Java Program to Count the Number of Repeated Characters in a String

This article is created to cover a program in Java that counts and prints the number of repeated or duplicate characters available in a given string. For example, if the given string is "Java Programming", then the output will be 4. Because the characters a, r, g, and m are available more than once.

Count the Number of Repeated Characters in a String: Basic Version

The question is: write a Java program to count the number of repeated characters in a string. The string must be received by the user at the runtime of the program. Here is its sample run:

import java.util.Scanner;

public class CodesCracker
{
   public static void main(String[] args)
   {
      String str;
      char ch;
      int strLen, i, count, j, k, repChars=0;
      Scanner s = new Scanner(System.in);
      
      System.out.print("Enter the String: ");
      str = s.nextLine();
      
      strLen = str.length();
      char[] arr = new char[strLen];
      
      for(i=0; i<strLen; i++)
         arr[i] = str.charAt(i);
      
      for(i=0; i<strLen; i++)
      {
         ch = arr[i];
         count = 0;
         for(j=(i+1); j<strLen; j++)
         {
            if(ch==arr[j])
            {
               count++;
               for(k=j; k<(strLen-1); k++)
                  arr[k] = arr[k+1];
               strLen--;
               j--;
            }
         }
         if(count>0)
            repChars++;
      }
      
      System.out.println("\nTotal Number of Repeated Characters = " +repChars);
   }
}

The snapshot given below shows the sample run of the above program with user input codescracker as a string to count and print the total number of duplicate characters available in it:

java count repeated characters in string

That is, the characters c, e, and r are the three characters available a repeated number of times. Therefore, the output was 3.

Count the Number of Repeated Characters in a String: Complete Version

The problem with the above program is that if the user enters a string containing white spaces, then white space duplication also gets counted. Therefore, let's modify the above program. Some other modifications are also done in the program given below:

import java.util.Scanner;

public class CodesCracker
{
   public static void main(String[] args)
   {
      int i, count, j, k, repChars=0;
      Scanner s = new Scanner(System.in);
      
      System.out.print("Enter the String: ");
      String str = s.nextLine();
      
      str = str.replaceAll("\\s+", "");
      str = str.toLowerCase();
      int strLen = str.length();
      char[] arr = new char[strLen];
      
      for(i=0; i<strLen; i++)
         arr[i] = str.charAt(i);
      
      for(i=0; i<strLen; i++)
      {
         count = 0;
         for(j=(i+1); j<strLen; j++)
         {
            if(arr[i]==arr[j])
            {
               count++;
               for(k=j; k<(strLen-1); k++)
                  arr[k] = arr[k+1];
               strLen--;
               j--;
            }
         }
         if(count>0)
            repChars++;
      }
      
      if(repChars==0)
         System.out.println("\nNo repeated character found.");
      else if(repChars==1)
         System.out.println("\nThere is only one repeated character found.");
      else
         System.out.println("\nThere are " +repChars+ " characters found in the String.");
   }
}

Here is its sample run with user input "Java" as a string:

count repeated characters in Java

Here is another sample run with user input, "codescracker dot com"

java program count number of repeatec characters in string

In the above program, the statement:

str = str.replaceAll("\\s+", "");

is used to remove all white spaces (single space, multiple space (tabs)). And the following statement:

is used to convert the whole string to lowercase. Because the same character in uppercase and lowercase gets treated as a different character.

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