Java Program to Remove Characters from the Input String which are Present in the Mask String

This is the Java Program to Remove Characters that belong to a Mask String from the Input String

Problem Description

Given a string of characters and a mask string remove all the characters of the mask string from the first string.

String x = “We belong to Russia”
Mask = “Wbos”

Output = “e elng t Ruia”

Problem Solution

Count the frequency of each character in the mask string in an array. Now, iterate through the user string and check each characters frequency in the frequency array. If it is zero, add that character to the output string.

Program/Source Code

Here is the source code of the Java Program to Remove Characters that belong to a Mask String from the Input String. The program is successfully compiled and tested using IDE IntelliJ Idea in Windows 7. The program output is also shown below.

  2. // Java Program to Remove Characters that belong
  3. // to a Mask String from the Input String
  5. import;
  6. import;
  7. import;
  9. public class RemoveMaskChar {
  10.     // Function to remove the mask characters
  11.     static String removeMaskCharacters(String str, String str2){
  12.         int[] array = new int[256];
  13.         int i;
  14.         for(i=0; i<str2.length(); i++){
  15.             array[str2.charAt(i)]++;
  16.         }
  17.         String output="";
  18.         for(i=0; i<str.length(); i++){
  19.             if(array[str.charAt(i)] == 0)
  20.                 output+=str.charAt(i);
  21.         }
  22.         return output;
  23.     }
  24.     // Function to read input and display the output
  25.     public static void main(String[] args) {
  26.         BufferedReader br = new BufferedReader(new InputStreamReader(;
  27.         String str;
  28.         System.out.println("Enter the string");
  29.         try {
  30.             str = br.readLine();
  31.         } catch (IOException e) {
  32.             System.out.println("An I/O error occurred");
  33.             return;
  34.         }
  35.         String str2;
  36.         System.out.println("Enter the second string");
  37.         try {
  38.             str2 = br.readLine();
  39.         } catch (IOException e) {
  40.             System.out.println("An I/O error occurred");
  41.             return;
  42.         }
  43.         String output = removeMaskCharacters(str,str2);
  44.         System.out.println("Final String is " + output);
  45.     }
  46. }
Program Explanation

1. In function removeMaskCharacters(), an array is created to count the frequency of each character in the mask string.
2. The loop for(i=0; i<str2.length(); i++) is used to count the above mentioned frequencies.
3. The loop for(i=0; i<str.length(); i++) is used to iterate through the original string.
4. The condition if(array[str.charAt(i)] == 0) checks if the frequency of the current character in the array is equal to zero.
5. If it is zero, then the element is added to the output string.

Time Complexity: O(n) where n is the length of the string.

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Runtime Test Cases
Case 1 (Simple Test Case):
Enter the string
We belong to Russia
Enter the second string
Final String is e elng t Ruia
Case 2 (Simple Test Case - another example):
Enter the string
Enter the second string
Final String is

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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