Java Program to Find Duplicate Characters in a String

Sometimes, you gotta find duplicate characters in a string, like 'programming'. In this guide, we're gonna check out a Java program that does just that. It's super useful when you're dealing with text data.

Prerequisites for the Java Program to Find Duplicate Characters

Here's what you need for this program:

• Understanding of Java collections like HashMap.
• Basic knowledge of loops and conditionals in Java.
• Java Development Kit (JDK) 8 or newer.
• Any IDE like IntelliJ IDEA or Eclipse.
• The EduSeekho online compiler.

Source Code of Java Program to Find Duplicate Characters in a String

Let's take a look at the complete source code.

import java.util.HashMap;// Import HashMap

public class FindDuplicates {
    public static void main(String[] args) {
        java.util.Scanner scanner = new java.util.Scanner(System.in);
        System.out.print("Enter a string: ");
        String str = scanner.nextLine();

        HashMap<Character, Integer> charCountMap = new HashMap<>();

        char[] chars = str.toCharArray();

        for (char c : chars) {
            if (charCountMap.containsKey(c)) {
                charCountMap.put(c, charCountMap.get(c) + 1);
            } else {
                charCountMap.put(c, 1);
            }
        }

        System.out.println("Duplicate characters:");

        for (Map.Entry entry : charCountMap.entrySet()) {
            if (entry.getValue() > 1) {
                System.out.print(entry.getKey() + ", ");
            }
        }
    }
}

Output of the Find Duplicate Characters Program

Here's what happens when you run the program and input 'programming'.

Enter a string: programming
Duplicate characters: r, g, m

Algorithm to Find Duplicate Characters in Java

Identifying duplicate characters involves checking each character's occurrence. Here's how the program works, step by step.

1. Read the input string from the user.
2. Convert the string to a character array.
3. Use a HashMap to store characters and their counts.
4. Iterate over the character array, updating counts in the map.
5. Print characters that appear more than once.

flowchart TD
  A([Start]) --> B[/Read string/]
  B --> C{More characters?}
  C -->|Yes| D[Check character count]
  D --> E{Is duplicate?}
  E -->|Yes| F[Add to duplicates list]
  E -->|No| G[Increment character count]
  F --> G
  G --> C
  C -->|No| H([End])

How the Java Program to Find Duplicate Characters Works

Let's walk through the code to understand how it identifies duplicates.

• import java.util.HashMap: This import allows us to use the HashMap class for counting characters.
• Scanner is initialized to read input from the user.
• We convert the input string into a character array to process it easily.
• For each character, the code checks if it already exists in the HashMap. If it does, the count is incremented.
• Characters with counts greater than 1 are considered duplicates and are printed.

Time and Space Complexity of the Java Program to Find Duplicate Characters

Understanding the complexity helps gauge the program's efficiency.

Time complexity: O(n), where n is the length of the string — 11 for 'programming'. The for loop iterates over the character array once to update counts in the HashMap.

Space complexity: O(n), because the HashMap can store up to all unique characters of the input string, like 'p', 'r', 'o', 'g', 'a', 'm', 'i', 'n', 'g'.

Key takeaway: Using a HashMap is a smart way to count occurrences of elements in Java.

FAQs About Java Program to Find Duplicate Characters

Here are some common queries students have.

Related Programs

If you found this program helpful, check out these related programs:

• Java Program to Print Fibonacci Series with Example Output
• Java Program to Find Prime Number | Full Code and Output

Conclusion

HashMaps are awesome for counting occurrences in Java. They make finding duplicates easy and quick. If you're feeling adventurous, try modifying the program to find duplicate words instead of characters. It’s a nice challenge that deepens your understanding of strings!

Practice finding duplicates with more Java programs in our Java quiz. Test and run them in the online compiler.