How to Find Duplicate Characters on String - Java Programming Problems

Today's programming exercise is to write a program to find repeated characters in a String. For example, if given input to your program is "Java", it should print all duplicates characters, i.e. characters appear more than once in String and their count e.g. a = 2 because character 'a' has appeared twice in String "Java". This is also a very popular coding question on the various level of Java interviews and written test, where you need to write code. On difficulty level, this question is at par with prime numbers or Fibonacci series. I personally like this exercise because it gives beginners an opportunity to familiar with the concept of Map data structure, which allows you store mappings in the form of key and value. Since Map is heavily used in any enterprise Java application, good knowledge of this data structure is highly desirable among any level of Java programmers.


By the way, there are a couple of variants of this problem, which you may want to look before going for an interview. Sometimes an interviewer will ask you to read a file and print all duplicate characters and their count, core logic will remain same, all you need to do is demonstrate how much you know about File IO in Java e.g. streaming file if it's very large rather than reading the whole file in memory.

Java Program to find Repeated Characters of String

The standard way to solve this problem is to get the character array from String, iterate through that and build a Map with character and their count. Then iterate through that Map and print characters which have appeared more than once. So you actually need two loops to do the job, the first loop to build the map and second loop to print characters and counts.

If you look at below example, there is only one static method called printDuplicateCharacters(), which does both this job. We first got the character array from String by calling toCharArray().

Next we are using HashMap to store characters and their count. We use containsKey() method to check if key, which is a character already exists or not, if already exists we get the old count from HashMap by calling get() method and store it back after incrementing it by 1.

Once we build our Map with each character and count, next task is to loop through Map and check each entry, if count, which is the value of Entry is greater than 1, then that character has occurred more than once. You can now print duplicate characters or do whatever you want with them.

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.Set;

/**
* Java Program to find duplicate characters in String.
*
*
* @author http://java67.blogspot.com
*/
public class FindDuplicateCharacters{

    public static void main(String args[]) {
        printDuplicateCharacters("Programming");
        printDuplicateCharacters("Combination");
        printDuplicateCharacters("Java");
    }

    /*
     * Find all duplicate characters in a String and print each of them.
     */
    public static void printDuplicateCharacters(String word) {
        char[] characters = word.toCharArray();

        // build HashMap with character and number of times they appear in String
        Map<Character, Integer> charMap = new HashMap<Character, Integer>();
        for (Character ch : characters) {
            if (charMap.containsKey(ch)) {
                charMap.put(ch, charMap.get(ch) + 1);
            } else {
                charMap.put(ch, 1);
            }
        }

        // Iterate through HashMap to print all duplicate characters of String
        Set<Map.Entry<Character, Integer>> entrySet = charMap.entrySet();
        System.out.printf("List of duplicate characters in String '%s' %n", word);
        for (Map.Entry<Character, Integer> entry : entrySet) {
            if (entry.getValue() > 1) {
                System.out.printf("%s : %d %n", entry.getKey(), entry.getValue());
            }
        }
    }

}

Output
List of duplicate characters in String 'Programming'
g : 2
r : 2
m : 2
List of duplicate characters in String 'Combination'
n : 2
o : 2
i : 2
List of duplicate characters in String 'Java'

That's all on how to find duplicate characters in a String. Next time if this question is asked to you in a programming job interview, you can confidently write a solution and can explain them. Remember this question is also asked as write a Java program to find repeated characters of a given String, so don't get confused yourself in wording, the algorithm will remain same.