检查以下代码错误并修改，代码如下： import java.util.Scanner; public class zzz1 {public class Main { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); String grade = scanner.next(); //write your code here...... StringBuffer sb = new StringBuffer("优秀"); switch (grade.toUpperCase()) { case "A": break; case "B": sb.replace(0, sb.length(), "良好"); break; case "C": sb.replace(0, sb.length(), "及格"); break; case "D": sb.replace(0, sb.length(), "不及格"); break; default: sb.replace(0, sb.length(), "未知等级"); break; } System.out.println(sb); } } }

这段代码有两个问题：

1. 类名与文件名不一致，应该将类名改为zzz1或者将文件名改为Main.java。

2. zzz1类中包含了Main类的定义，应该将Main类与zzz1类分开，或者将Main类定义为静态内部类。

修改后的代码如下：

import java.util.Scanner;

public class zzz1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String grade = scanner.next();

        StringBuffer sb = new StringBuffer("优秀");
        switch (grade.toUpperCase()) {
            case "A":
                break;
            case "B":
                sb.replace(0, sb.length(), "良好");
                break;
            case "C":
                sb.replace(0, sb.length(), "及格");
                break;
            case "D":
                sb.replace(0, sb.length(), "不及格");
                break;
            default:
                sb.replace(0, sb.length(), "未知等级");
                break;
        }
        System.out.println(sb);
    }
}