java使用带枚举的switch语句

Question

我从这个网站上找到了以下内容。我添加了一些代码来获取用户输入。但在switch语句的开头有一个名为NullPointerExecption的异常

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

        double gpa;
        System.out.println("Enter your GPA: ");
        gpa = input.nextDouble();
        Grade grade = Grade.getGradeFromGPA(gpa);



        switch (grade) {
            case FIRST_CLASS:   System.out.println("First class Hons"); break;
            case TWO_ONE:       System.out.println("upper Second class Hons"); break;
            case TWO_TWO:       System.out.println("Lower Second class Hons"); break;
            case PASS:          System.out.println("Pass"); break;
            case FAIL:          System.out.println("Fail"); break;
            default:            System.out.println("Invalied GPA");
        }
    }

    private enum Grade {
        FIRST_CLASS(3.5, 4),
        TWO_ONE(3, 3.49),
        TWO_TWO(2.5, 2.99),
        PASS(2.49, 2),
        FAIL(0, 1.99);

        private final double minGPA;
        private final double maxGPA;

        Grade(final double minGPA, final double maxGPA) {
            this.minGPA = minGPA;
            this.maxGPA = maxGPA;
        }

        static Grade getGradeFromGPA(double gpa) {
            for (Grade grade : Grade.values()) {
                if (grade.maxGPA >= gpa && grade.minGPA <= gpa) {
                    return grade;
                }
            }
            //invalid GPA
            return null;
        }
    }

我认为它不会通过getGradeFromGPA()方法返回任何内容

Answer 1

I think it doesn't return anything by getGradeFromGPA() method.

没错。例如，如果输入55，getGradeFromGPA将返回null

请注意，一些double无法准确表示，因此，如果您输入2.49，这也将产生null

（另外，您似乎交换了PASSenum上的参数。）

How to fix this?

关于double的问题，我建议您：

private enum Grade {
    FIRST_CLASS(3.5),
    TWO_ONE(3),
    TWO_TWO(2.5),
    PASS(2),
    FAIL(0);

    private final double minGPA;

    Grade(final double minGPA) {
        this.minGPA = minGPA;
    }

    static Grade getGradeFromGPA(double gpa) {
        for (Grade grade : Grade.values()) {
            if (gpa > grade.minGPA)
                return grade;
        }
        //invalid GPA
        return null;
    }
}

然而，当你进入时，比如说，55的情况需要处理。最好的方法可能是使用异常：

static Grade getGradeFromGPA(double gpa) {
    for (Grade grade : Grade.values()) {
        if (gpa > grade.minGPA)
            return grade;
    }
    //invalid GPA
    throw new IllegalArgumentException();  // <     
}

及

try {
    Grade grade = Grade.getGradeFromGPA(gpa);
    switch (grade) {
        case FIRST_CLASS:   System.out.println("First class Hons"); break;
        case TWO_ONE:       System.out.println("upper Second class Hons"); break;
        case TWO_TWO:       System.out.println("Lower Second class Hons"); break;
        case PASS:          System.out.println("Pass"); break;
        case FAIL:          System.out.println("Fail"); break;
    }
} catch (IllegalArgumentException e) {
    System.out.println("Invalied GPA");
}