JavaAlgorithms/Ciphers/Caesar.java

package Ciphers;

import java.util.Scanner;

/**
 * A Java implementation of Caesar Cipher. /It is a type of substitution cipher in which each letter
 * in the plaintext is replaced by a letter some fixed number of positions down the alphabet. /
 *
 * @author FAHRI YARDIMCI
 * @author khalil2535
 */
public class Caesar {

  /**
   * Encrypt text by shifting every Latin char by add number shift for ASCII Example : A + 1 -> B
   *
   * @return Encrypted message
   */
  public static String encode(String message, int shift) {
    StringBuilder encoded = new StringBuilder();

    shift %= 26;

    final int length = message.length();
    for (int i = 0; i < length; i++) {

      //            int current = message.charAt(i); //using char to shift characters because ascii
      // is in-order latin alphabet
      char current = message.charAt(i); // Java law : char + int = char

      if (IsCapitalLatinLetter(current)) {

        current += shift;
        encoded.append((char) (current > 'Z' ? current - 26 : current)); // 26 = number of latin letters

      } else if (IsSmallLatinLetter(current)) {

        current += shift;
        encoded.append((char) (current > 'z' ? current - 26 : current)); // 26 = number of latin letters

      } else {
        encoded.append(current);
      }
    }
    return encoded.toString();
  }

  /**
   * Decrypt message by shifting back every Latin char to previous the ASCII Example : B - 1 -> A
   *
   * @return message
   */
  public static String decode(String encryptedMessage, int shift) {
    StringBuilder decoded = new StringBuilder();

    shift %= 26;

    final int length = encryptedMessage.length();
    for (int i = 0; i < length; i++) {
      char current = encryptedMessage.charAt(i);
      if (IsCapitalLatinLetter(current)) {

        current -= shift;
        decoded.append((char) (current < 'A' ? current + 26 : current)); // 26 = number of latin letters

      } else if (IsSmallLatinLetter(current)) {

        current -= shift;
        decoded.append((char) (current < 'a' ? current + 26 : current)); // 26 = number of latin letters

      } else {
        decoded.append(current);
      }
    }
    return decoded.toString();
  }

  /**
   * @return true if character is capital Latin letter or false for others
   */
  private static boolean IsCapitalLatinLetter(char c) {
    return c >= 'A' && c <= 'Z';
  }

  /**
   * @return true if character is small Latin letter or false for others
   */
  private static boolean IsSmallLatinLetter(char c) {
    return c >= 'a' && c <= 'z';
  }

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    System.out.println("Please enter the message (Latin Alphabet)");
    String message = input.nextLine();
    System.out.println(message);
    System.out.println("Please enter the shift number");
    int shift = input.nextInt() % 26;
    System.out.println("(E)ncode or (D)ecode ?");
    char choice = input.next().charAt(0);
    switch (choice) {
      case 'E':
      case 'e':
        System.out.println(
            "ENCODED MESSAGE IS \n" + encode(message, shift)); // send our function to handle
        break;
      case 'D':
      case 'd':
        System.out.println("DECODED MESSAGE IS \n" + decode(message, shift));
      default:
        System.out.println("default case");
    }
    input.close();
  }
}