Question

Write a java program that prompts the user to see if they wish to encode or decode a message.

(a) If they choose to encode a message:

i. Ask them to enter the message and store it in a String variable.

ii. Once the message is entered, ask them for a “shift” integer, similar to Lab 6.

iii. Using that shift, move all alphabetic characters in the message (i.e. excluding spaces and punctuation) using char and int arithmetic. Be sure to account for alphabetical wrap-

around!

iv. Ask the user for a filename (no spaces), and then output the encrypted message to that file

on a single line. Then you may terminate the program, or loop back to the beginning.

(b) If they choose to decode a message:

i. Ask them for the file name that stores their encrypted message.

ii. Ask them for the “shift” that was used to encrypt the message.

iii. Read in the message from the file into a String (assuming it is all on one line).

iv. Using the shift, reverse the encryption procedure by subtracting the shift from each character

of the message, taking into account wrap-around.

v. Print the decrypted message to the user. You may then terminate the program or loop back

to the beginning.

Answer 1

import java.util.Scanner; // Import the Scanner class
import java.io.IOException; // Import the IOException class
import java.io.FileWriter;   // Import the FileWriter class
import java.nio.file.*;
class Main {
  //function to encode the message
  public static void encode()
  {
    Scanner sc = new Scanner(System.in);
    System.out.println(" Input the plaintext message : ");
    String plaintext = sc.nextLine();
    System.out.println(" Enter the value by which each character in the plaintext message gets shifted:"); int shift = sc.nextInt();
    String ciphertext = "";
    char alphabet;
    for(int i=0; i < plaintext.length();i++) 
    {
      // Shift one character at a time
      alphabet = plaintext.charAt(i);
            
      // if alphabet lies between a and z 
      if(alphabet >= 'a' && alphabet <= 'z') 
      {
        // shift alphabet
        alphabet = (char) (alphabet + shift);
       // if shift alphabet greater than 'z'
        if(alphabet > 'z') 
        {
          // reshift to starting position 
            alphabet = (char) (alphabet+'a'-'z'-1);
        }
             ciphertext = ciphertext + alphabet;
        }
            
      // if alphabet lies between 'A'and 'Z'
      else if(alphabet >= 'A' && alphabet <= 'Z') {
      // shift alphabet
      alphabet = (char) (alphabet + shift);    
      // if shift alphabet greater than 'Z'
      if(alphabet > 'Z') 
      {
        //reshift to starting position 
        alphabet = (char) (alphabet+'A'-'Z'-1);
      }
      ciphertext = ciphertext + alphabet;
      }
      else 
      {
        ciphertext = ciphertext + alphabet;   
      }
        
    }
    //user input for filename to store encrypted message
   System.out.println("Enter the file name where you want to store encrypted message:");
   String filename = sc.next();
    //writing encrypted message to the file
    try
    {
      FileWriter fp = new FileWriter(filename);
      fp.write(ciphertext);
      fp.close();
    }
    catch(IOException e)
    {
      System.out.println("Error Occured");
    }
  }
//function to decode the message
  public static void decode() throws Exception
  {
    Scanner sc = new Scanner(System.in);
    //user input for filename
    System.out.println("Enter the file name which have stored encrypted message:");
    String filename = sc.next();
    //shift
     System.out.println(" Enter the value by which each character in the plaintext message gets shifted:"); int shift = sc.nextInt();
     String ciphertext ;
     String decryptMessage = "";
     //reading the encrypted string from the file and store in ciphertext variable
    ciphertext  = new String(Files.readAllBytes(Paths.get(filename)));

    for(int i=0; i < ciphertext.length();i++)  

        {
            // Shift one character at a time
            char alphabet = ciphertext.charAt(i);
            // if alphabet lies between a and z 
            if(alphabet >= 'a' && alphabet <= 'z')
            {
                // shift alphabet
                alphabet = (char) (alphabet - shift);
            
                // shift alphabet lesser than 'a'
                if(alphabet < 'a') {
                    //reshift to starting position 
                    alphabet = (char) (alphabet-'a'+'z'+1);
                }
                decryptMessage = decryptMessage + alphabet;
            }    
                // if alphabet lies between A and Z
            else if(alphabet >= 'A' && alphabet <= 'Z')
            {
             // shift alphabet
                alphabet = (char) (alphabet - shift);
                
                //shift alphabet lesser than 'A'
                if (alphabet < 'A') {
                    // reshift to starting position 
                    alphabet = (char) (alphabet-'A'+'Z'+1);
                }
                decryptMessage = decryptMessage + alphabet;            
            }
            else 
            {
             decryptMessage = decryptMessage + alphabet;            
            } 
        }
        System.out.println(" Decrypted message : " + decryptMessage);
    }

  
  public static void main(String[] args) throws Exception {
    int ch;
    Scanner sc = new Scanner(System.in);
    //choose the operation
    System.out.println("1.Encode Message");
    System.out.println("1.Decode Message");
    System.out.println("Enter your choice:");
    ch = sc.nextInt();
    sc.nextLine();
    //if ch is 1 perform encode operation
    if(ch==1)
    {
      encode();
    }
        //if ch is 2 perform decode operation

    else if(ch==2)
    {
      decode();
    }
    else
    {
      System.out.println("Invalid choice");
    }
  }
}

Output::

Encode operation:

Decode operation::

The encrypt.txt which stores encrypted text::