Java ArrayList lastIndexOf() Method

The lastIndexOf() method is a valuable operation available in the Java ArrayList class. It allows you to find the last occurrence index of a specified element in the list. This is especially useful when dealing with lists containing duplicate elements and you need to locate the most recent position where a value appears.

What Is the lastIndexOf() Method in ArrayList?

• The lastIndexOf() method returns the index of the last occurrence of a specified element in the list.
• If the element is not present, it returns -1.
• The search checks elements from the end of the list to the start, using the equals() method for comparison.
• It supports all types: primitives (as wrappers), strings, objects, and also null values.

Syntax

public int lastIndexOf(Object o)

Where:

• o is the element for which to find the last occurrence.

Parameters

Parameter	Description
o	The element whose last index you want to find

• The method accepts null and will return the last index of null if present.

Return Value

• Returns the index of the last occurrence of the specified element.
• Returns -1 if the element is not found in the list.

Exceptions

• The method does not throw any checked exceptions.
• No error for invalid input; simply returns -1 if not found.

How Does lastIndexOf() Work Internally?

• lastIndexOf() loops through the ArrayList in reverse order (from the last item to the first).
• Each element is compared with the argument using equals().
• The index of the first matching occurrence found (going backward) is returned.
• If no match is found, -1 is returned.
• Time complexity is O(n), where n is the size of the list.

Examples of the lastIndexOf() Method

1. Find the Last Occurrence of a String

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> animals = new ArrayList<>();
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Dog");
        animals.add("Rabbit");
        animals.add("Dog");

        int lastDog = animals.lastIndexOf("Dog");
        System.out.println("Last 'Dog' is at index: " + lastDog); // Output: 4
    }
}

Output:

Last 'Dog' is at index: 4

2. Element Not Present in the List

import java.util.ArrayList;

public class Demo {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);

        int lastIndex = numbers.lastIndexOf(40); // Not in list
        System.out.println("Index of 40: " + lastIndex); // Output: -1
    }
}

3. Using lastIndexOf() with Custom Objects

import java.util.ArrayList;

class Book {
    String title;
    Book(String title) {
        this.title = title;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (!(obj instanceof Book)) return false;
        Book other = (Book) obj;
        return this.title.equals(other.title);
    }
}

public class Example {
    public static void main(String[] args) {
        ArrayList<Book> books = new ArrayList<>();
        books.add(new Book("Java"));
        books.add(new Book("Python"));
        books.add(new Book("Java"));

        Book search = new Book("Java");
        int index = books.lastIndexOf(search);
        System.out.println("Last 'Java' book index: " + index); // Output: 2
    }
}

4. Find Last Occurrence of null

import java.util.ArrayList;

public class NullTest {
    public static void main(String[] args) {
        ArrayList<String> items = new ArrayList<>();
        items.add("Pen");
        items.add(null);
        items.add("Book");
        items.add(null);

        System.out.println(items.lastIndexOf(null)); // Output: 3
    }
}

Important Notes

• Last Occurrence: If the element appears multiple times, lastIndexOf() returns only the last occurrence’s index.
• Not Found: Returns -1 if the element does not exist in the list.
• Custom Objects: For correct results, custom classes should override the equals() method.
• null Support: Handles searching for null values as well.
• Difference with indexOf(): indexOf() searches from the beginning, lastIndexOf() from the end.

Summary

Aspect	Details
Method Purpose	Find the index of last occurrence of an element
Syntax	list.lastIndexOf(Object o)
Return Value	Index if found, -1 if not found
Supports null?	Yes
Throws Exception	None (for standard use)
Case Sensitivity	Yes (e.g., "Dog" ≠ "dog")
Time Complexity	O(n) (linear search, from the end)
Typical Use Cases	Locating last duplicate, reverse searches

Typical Use Cases

• Finding the position of the last duplicate in a list.
• Determining the last input or action of a specific value in a sequence.
• Reverse searches in ordered data or history logs.