java简单实现

import java.util.*;

public class Main {
    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) {
        var x = scanner.nextInt();
        if(x < 60)
            System.out.println('E');
        else if(x < 70)
            System.out.println('D');
        else if(x < 80)
            System.out.println('C');
        else if(x < 90)
            System.out.println('B');
        else
            System.out.println('A');
    }
}

讲真，java的条件判断真难用。C#直接写成下面就行

Console.WriteLine(int.Parse(Console.ReadLine()) switch {
    < 60 => "E",
// 省略不写
});