Java ArrayList lastIndexOf() Method

The ArrayList.lastIndexOf(Object o) method in Java is used to find the index of the last occurrence of a specified element in an ArrayList. This guide will cover the method's usage, explain how it works, and provide examples to demonstrate its functionality.

Table of Contents

1. Introduction
2. lastIndexOf Method Syntax
3. Examples
   • Finding the Last Index of an Element
   • Handling Elements Not Present in the ArrayList
   • Handling Null Values
4. Conclusion

Introduction

The lastIndexOf(Object o) method is a member of the ArrayList class in Java. It allows you to find the index of the last occurrence of a specified element in the ArrayList. If the element is not present, the method returns -1.

lastIndexOf Method Syntax

The syntax for the lastIndexOf method is as follows:

public int lastIndexOf(Object o)

• o: The element whose last occurrence index is to be found.

The method returns the index of the last occurrence of the specified element, or -1 if the element is not present in the ArrayList.

Examples

Finding the Last Index of an Element

The lastIndexOf method can be used to find the index of the last occurrence of a specific element in the ArrayList.

Example

import java.util.ArrayList;

public class LastIndexOfExample {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Orange");
        list.add("Banana");

        // Find the last index of an element
        int lastIndex = list.lastIndexOf("Banana");

        System.out.println("Last index of 'Banana': " + lastIndex);
    }
}

Output:

Last index of 'Banana': 3

Handling Elements Not Present in the ArrayList

If the specified element is not present in the ArrayList, the lastIndexOf method returns -1.

Example

import java.util.ArrayList;

public class LastIndexOfNotFoundExample {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Orange");

        // Try to find the last index of an element not in the list
        int lastIndex = list.lastIndexOf("Grapes");

        System.out.println("Last index of 'Grapes': " + lastIndex);
    }
}

Output:

Last index of 'Grapes': -1

Handling Null Values

The lastIndexOf method can handle null values. If null is present in the ArrayList, the method returns the index of the last occurrence of null.

Example

import java.util.ArrayList;

public class LastIndexOfNullExample {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add(null);
        list.add("Orange");
        list.add(null);

        // Find the last index of null
        int lastIndex = list.lastIndexOf(null);

        System.out.println("Last index of null: " + lastIndex);
    }
}

Output:

Last index of null: 3

Conclusion

The ArrayList.lastIndexOf(Object o) method in Java is a simple and effective way to find the index of the last occurrence of a specified element in an ArrayList. By understanding how to use this method, you can efficiently search for elements and handle cases where the element is not present or is null in your Java applications. The lastIndexOf method provides a reliable solution for these tasks.