java - 在 Java 中使用类时遇到很多麻烦

Question

我正在学习如何使用类和方法，但我什么也学不会。我已经阅读了教科书中的章节和大量网页。我正在用 Java 编写一个程序，它将从 2 个测验、一个期中考试和一个期末考试中获取成绩，并输出所有成绩、最终加权成绩和字母成绩。我已经尝试了很多次，但无法使用类来完成。我能够编写一个无需实现类即可运行的程序，但我真的很想了解类。

这是我在没有类(class)的情况下做的:

import java.util.Scanner;

public class grade {
    public static void main(String args[]) {


        //quiz method       
        Scanner keyboard = new Scanner(System.in);
        System.out.printf("Please enter the grade of Quiz 1: ");
        double a = keyboard.nextInt();

        System.out.printf("Please enter the grade of Quiz 2: ");
        double b = keyboard.nextInt();

        double c = (a + b);
        double d = (c / 20);
        double quizGrade = (d * 25);


        //midterm method
        System.out.printf("Please enter the grade of the Midterm: ");
        double aa = keyboard.nextInt();
        double bb = (aa / 100);
        double midtermGrade = (bb * 25);


        //final method
        System.out.printf("Please Enter the grade of the Final: ");
        double aaa = keyboard.nextInt();
        double bbb = (aaa / 100);
        double finalGrade = (bbb * 50);

        double overallGrade = (quizGrade + midtermGrade + finalGrade);

        System.out.printf("Score for Quiz 1 is: %f\n", a);
        System.out.printf("Score for Quiz 2 is: %f\n", b);
        System.out.printf("Score for Midterm is: %f\n", aa);
        System.out.printf("Score for Final is: %f\n", aaa);


        //figuring letter grade
        if (overallGrade >= 90) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: A\n");
        }
        if (overallGrade >= 80 && overallGrade <= 89) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: B\n");
        }
        if (overallGrade >= 70 && overallGrade<= 79) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: C\n");
        }
        if (overallGrade >= 60 && overallGrade<= 69) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: D\n");
        }
        if (overallGrade < 60) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: F\n");
        }
    }

}

这就是我在类里面尝试的方式...我认为可以肯定地说我不知道​​自己在做什么:

import java.util.Scanner;

public class grade2 {
    Scanner keyboard = new Scanner(System.in);
    System.out.printf("Please enter the grade of Quiz 1: ");
    double grade1 = keyboard.nextInt();
    System.out.printf("Please enter the grade of Quiz 2: ");
    double grade11 = keyboard.nextInt();
    System.out.printf("Please enter the grade of the Midterm: ");
    double grade2 = keyboard.nextInt();
    System.out.printf("Please enter the grade of the Final: ");
    double grade3 = keyboard.nextInt();

    public static double quizgrade(double grade1, double grade11) {
        //quiz      
        double a = grade1
        double b = grade11

        double c = (a + b);
        double d = (c / 20);
        double quizGrade = (d * 25);
        return quizGrade;
    }

    public static double midtermgrade(double grade2) {
        //midterm
        double aa = grade2
        double bb = (aa / 100);
        double midtermGrade = (bb * 25);
        return midtermGrade;
    }

    public static double finalgrade(double grade3) {
        //final
        double aaa = grade3
        double bbb = (aaa / 100);
        double finalGrade = (bbb * 50);
        return finalGrade;
    }

        double overallGrade = (quizgrade + midtermgrade + finalgrade);

        System.out.printf("Quiz 1 grade is: %f\n", grade1);
        System.out.printf("Quiz 2 grade is: %f\n", grade11);
        System.out.printf("Midterm grade is: %f\n", grade2);
        System.out.printf("Final grade is: %f\n", grade3);


        //figuring letter grade
        if (overallGrade >= 90) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: A\n");
        }
        if (overallGrade >= 80 && overallGrade <= 89) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: B\n");
        }
        if (overallGrade >= 70 && overallGrade<= 79) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: C\n");
        }
        if (overallGrade >= 60 && overallGrade<= 69) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: D\n");
        }
        if (overallGrade < 60) {
            System.out.printf("Your overall grade is: %f\n", overallGrade);
            System.out.printf("Your letter grade is: F\n");
        }
}

有什么帮助吗？这种 Material 令人困惑不已!

Answer 1

您的首要目标应该是让您的代码执行，以便您可以检查它并了解正在发生的事情，然后您可以在此之后了解 OOP 的其他方面。

在您的第一段代码中，您围绕过程代码包装了一个类，因为它位于您的静态 main 方法中，所以它将在您运行程序时执行。但是，在您的第二次尝试中，您没有在任何地方创建类(对象)的实例，那么如何执行呢？没有 main 方法，因此除非您在其他地方创建 grade2 的实例，否则您的代码将永远不会执行。

对于单个类的实现，您将在 grade2 对象中有一个静态 main 方法，它创建自己的一个实例(我建议使用大写名称的类名)，如下所示:

public class Grade2 {

    static void main(String[] args) {

        Grade2 grade2 = new Grade2(); // Construct (instantiate) the object.

        // Access methods etc. on the grade2 instance.
        grade2.doStuff(args);
    }

    // Other methods etc.
}

因此，通过创建您的类的一个实例，您可以使用实例 (grade2) 调用方法和访问变量等。我不想让您重载访问(公共(public)、私有(private)、 protected )。但是，如果您能全神贯注于此，那么您将开始对 OOP 的其他方面有所了解。

祝你好运!