Question

I can't figure out why this won't run.

//CryptographyTest.java
//package cryptography;

import java.util.Scanner;

public class CryptographyTest {
  
public static void main(String[] args) {
// create a scanner object to read from user
Scanner s = new Scanner(System.in);
// prompt user for input option
while(true) {
// loop till user say exit
System.out.println("Select Option:");
System.out.println("1. Encrypt");
System.out.println("2. Decrypt");
System.out.println("3. Exit");
String input = s.nextLine(); // get user input
int option = 0;
try {
// check for valid input
option = Integer.parseInt(input);
if(option < 1 || option > 3) {
throw new IllegalArgumentException();
}
} catch (Exception e) {
// print error message
System.out.println("Invalid Input!");
}
      
       // check for user option
if(option == 1) {
// ask user for data to encrypt
System.out.print("Enter 4 digit number to Encrypt: ");
int data = getInput(s); // get user input
// check for valid input
if(data == -1) {
// print error message
System.out.println("Invalid Input!");
}
else {
System.out.print("Encrypted number is: ");
String encrypt_number = "" + Cryptography.encrypt(data);
// add zeroes at start of number if needed
int len = encrypt_number.length();
for(int i=4;i>len;i--) {
encrypt_number = "0" + encrypt_number;
}
System.out.println(encrypt_number);
}
}
else if(option == 2) {
// ask user for data to decrypt
System.out.print("Enter 4 digit number to Decrypt: ");
int data = getInput(s); // get user input
// check for valid input
if(data == -1) {
// print error message
System.out.println("Invalid Input!");
}
else {
System.out.print("Decrypted number is: ");
String decrypt_number = "" + Cryptography.decrypt(data);
// add zeroes at start of number if needed
int len = decrypt_number.length();
for(int i=4;i>len;i--) {
decrypt_number = "0" + decrypt_number;
}
System.out.println(decrypt_number);
}
}
else if(option == 3) {
// break the loop and exit program
System.out.println("Bye!");
break;
}
}
  
}

private static int getInput(Scanner s) {
String input;
// get user input
input = s.nextLine();
// check for input length
if(input.length() != 4) {
// return as input is not valid
return -1;
}
else {
// check for integer input
try {
int data = Integer.parseInt(input);
// return the input in integer form
return data;
} catch (Exception e) {
// return as input is not integer type
return -1;
}
}
}

}

(goes with this:)

//Cryptography.java

//package cryptography;

public class Cryptography {
  
public static int encrypt(int input) {
// input is a 4 digit integer
// convert integer to string
String str = "" + input;
// add zeroes at start of number if needed
int len = str.length();
for(int i=4;i>len;i--) {
str = "0" + str;
}
// get individual digits
int first_digit = str.charAt(0) - '0'; // remove '0' to convert char to digit
int second_digit = str.charAt(1) - '0';
int third_digit = str.charAt(2) - '0';
int fourth_digit = str.charAt(3) - '0';
// add 7 to each digit and take reminder by dividing 10
first_digit = (first_digit + 7) % 10;
second_digit = (second_digit + 7) % 10;
third_digit = (third_digit + 7) % 10;
fourth_digit = (fourth_digit + 7) % 10;
// replace first digit with third and second with fourth
String encrypt_number = "" + third_digit + fourth_digit + first_digit + second_digit;
// convert string to integer and return it
return Integer.parseInt(encrypt_number);
}
  
public static int decrypt(int input) {
// input is 4 digit integer
// convert integer to string
String str = "" + input;
// add zeroes at start of number if needed
int len = str.length();
for(int i=4;i>len;i--) {
str = "0" + str;
}
// get individual digits
int first_digit = str.charAt(0) - '0'; // remove '0' to convert char to digit
int second_digit = str.charAt(1) - '0';
int third_digit = str.charAt(2) - '0';
int fourth_digit = str.charAt(3) - '0';
// add 3 to each digit and take reminder by dividing 10
first_digit = (first_digit + 3) % 10;
second_digit = (second_digit + 3) % 10;
third_digit = (third_digit + 3) % 10;
fourth_digit = (fourth_digit + 3) % 10;
// replace first digit with third and second with fourth
String encrypt_number = "" + third_digit + fourth_digit + first_digit + second_digit;
// convert string to integer and return it
return Integer.parseInt(encrypt_number);
}

}

Answer 1

Please find your solutino below the code is corrected if any doubt comment and do upvote.

