import java.util.Scanner;

public class Main {

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

    //write your code here......
    if(grade.equals("A")){
        System.out.println("优秀");
    }else if(grade.equals("B")){
        System.out.println("良好");
    }else if(grade.equals("C")){
        System.out.println("及格");
    }else if(grade.equals("D")){
        System.out.println("不及格");
    }else{
        System.out.println("未知等级");
    }
}

}

在if里用equals比较出输入的东西然后输出就可以了