Scytale密码明文和Z实现

Question

在显示纯文本时，我在Scytale密码中遇到了麻烦，在行和列中缺少时，也遇到了“ z”字母。

为了使说明更容易，我希望这样显示明文：

---

H E L L O

W O R L D

Z Z Z Z Z

---

这是我的代码：

import java.util.*;

public class scytale{

    public static String scytaleEncode(String plaintext, int numRows){
        String encodedText = "";
        if(numRows>=plaintext.length() || numRows <=0){
            System.out.println(plaintext + "z");
        }
        else{
            while(plaintext.length()%numRows != 0){
                plaintext += " ";
            }
            int numCols = plaintext.length()/numRows;
            for(int i = 0; i<numCols; i++){
                for (int y = i; y <plaintext.length(); y+=numCols;){
                    encodedText += plaintext.charAt(y);
                }
                System.out.println();
            }
        return encodedText;
        }

    }

    public static void main(String[] args){
        System.out.println("-------WELCOME TO SCYTALE CIPHER-------");
        String cont;
        do{
            Scanner sc = new Scanner(System.in);
            System.out.println("=================================");
            System.out.println("Enter the String for Encryption: ");
            String message = new String();
            message = sc.nextLine();
            System.out.println("\nEnter your key: ");
            int key;
            key = sc.nextInt();
            System.out.println("\nCiphered:" + scytaleEncode(message.replaceAll("\\s",""), key));
            System.out.println("\nWould you like to continue? (y/n): ");
            cont = sc.next();
            System.out.println("\n");
        }while(cont.equalsIgnoreCase("y"));

        System.out.println("Thank you for using SCYTALE CIPHER!\n");
    }
}

我应该怎么做？