Java 8 – Reverse a String

Introduction

Reversing a string is a common programming task. Java 8 introduced the Stream API, which makes this task straightforward and efficient. In this guide, we will learn how to reverse a string using Java 8 Streams.

Problem Statement

Write a Java program that:

• Takes a string as input.
• Reverses the string using Java 8 Streams.
• Displays the reversed string.

Example:

• Input: "Java"
• Output: "avaJ"

Solution Steps

1. Input String: Define the input string that needs to be reversed.
2. Convert String to Stream: Convert the string into a stream of characters.
3. Reverse the Stream: Reverse the stream of characters.
4. Collect Reversed Characters: Collect the reversed characters and form the reversed string.
5. Display the Result: Print the reversed string.

Java Program

import java.util.stream.Collectors;

public class ReverseString {
    public static void main(String[] args) {
        // Step 1: Define the input string
        String input = "Java";

        // Step 2: Reverse the string using Streams
        String reversed = input.chars()                        // Convert the string to an IntStream of character codes
                .mapToObj(c -> (char) c)                       // Convert IntStream to Stream<Character>
                .collect(Collectors.collectingAndThen(         // Collect the characters
                        Collectors.toList(),                   // Collect them into a List
                        list -> {                              // Then reverse the list
                            java.util.Collections.reverse(list);
                            return list.stream();
                        }
                ))                                             // Join the reversed list into a new string
                .map(String::valueOf)
                .collect(Collectors.joining());

        // Step 3: Display the result
        System.out.println("Reversed String: " + reversed);
    }
}

Output

Reversed String: avaJ

Explanation

Step 1: Define the Input String

We start by defining the string:

String input = "Java";

This is the string we want to reverse.

Step 2: Reverse the String Using Streams

To reverse the string:

1. Convert the String to a Stream of Characters: input.chars() converts the string into a stream of character codes (as an IntStream), and mapToObj(c -> (char) c) converts those character codes into a stream of Character objects.
2. Collect into a List: The stream of characters is collected into a list using Collectors.toList().
3. Reverse the List: We reverse the list using java.util.Collections.reverse().
4. Join the Characters Back into a String: The reversed characters are streamed back and joined into a string using Collectors.joining().

Step 3: Display the Result

The reversed string is printed using:

System.out.println("Reversed String: " + reversed);

Output Example

Example 1:

For the input "Java", the output is:

Reversed String: avaJ

Example 2:

For the input "Hello", the output is:

Reversed String: olleH

Conclusion

This program demonstrates how to reverse a string using Java 8 Streams. By converting the string into a stream of characters, reversing the stream, and then joining the characters back together, we efficiently reverse the string. This approach leverages modern Java features to solve a classic programming problem.