Caesar Cipher in Java - printing blank line

Question

I'm currently trying to write a Caesar Cipher in Java with key 18.

The code I have so far is as follows, but for some reason I cannot work out, it isn't working.

import java.util.Objects;
import java.util.Scanner;
class MainApplication {
private static final Scanner input = new Scanner(System.in);

private static String CryptMessage(int key, String message) {
    StringBuilder temp = new StringBuilder();
    for (int i = 0; i == message.length(); i++) {
        if (Character.isLetter(message.charAt(i))) {
            int num = (int) message.charAt(i);
            num = num + key;
            if (Character.isUpperCase(message.charAt(i))) {
                if (num > (int) ('Z')) {
                    num = num - 26;
                } else if (num < (int) ('A')) {
                    num = num + 26;
                }
            }
            if (Character.isLowerCase(message.charAt(i))) {
                if (num > (int) ('z')) {
                    num = num - 26;
                } else if (num < (int) ('a')) {
                    num = num + 26;
                }
            }
            temp.append((char) num);
        } else {
            temp.append(message.charAt(i));
        }
    }
    message = temp.toString();
    return message;
}
private static void encrypt(){
    int key = 18;
    System.out.println("Please enter a message to encrypt: ");
    String message = input.nextLine();
    System.out.println(CryptMessage(key, message));
}
private static void decrypt(){
    int key = -18;
    System.out.println("Please enter a message to decrypt: ");
    String message = input.nextLine();
    System.out.println(CryptMessage(key, message));
}
public static void main(String args[]){
    System.out.println("Message Encryption System 3.0");
    System.out.println("Please select an option:");
    System.out.println("[1] - Encrypt Message");
    System.out.println("[2] - Decrypt Message");
    String opt = input.nextLine();
    if(Objects.equals(opt, "1")){
        encrypt();
    }
    else if(Objects.equals(opt, "2")){
        decrypt();
    }
    else{
        System.out.println("Invalid input.");
    }
}
}

The program runs, however, it prints a blank line where the encrypted (or decrypted) message should be.

Answer 1

The loop condition i == message.length() should be i < message.length().

The condition in a for loop determines when to continue to the next iteration. Your condition is false on the first iteration and the loop terminates immediately (after the first iteration).