Java Interview Question: Count Frequency of Characters in a String (HashMap & Streams Explained)

🔹 Introduction

Counting character frequency is a very common Java interview question.
It tests your understanding of strings, HashMap, and Java Streams.

👉 In this article, we will cover:

• HashMap approach (most asked in interviews)
• Java Streams approach (modern & advanced)

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🔹 Problem Statement

Given a string, count the occurrence of each character.

Example:

Input: aabbcc  
Output: a=2, b=2, c=2  

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🔹 Approach 1: Using HashMap (Most Important)

💡 Explanation

• Traverse the string
• Store character count in HashMap
• Use getOrDefault() for clean code

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👨‍💻 Java Code (HashMap)

import java.util.*;

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

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

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

        map.forEach((key, value) -> 
            System.out.println(key + " = " + value)
        );
    }
}

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🔹 Approach 2: Using Java Streams (Advanced)

💡 Explanation

• Convert string into stream of characters
• Group characters using groupingBy()
• Count occurrences using counting()

👉 This is a functional programming approach

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👨‍💻 Java Code (Streams)

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

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

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

        frequencyMap.forEach((key, value) ->
                System.out.println(key + " = " + value));
    }
}

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🔍 Internal Working (Important for Interviews)

• chars() → converts string to IntStream
• mapToObj() → converts int → Character
• groupingBy() → groups same characters
• counting() → counts frequency

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🔹 Output

a = 2  
b = 2  
c = 2  

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🔹 Time Complexity

• O(n) for both approaches

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🔹 HashMap vs Streams

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🔹 Key Takeaways

✔ HashMap is must-know
✔ Streams show advanced skills
✔ groupingBy() is very important

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🔹 Conclusion

This problem is a foundation for many string-based interview questions.
Master both approaches to stand out in interviews.

🔗 Also Read

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