CryptographyTest.java:

import java.util.Scanner;
public class CryptographyTest {
          
        private static int getInput(Scanner s) {
                String input;
                // get user input
                input = s.nextLine();
                // check for input length
                if(input.length() != 4||Integer.parseInt(input)<0) {
                // return as input is not valid
                return -1;
                }
                else {
                // check for integer input
                try {
                int data = Integer.parseInt(input);
                // return the input in integer form
                return data;
                } catch (Exception e) {
                // return as input is not integer type
                return -1;
                }
                }
                }
        public static void main(String[] args) {
                // create a scanner object to read from user
                 Scanner s = new Scanner(System.in);
                 // prompt user for input option
                 while(true) {
                         // loop till user say exit
                         System.out.println("Select Option:");
                         System.out.println("1. Encrypt");
                         System.out.println("2. Decrypt");
                         System.out.println("3. Exit");
                         String input = s.nextLine(); // get user input
                         int option = 0;
                         try {
                                 // check for valid input
                         
                                 option = Integer.parseInt(input);
                                 if(option < 1 || option > 3) {
                                         throw new IllegalArgumentException();
                                 }
                                }
                          catch (Exception e) {
                                  // print error message
                             System.out.println("Invalid Input!");
                          }
                         
                 // check for user option
                 if(option == 1) {
                         // ask user for data to encrypt
                         System.out.print("Enter 4 digit number to Encrypt: ");
                         int data = getInput(s); // get user input
                         // check for valid input
                         if(data == -1) {
                                 // print error message
                                 System.out.println("Invalid Input!");
                                 }
                         else {
                                 System.out.print("Encrypted number is: ");
                                 String encrypt_number = "" + Cryptography.encrypt(data);
                                 // add zeroes at start of number if needed
                                 int len = encrypt_number.length();
                                 for(int i=4;i>len;i--) {
                                         encrypt_number = "0" + encrypt_number;
                                         }
                                 System.out.println(encrypt_number);
                                 }
                         }
                 else if(option == 2) {
                         // ask user for data to decrypt
                         System.out.print("Enter 4 digit number to Decrypt: ");
                         int data = getInput(s); // get user input
                         // check for valid input
                         if(data == -1) {
                                 // print error message
                                 System.out.println("Invalid Input!");
                                 }
                         else {
                                 System.out.print("Decrypted number is: ");
                                 String decrypt_number = "" + Cryptography.decrypt(data);
                                 // add zeroes at start of number if needed
                                 int len = decrypt_number.length();
                                 for(int i=4;i>len;i--) {
                                         decrypt_number = "0" + decrypt_number;
                                         }
                                 System.out.println(decrypt_number);
                                 }
                         }
                 else if(option == 3) {
                         // break the loop and exit program
                         System.out.println("Bye!");
                         break;
                         }
}
  
}
}

Cryptography.java:

public class Cryptography {
          
        public static int decrypt(int input) {
                // input is 4 digit integer
                // convert integer to string
                String str = "" + input;
                // add zeroes at start of number if needed
                int len = str.length();
                for(int i=4;i>len;i--) {
                str = "0" + str;
                }
                // get individual digits
                int first_digit = str.charAt(0) - '0'; // remove '0' to convert char to digit
                int second_digit = str.charAt(1) - '0';
                int third_digit = str.charAt(2) - '0';
                int fourth_digit = str.charAt(3) - '0';
                // add 3 to each digit and take reminder by dividing 10
                first_digit = (first_digit + 3) % 10;
                second_digit = (second_digit + 3) % 10;
                third_digit = (third_digit + 3) % 10;
                fourth_digit = (fourth_digit + 3) % 10;
                // replace first digit with third and second with fourth
                String encrypt_number = "" + third_digit + fourth_digit + first_digit + second_digit;
                // convert string to integer and return it
                return Integer.parseInt(encrypt_number);
                }

                

        

        
        public static int encrypt(int input) {
                // input is a 4 digit integer
                // convert integer to string
                String str = "" + input;
                // add zeroes at start of number if needed
                int len = str.length();
                for(int i=4;i>len;i--) {
                        str = "0" + str;
                }
                
                // get individual digits
                int first_digit = str.charAt(0) - '0'; 
                // remove '0' to convert char to digit
                int second_digit = str.charAt(1) - '0';
                int third_digit = str.charAt(2) - '0';
                int fourth_digit = str.charAt(3) - '0';
                // add 7 to each digit and take reminder by dividing 10
                first_digit = (first_digit + 7) % 10;
                second_digit = (second_digit + 7) % 10;
                third_digit = (third_digit + 7) % 10;
                fourth_digit = (fourth_digit + 7) % 10;
                // replace first digit with third and second with fourth
                String encrypt_number = "" + third_digit + fourth_digit + first_digit + second_digit;
                // convert string to integer and return it
                return Integer.parseInt(encrypt_number);
        }
}