import java.util.Scanner;
public class PrintGrade{
    public static void main(String[] args){   //主方法
        Scanner input  = new Scanner(System.in);
       int sc;
       System.out.println("please enter your score:");
       sc = input.nextInt();
       
       while (sc!=999){ //不固定次数循环（先调用方法后输入）
           printGrade3(sc);
           System.out.println("please enter your score:");
           sc = input.nextInt();
......................

import java.util.Scanner;
public class PrintGrade{
    public static void main(String[] args){
        Scanner input  = new Scanner(System.in);
       int sc;
       System.out.println("please enter your score:");
       sc = input.nextInt();
       
       while (sc!=999){ //不固定次数循环（先调用方法后）
           printGrade3(sc);
           System.out.println("please enter your score:");
           sc = input.nextInt();
......................