Write a program that produces a Caesar cipher of a given message string. A Caesar cipher is formed by rotating each letter of a message by a given amount. For example, if your rotate by 3, every A becomes D; every B becomes E; and so on. Toward the end of the alphabet, you wrap around: X becomes A; Y becomes B; and Z becomes C. Your program should prompt for a message and an amount by which to rotate each letter and should output the encoded message.

Here is the JAVA program that produces Caeser cipher o given message string:

import java.util.Scanner;  //to accept input from user

public class Main {  

   public static void main(String args[]) {  //start of main function

       Scanner input = new Scanner(System.in);  //creates object of Scanner

       System.out.println("Enter the message : ");  //prompts user to enter a plaintext (message)

       String message = input.nextLine();  //reads the input message

       System.out.println("Enter the amount by which by which to rotate each letter : ");  //prompts user to enter value of shift to rotate each character according to shift

       int rotate = input.nextInt();  // reads the amount to rotate from user

       String encoded_m = "";  // to store the cipher text

       char letter;  // to store the character

       for(int i=0; i < message.length();i++)  {  // iterates through the message string until the length of the message string is reached

           letter = message.charAt(i);  // method charAt() returns the character at index i in a message and stores it to letter variable

           if(letter >= 'a' && letter <= 'z')  {  //if letter is between small a and z

            letter = (char) (letter + rotate);  //shift/rotate the letter

            if(letter > 'z') {  //if letter is greater than lower case z

               letter = (char) (letter+'a'-'z'-1); }  // re-rotate to starting position  

            encoded_m = encoded_m + letter;}  //compute the cipher text by adding the letter to the the encoded message

           else if(letter >= 'A' && letter <= 'Z') {  //if letter is between capital A and Z

            letter = (char) (letter + rotate);  //shift letter

            if(letter > 'Z') {  //if letter is greater than upper case Z

                letter = (char) (letter+'A'-'Z'-1);}  // re-rotate to starting position  

            encoded_m = encoded_m + letter;}  //computes encoded message

           else {  

         encoded_m = encoded_m + letter;  } }  //computes encoded message

System.out.println("Encoded message : " + encoded_m.toUpperCase());  }} //displays the cipher text (encoded message) in upper case letters

Explanation:

The program prompts the user to enter a message. This is a plaintext. Next the program prompts the user to enter an amount by which to rotate each letter. This is basically the value of shift. Next the program has a for loop that iterates through each character of the message string. At each iteration it uses charAt() which returns the character of message string at i-th index. This character is checked by if condition which checks if the character/letter is an upper or lowercase letter. Next the statement letter = (char) (letter + rotate);   is used to shift the letter up to the value of rotate and store it in letter variable. This letter is then added to the variable encoded_m. At each iteration the same procedure is repeated. After the loop breaks, the statement     System.out.println("Encoded message : " + encoded_m.toUpperCase()); displays the entire cipher text stored in encoded_m in uppercase letters on the output screen.

The logic of the program is explained here with an example in the attached document.

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