In this quick post, we will write a Java Program to Count Duplicate Characters in a String.

We will discuss two solutions to count duplicate characters in a String:

1. HashMap based solution
2. Java 8, functional-style solution

Java Program to Count Duplicate Characters in a String

package net.javaguides.corejava.programs;

import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;

public class CountDuplicateCharacters {
 private static final String TEXT = "Be strong, be fearless, be beautiful. "
            + "And believe that anything is possible when you have the right "
            + "people there to support you.";
       
    public static void main(String[] args) {
        
        System.out.println("Input text: \n" + TEXT + "\n");
        
        System.out.println("HashMap based solution:");
        long startTimeV1 = System.nanoTime();
        
        Map<Character, Integer> duplicatesV1 = countDuplicateCharactersV1(TEXT);
        
        displayExecutionTime(System.nanoTime()-startTimeV1);
        System.out.println(Arrays.toString(duplicatesV1.entrySet().toArray()));
        // or: duplicatesV1.forEach( (k, v) -> System.out.print(k + "="+ v + ", "));
        
        System.out.println();        
        System.out.println("Java 8, functional-style solution:");
        long startTimeV2 = System.nanoTime();
        
        Map<Character, Long> duplicatesV2 = countDuplicateCharactersV2(TEXT);
        
        displayExecutionTime(System.nanoTime()-startTimeV2);
        System.out.println(Arrays.toString(duplicatesV2.entrySet().toArray()));
        // or: duplicatesV2.forEach( (k, v) -> System.out.print(k + "="+ v + ", "));
    }
    
    private static void displayExecutionTime(long time) {
        System.out.println("Execution time: " + time + " ns" + " (" +
                TimeUnit.MILLISECONDS.convert(time, TimeUnit.NANOSECONDS) + " ms)");
    }
    
 // Note: For Unicode supplementary characters use codePointAt() instead of charAt()
    //       and codePoints() instead of chars()

    public static Map<Character, Integer> countDuplicateCharactersV1(String str) {

        if (str == null) {
            // or throw IllegalArgumentException
            return Collections.EMPTY_MAP;
        }

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

        for (int i = 0; i < str.length(); i++) {
            
            char ch = str.charAt(i);

            Integer count = result.get(ch);
            if (count != null) {
                result.put(ch, ++count);
            } else {
                result.put(ch, 1);
            }
        }

        return result;
    }

    public static Map<Character, Long> countDuplicateCharactersV2(String str) {
        
        if (str == null) {
            // or throw IllegalArgumentException
            return Collections.EMPTY_MAP;
        }
     
        Map<Character, Long> result = str.chars()                
                .mapToObj(c -> (char) c)
                .collect(Collectors.groupingBy(c -> c, Collectors.counting()));

        return result;
    }
}

Output

Input text: 
Java is a popular  general-purpose programming language and computing platform. It is fast, reliable, and secure.

HashMap based solution:
[ =16, a=13, b=1, c=2, d=2, e=8, f=2, g=6, i=5, I=1, J=1, l=6, ,=2, -=1, m=4, n=6, .=2, o=5, p=7, r=8, s=5, t=4, u=5, v=1]

Java 8, functional-style solution:
[ =16, a=13, b=1, c=2, d=2, e=8, f=2, g=6, I=1, i=5, J=1, ,=2, l=6, m=4, -=1, .=2, n=6, o=5, p=7, r=8, s=5, t=4, u=5, v=1]

Related Java String Programs with Output

1. Java Program to Find the First Non-repeated Character in a String
2. Java Program to Check Palindrome String
3. Java Program to Find Duplicate Characters in a String
4. Java Program to Find Duplicate Words in a String
5. Java Program to Find All the Permutations of a String
6. Java Program to Count Occurrences of Words in a String
7. Java Program to Count the Occurrences of Each Character
8. Java Program to Count Vowels and Consonants in a String
9. Java program to Count the Number of Duplicate Words in a String
10. Java Program to Count Number of Words in Given String
11. Java Program to Count the Number of Occurrences of Substring in a String
12. Java Program to Count the Occurrences of Each Character in String
13. Java Program to Merge Two String Arrays
14. Java Program to Remove Duplicate Words from String
15. Java Program to Reverse a String(5 ways)
16. Java Program to Reverse Each Word of a String
17. Java Program to Swap Two Strings
18. How to Check if the String Contains Only Digits
19. How to Check if the String Contains Only Letters
20. How to Check If the String Contains Only Letters or Digits
21. Java Program to Swap Two Strings Without Using Third Variable