Java Interview Question: Find Duplicate Characters in a String (3 Methods Explained)

 

🔹 Introduction

Finding duplicate characters in a string is a frequently asked Java interview question.
It helps test your understanding of strings, HashMap, and Java Streams.

👉 In this article, we will explore 3 different approaches to solve this problem.

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🔹 Problem Statement

Given a string, find all duplicate characters.

Example:

Input: programming  
Output: r, g, m  

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🔹 Approach 1: Using HashMap (Most Asked)

💡 Explanation

• Count frequency using HashMap
• Print characters with frequency > 1

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👨‍💻 Java Code

import java.util.*;

public class DuplicateCharacters {
    public static void main(String[] args) {
        String str = "programming";

        Map<Character, Integer> map = new HashMap<>();

        for (char ch : str.toCharArray()) {
            map.put(ch, map.getOrDefault(ch, 0) + 1);
        }

        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            if (entry.getValue() > 1) {
                System.out.println(entry.getKey());
            }
        }
    }
}

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🔹 Approach 2: Using Set

💡 Explanation

• Use one set to track seen characters
• Another set to store duplicates

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👨‍💻 Java Code

import java.util.*;

public class DuplicateCharacters {
    public static void main(String[] args) {
        String str = "programming";

        Set<Character> seen = new HashSet<>();
        Set<Character> duplicates = new HashSet<>();

        for (char ch : str.toCharArray()) {
            if (!seen.add(ch)) {
                duplicates.add(ch);
            }
        }

        duplicates.forEach(System.out::println);
    }
}

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🔹 Approach 3: Using Java Streams (Advanced)

💡 Explanation

• Convert string into stream
• Use groupingBy + counting
• Filter duplicates

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👨‍💻 Java Code

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

public class DuplicateCharacters {
    public static void main(String[] args) {
        String str = "programming";

        Map<Character, Long> map =
                str.chars()
                   .mapToObj(c -> (char) c)
                   .collect(Collectors.groupingBy(
                       Function.identity(),
                       Collectors.counting()
                   ));

        map.entrySet().stream()
           .filter(entry -> entry.getValue() > 1)
           .forEach(entry -> System.out.println(entry.getKey()));
    }
}

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🔹 Output

r  
g  
m  

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🔹 Time Complexity

• O(n) for all approaches

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🔹 Key Takeaways

✔ HashMap is most important for interviews
✔ Set approach is clean and efficient
✔ Streams show advanced knowledge

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🔹 Conclusion

This is a must-know Java interview problem.
Understanding multiple approaches helps you stand out in interviews.

🔗 Also Read

👉 First Non-Repeating Character in Java
👉 Check Palindrome String in Java
👉 Reverse a String
👉 Count Character Frequency in Java