Java Program to Find Duplicate Characters in a String

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Introduction

Finding duplicate characters in a string is a common task in programming, especially in text processing and data validation. This guide will show you how to create a Java program that identifies and displays duplicate characters in a given string.

Problem Statement

Create a Java program that:

• Takes a string as input.
• Finds and displays all duplicate characters in the string.

Example 1:

• Input: "programming"
• Output: Duplicate characters: r, g, m

Example 2:

• Input: "hello"
• Output: Duplicate characters: l

Solution Steps

1. Prompt for Input: Use the Scanner class to read a string input from the user.
2. Use a HashMap to Track Character Frequencies: Iterate through the string and store the frequency of each character in a HashMap.
3. Identify Duplicate Characters: Traverse the HashMap to identify characters with a frequency greater than 1.
4. Display the Duplicate Characters: Print the duplicate characters found in the string.

Java Program

Java Program to Find Duplicate Characters in a String

import java.util.HashMap; import java.util.Map; import java.util.Scanner; /** * Java Program to Find Duplicate Characters in a String * Author: https://www.javaguides.net/ */ public class DuplicateCharactersInString { 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 to find duplicate characters: "); String input = scanner.nextLine(); // Step 2: Use a HashMap to track character frequencies Map<Character, Integer> charCountMap = new HashMap<>(); for (char c : input.toCharArray()) { charCountMap.put(c, charCountMap.getOrDefault(c, 0) + 1); } // Step 3: Identify and display duplicate characters System.out.print("Duplicate characters: "); boolean hasDuplicates = false; for (Map.Entry<Character, Integer> entry : charCountMap.entrySet()) { if (entry.getValue() > 1) { System.out.print(entry.getKey() + " "); hasDuplicates = true; } } if (!hasDuplicates) { System.out.print("No duplicates found."); } System.out.println(); } } 

Explanation

• Input: The program prompts the user to enter a string.
• Tracking Character Frequencies: The program uses a HashMap to count the occurrences of each character in the string. The getOrDefault method is used to simplify the process of incrementing the count.
• Finding Duplicates: The program then iterates over the HashMap to identify characters with a frequency greater than 1, indicating that they are duplicates.
• Output: The program prints the duplicate characters found in the string. If no duplicates are found, it prints a message indicating this.

Output Example

Example 1:

Enter a string to find duplicate characters: programming Duplicate characters: r g m 

Example 2:

Enter a string to find duplicate characters: hello Duplicate characters: l 

Example 3:

Enter a string to find duplicate characters: java Duplicate characters: a 

Example 4:

Enter a string to find duplicate characters: abcdefg Duplicate characters: No duplicates found. 

Conclusion

This Java program effectively identifies duplicate characters in a string using a HashMap to track character frequencies. By counting the occurrences of each character and then filtering out those with a frequency greater than 1, the program efficiently finds and displays any duplicates. This approach is both simple and efficient, making it suitable for a wide range of text-processing tasks.