CaesarCipher.java

import java.util.Scanner;

// Caesar Cipher program that encrypts and decrypts text
public class CaesarCipher {
    // Stores the result of the encryption or decryption
    public static StringBuilder result = new StringBuilder();
    
    // Encrypts text using a Caesar Cipher with the given shift key
    public static String encrypt(String text, int shift) {
        for (char character : text.toCharArray()) { // Loops through each character in the text
            if (Character.isLetter(character)) { // Checks if the character is a letter
                char base = Character.isLowerCase(character) ? 'a' : 'A'; // Checks if the character is lowercase or uppercase and sets the base
                char shifted = (char) ((character - base + shift) % 26 + base); // Shift calculation
                result.append(shifted); // Adds the shifted character to the result
            }
            else {
                result.append(character); // If the character is not a letter then keep it the same
            }
        }
        return result.toString(); // Returns the result as a string
    }
    
    // Decrypts text using a Caesar Cipher with the given shift key
    public static String decrypt(String text, int shift) {
        for (char character : text.toCharArray()) {
            if (Character.isLetter(character)) {
                char base = Character.isLowerCase(character) ? 'a' : 'A';
                char unshifted = (char) ((character - shift - base + 26) % 26 + base); // Unshift calculation
                result.append(unshifted);
            }
            else {
                result.append(character);
            }
        }
        return result.toString();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.println("Would you like to encrypt or decrypt text? (E/D): ");
        String choice = scanner.nextLine();
        
        // Encrypt
        if (choice.equalsIgnoreCase("E")) {
            System.out.print("Enter text to encrypt: ");
            String inputText = scanner.nextLine();
            
            System.out.print("Enter shift key (0-25): ");
            int shiftKey = scanner.nextInt();
            
            String encrypted = encrypt(inputText, shiftKey);
            System.out.println("Encrypted text: " + encrypted);
        }
        // Decrypt
        else if (choice.equalsIgnoreCase("D")) {
            System.out.print("Enter text to decrypt: ");
            String inputText = scanner.nextLine();

            System.out.print("Enter shift key (0-25): ");
            int shiftKey = scanner.nextInt();
            
            String decrypted = decrypt(inputText, shiftKey);
            System.out.println("Decrypted text: " + decrypted);
        }
        // Invalid input (ends the program)
        else {
            System.out.println("Invalid input");
        }
        
        scanner.close();
    }
}