Java 8 Program To Find First Repeated Character in a String

Introduction

Finding the first repeated character in a string is a common problem in text processing. With Java 8, this task can be efficiently handled using streams. This guide will show you how to create a Java program that identifies the first repeated character in a given string.

Problem Statement

Create a Java program that:

• Takes a string as input.
• Finds and returns the first repeated character in the string.
• If no character is repeated, returns an appropriate message.

Example 1:

• Input: "programming"
• Output: The first repeated character is 'r'

Example 2:

• Input: "abcdef"
• Output: No repeated characters found.

Solution Steps

1. Prompt for Input: Use the Scanner class to read a string input from the user.
2. Use Java 8 Streams to Find Repeated Character:
   • Convert the string to a stream of characters.
   • Use a Set to track characters that have been seen.
   • Find the first character that has already been seen.
3. Display the Result: Print the first repeated character or an appropriate message if no character is repeated.

Java Program

Java 8 Program to Find the First Repeated Character in a String

import java.util.HashSet;
import java.util.Optional;
import java.util.Scanner;
import java.util.Set;
import java.util.stream.Collectors;

/**
 * Java 8 Program to Find the First Repeated Character in a String
 * Author: https://www.javaguides.net/
 */
public class FirstRepeatedCharacter {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Step 1: Prompt the user for input
        System.out.print("Enter a string: ");
        String input = scanner.nextLine();

        // Step 2: Find the first repeated character using Java 8 streams
        Optional<Character> firstRepeated = findFirstRepeatedCharacter(input);

        // Step 3: Display the result
        if (firstRepeated.isPresent()) {
            System.out.println("The first repeated character is '" + firstRepeated.get() + "'");
        } else {
            System.out.println("No repeated characters found.");
        }
    }

    // Method to find the first repeated character in a string
    public static Optional<Character> findFirstRepeatedCharacter(String input) {
        Set<Character> seenCharacters = new HashSet<>();

        return input.chars()  // Convert the string to an IntStream of character codes
                .mapToObj(c -> (char) c)  // Convert character codes to characters
                .filter(c -> !seenCharacters.add(c))  // Filter characters that are already in the set
                .findFirst();  // Return the first repeated character if found
    }
}

Explanation

• Input: The program prompts the user to enter a string.

• Finding the First Repeated Character Using Streams:

  • The chars() method converts the string into an IntStream of character codes.
  • mapToObj(c -> (char) c) converts these character codes back to characters.
  • The filter() method is used to check if the character has already been seen by trying to add it to a Set. If the character is already in the Set, add() will return false, making it a repeated character.
  • findFirst() returns the first repeated character if it exists, wrapped in an Optional.

• Output: The program prints the first repeated character or a message indicating that no repeated characters were found.

Output Example

Example 1:

Enter a string: programming
The first repeated character is 'r'

Example 2:

Enter a string: abcdef
No repeated characters found.

Example 3:

Enter a string: javaprogram
The first repeated character is 'a'

Explanation of Edge Cases:

• Example 1: The string contains repeated characters, and the first repeated character is 'r'.
• Example 2: The string contains all unique characters, so no repeated characters are found.
• Example 3: The string contains multiple repeated characters, and the first repeated character is 'a'.

Conclusion

This Java 8 program efficiently finds the first repeated character in a string using streams and a Set to track seen characters. The program handles both cases where a repeated character exists and where all characters are unique. The use of streams makes the solution concise and leverages Java 8's functional programming capabilities.