ArrayList lastIndexOf() method - Syntax and Examples

Learn how to use the Java ArrayList's `lastIndexOf()` method with clear syntax, parameters, return value explanation, and practical examples.

This reference explains ArrayList lastIndexOf() method - Syntax and Examples through its syntax, examples and boundaries. Use the linked course lessons when the surrounding Java concepts are unfamiliar.

Use this reference
Identify the documented behavior, read a complete example and check its inputs before applying it.

Introduction

The lastIndexOf() method in Java's ArrayList lets you find the *last* occurrence of a specific object within the list. Think of it like searching for something and knowing you want to find the most recent time it appeared, not just any time.

Syntax

public int lastIndexOf(Object o)

Parameters

Parameter	Description
`o`	The object to search for within the ArrayList. Can be `null`.

Return Value

The method returns the index of the last occurrence of the specified element in this list, or -1 if it is not found.

Examples

Example 1: Finding a String

This example demonstrates how to find the last occurrence of a specific string within an ArrayList.

import java.util.ArrayList;

public class LastIndexOfExample {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>(
                Arrays.asList("Alice", "Bob", "Charlie", "Alice", "Ram")
        );

        int lastIndex = names.lastIndexOf("Alice");

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

Last index of 'Alice': 3

Explanation: The ArrayList `names` contains the string “Alice” multiple times. The lastIndexOf() method returns 3, which is the index of the last occurrence of "Alice" in the list.

Example 2: Finding a Null Value

This example shows how to use lastIndexOf() when searching for a null value.

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

public class LastIndexOfNullExample {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>(
                Arrays.asList("Alice", null, "Charlie", null, "Ram")
        );

        int lastIndex = names.lastIndexOf(null);

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

Last index of null: 3

Explanation: The ArrayList `names` contains two null values. The lastIndexOf(null) method returns 3, which is the index of the last occurrence of null in the list.

Example 3: Object Not Found

This example demonstrates what happens when you try to find an object that isn't present in the ArrayList.

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

public class LastIndexOfNotFoundExample {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>(
                Arrays.asList("Alice", "Bob", "Charlie")
        );

        int lastIndex = names.lastIndexOf("Eve");

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

Last index of 'Eve': -1

Explanation: The ArrayList `names` does not contain the string "Eve". Therefore, lastIndexOf("Eve") returns -1 to indicate that the object was not found.

Using this reference

Check the example’s types, inputs and documented behavior before copying it into an application. Some APIs require external libraries, files or platform services; consult their setup notes.

Supporting documentation: Official Java learning guide.

Reference checks are optional and do not count toward course progress.

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ArrayList lastIndexOf() method - Syntax and Examples