java - '无效的'？类(class)分配问题

Question

问题是我从影评人那里收到了意外错误。

import java.util.Scanner;

public class deTest {

public static void main(String[] args) {
    Scanner kbScanner=new Scanner(System.in);
    aClass bClass=new aClass();

    System.out.print("What is the movie you want to review?... ");
    bClass.movies=kbScanner.nextLine();

    do{
        System.out.print("What would you give it out of 10?... ");
        bClass.ratings=kbScanner.nextInt();
        if (bClass.ratings>10){
            System.out.println("Sorry, your rating went over 10. Please try again...");
        }
        if (bClass.ratings<=10){
            System.out.println("You gave a rating of "+bClass.ratings+".");
        }
    }while (bClass.ratings>10);

    bClass.graderate(bClass.ratings, bClass.grade);
    System.out.println("This movie got a grade of "+bClass.grade);
    bClass.printInfo(bClass.movies, bClass.ratings, bClass.grade);

}
}

这是类(class):

public class aClass {
String movies;
int ratings;
String grade;

void graderate(int ratings,String grade){
    switch (ratings){
    case 1:
        grade=("C");
        break;
    case 2:
        grade=("C");
        break;
    case 3:
        grade=("C");
        break;
    case 4:
        grade=("B");
        break;
    case 5:
        grade=("B");
        break;
    case 6:
        grade=("B");
        break;
    case 7:
        grade=("A");
        break;
    case 8:
        grade=("A");
        break;
    case 9:
        grade=("A");
        break;
    case 10:
        grade=("A+");
        break;
    default:
        grade=("?");
    }


}
void printInfo(String movies,int ratings,String grade){
    System.out.println("The movie "+movies+" was given rating of "+ratings+" out of 10 and received a grade of "+ grade);
}
}

我收到的错误是这样的:

What is the movie you want to review?... Titanic
What would you give it out of 10?... 7
You gave a rating of 7.
This movie got a grade of null
The movie Titanic was given rating of 7 out of 10 and received a grade of null

控制台没有给我成绩(它是一个字符串)，而是不断将 null 放在成绩的位置。这个错误是在我将 bClass.grade 从字符更改为字符串之后发生的。当 bClass.grade 是一个字符时，我得到:

What is the movie you want to review?... Real Steel
What would you give it out of 10?... 10
You gave a rating of 10.
This movie got a grade of 
The movie Titanic was given rating of 10 out of 10 and received a grade of 

没有任何东西可以代替成绩。那么，我对我的影评人做错了什么？

Answer 1

graderate(...) 中的参数 grade 隐藏了类 aClass 的 grade 实例，所以您在 switch 中修改的是局部变量 grade(parameter) ，而不是字段。没有必要传递它，因为 grade 是类的一个字段:

void graderate(int ratings)
{
    switch (ratings)
    {
    case 1:
    case 2:
    case 3:
        grade = ("C");
        break;
    case 4:
    case 5:
    case 6:
        grade = ("B");
        break;
    case 7:
    case 8:
    case 9:
        grade = ("A");
        break;
    case 10:
        grade = ("A+");
        break;
    default:
        grade = ("?");
    }
}

并在您的 main 方法中调用它:

bClass.graderate(bClass.ratings);

注意，您可以通过“分组案例”来简化您的switch，这样您就不会重复代码(看看上面的修改，很简单)。