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Java program to print all duplicate characters in a string

Last Updated : 15 Jul, 2025
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Given a string, the task is to write Java program to print all the duplicate characters with their frequency Example: 

Input: str = "geeksforgeeks" Output: s : 2 e : 4 g : 2 k : 2 Input: str = "java" Output: a : 2

Approach: The idea is to do hashing using HashMap.

  • Create a hashMap of type {char, int}.
  • Traverse the string, check if the hashMap already contains the traversed character or not.
  • If it is present, then increment the count or else insert the character in the hashmap with frequency = 1.
  • Now traverse through the hashmap and look for the characters with frequency more than 1. Print these characters with their respective frequencies.

Below is the implementation of the above approach:

Java 
// Java program for the above approach
import java.util.*;
class GFG {

    // Function to print all duplicate
    // characters in string using HashMap
    public static void
    countDuplicateCharacters(String str)
    {

        // Creating a HashMap containing char
        // as a key and occurrences as a value
        Map<Character, Integer> map
            = new HashMap<Character, Integer>();

        // Converting given string into
        // a char array
        char[] charArray = str.toCharArray();

        // Checking each character
        // of charArray
        for (char c : charArray) {

            if (map.containsKey(c)) {

                // If character is present
                // in map incrementing it's
                // count by 1
                map.put(c, map.get(c) + 1);
            }
            else {

                // If character is not present
                // in map putting this
                // character into map with
                // 1 as it's value.
                map.put(c, 1);
            }
        }

        // Traverse the HashMap, check
        // if the count of the character
        // is greater than 1 then print
        // the character and its frequency
        for (Map.Entry<Character, Integer> entry :
             map.entrySet()) {

            if (entry.getValue() > 1) {
                System.out.println(entry.getKey()
                                   + " : "
                                   + entry.getValue());
            }
        }
    }

    // Driver Code
    public static void
    main(String args[])
    {
        // Given String str
        String str = "geeksforgeeks";

        // Function Call
        countDuplicateCharacters(str);
    }
}
Output:
s : 2
e : 4
g : 2
k : 2

Time Complexity: O(NlogN) 

Auxiliary Space: O(N) since using Map


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