name) { this.name = name; }
}

public class PitfallExample {
    public static void main(String[] args) {
        ArrayList<Person> people = new ArrayList<>();
        people.add(new Person("Alice"));
        people.add(new Person("Bob"));

        Person searchPerson = new Person("Alice");
        int index = people.indexOf(searchPerson); // Will be -1 because equals() not overridden

        System.out.println("Index found: " + index);
    }
}

Output

Index found: -1

Quick Reference

Remember these points when using indexOf() with ArrayList:

Method	Description
indexOf(element)	Returns index of first occurrence or -1 if not found
lastIndexOf(element)	Returns index of last occurrence or -1 if not found
contains(element)	Returns true if element exists in the list
equals()	Must be overridden in custom objects for correct indexOf behavior

Key Takeaways

Use ArrayList's indexOf() method to find the first index of an element.

indexOf() returns -1 if the element is not present in the list.

For custom objects, override equals() to ensure indexOf() works correctly.

indexOf() only finds the first occurrence if duplicates exist.

Always check the returned index before using it to avoid errors.