자바 무한반복 사칙연산 조건문을 수정하고 싶습니다.

package project;
 
import java.io.*;
import java.util.StringTokenizer;
import java.util.Scanner;
 
 
//Class Scanner
//https://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html
 
//[JAVA] 문자열 입력과 출력 (Scanner)
//http://hyeonstorage.tistory.com/136
 
public class Main
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
 
        double x = 0;
        double y = 0;
        String z = "";
 
        while (true)
        {
        	System.out.println("Hi, I am really good at math! Put me to the test.");
            System.out.print("Please enter number and then press Enter: ");
            x = scan.nextDouble();
            System.out.println(x);
            y = scan.nextDouble();
            System.out.println(y);
 
            if (x == 999) 
            {
                System.out.print("Bye bye!");
                return;
            }
            else
            {
//                 System.out.print("Please enter number and then press Enter: ");
//                y = scan.nextDouble();
 
                System.out.print("Please enter one of the operations +, -, * or / and press Enter:");
                z = scan.next();
 
                switch (z)
                {
                case "+":
                    System.out.println( + x + " + " + y + " = " + (x + y) );
                    System.out.println("I'm great at subtraction!\n");
                    break;
                case "-":
                    System.out.println( + x + " - " + y + " = " + (x - y) );
                    System.out.println("I'm great at subtraction!\n");
                    break;
                case "*":
                    System.out.println( + x + " * " + y + " = " + (x * y) );
                    System.out.println("I'm great at subtraction!\n");
                    break;
                case "/":
                    System.out.println( + x + " / " + y + " = " + (x / y) );
                    System.out.println("I'm great at subtraction!\n");
                    break;
                default:
                    System.out.println("잘못된 연산자입니다.\n");
                }
            }
        }
    }
}