半径变量未从构造函数传递

Question

我应该使用构造函数来接受圆的半径作为参数，然后使用各种访问器和修改器方法来显示面积、周长和直径。当我输入半径值时，比如 4，它表示半径和所有其他值（面积、周长、直径）都是 0.0。我是 java 新手，想知道我缺少什么。

// import scanner to read keyboard input
import java.util.Scanner;

class CircleCalculator
{
    private double radius;

    public CircleCalculator(double r)
    {
        radius = r;
    }

    public static void main(String[] args) 
    {
        // declare the variable keyboard that can reference an object of the scanner class 
        // create a new Scanner object in the memory that reads from the input System.in
        Scanner keyboard = new Scanner(System.in);
        System.out.println("\nWelcome to Cam's Magic Circle Calculator!");
        System.out.println("\nEnter the radius of your circle");
        
        double r, a, d, c;

        CircleData circledata = new CircleData();

        System.out.println("\nThe measurements of a circle with the radius " + circledata.getRadius() + " are:\n" +
                    "\tArea: " + circledata.getArea() +" units squared\n" +
                    "\tDiameter: " + circledata.getDiameter() +" units\n" +
                    "\tCircumference: " + circledata.getCircumference() +" units\n");
    }

    public static class CircleData //create class for mutator and accessor methods
    {

    Scanner keyboard = new Scanner(System.in);
    double r = keyboard.nextDouble();
    CircleCalculator setRadius = new CircleCalculator(r);

        final double pi = 3.1415;

        private double radius, area, diameter, circumference;

        private double getRadius()
        {
        return radius;
        }

        private double getArea()
        {
        area = pi * radius * radius;
        return area;
        }

        private double getDiameter()
        {
        diameter = 2 * radius;
        return diameter;
        }

        private double getCircumference()
        {
        circumference = 2 * pi * radius;
        return circumference;
        }
    }
}

给予：

Welcome to Cam's Magic Circle Calculator!

Enter the radius of your circle
4

The measurements of a circle with the radius 0.0 are:
        Area: 0.0 units squared
        Diameter: 0.0 units
        Circumference: 0.0 units

Answer 1

由于类的嵌套，您当前的代码很混乱。我建议遵循良好的代码实践，将它们分离到自己的类文件中。让CircleData处理实际的圆数据和计算，让CircleCalculator成为你的整体程序序列逻辑。

圆形计算器

import java.util.Scanner;

public class CircleCalculator
{
  public static void main(String[] args) 
  {
      System.out.println("\nWelcome to Cam's Magic Circle Calculator!");
      System.out.println("\nEnter the radius of your circle");
      
      Scanner keyboard = new Scanner(System.in);
      CircleData circledata = new CircleData(keyboard.nextDouble());
      keyboard.close();

      System.out.println("\nThe measurements of a circle with the radius " + circledata.getRadius() + " are:\n" +
                  "\tArea: " + circledata.getArea() +" units squared\n" +
                  "\tDiameter: " + circledata.getDiameter() +" units\n" +
                  "\tCircumference: " + circledata.getCircumference() +" units\n");
  }
}

CircleData（在单独的文件中，因此方法现在是公开的）

public class CircleData
{
  final double pi = 3.1415;

  private double radius, area, diameter, circumference;
  
  public CircleData(double radius) {
    this.radius = radius;
  }

  public double getRadius()
  {
  return radius;
  }

  public double getArea()
  {
  area = pi * radius * radius;
  return area;
  }

  public double getDiameter()
  {
  diameter = 2 * radius;
  return diameter;
  }

  public double getCircumference()
  {
  circumference = 2 * pi * radius;
  return circumference;
  }
}

Answer 2

1. 因为来自CircleCalculator 和CircleData 类的半径字段是两个不同的字段。因此，您不能使用CircleData 中的CircleCalculator 的半径，因为CircleData 已经包含此类字段。
2. CircleData 实际上不应该有 Scanner 字段，它破坏了 SRP https://en.wikipedia.org/wiki/Single-responsibility_principle。

我建议这样写：

class CircleCalculator {

    public static void main(String[] args) {
        // declare the variable keyboard that can reference an object of the scanner class
        // create a new Scanner object in the memory that reads from the input System.in
        Scanner keyboard = new Scanner(System.in);
        System.out.println("\nWelcome to Cam's Magic Circle Calculator!");
        System.out.println("\nEnter the radius of your circle");
        CircleData circledata = new CircleData(keyboard.nextDouble());
        System.out.println("\nThe measurements of a circle with the radius " + circledata.getRadius() + " are:\n" +
                "\tArea: " + circledata.getArea() + " units squared\n" +
                "\tDiameter: " + circledata.getDiameter() + " units\n" +
                "\tCircumference: " + circledata.getCircumference() + " units\n");
    }

    public static class CircleData //create class for mutator and accessor methods
    {

        final double pi = 3.1415;

        private final double radius;

        public CircleData(double radius) {
            this.radius = radius;
        }

        private double getRadius() {
            return radius;
        }

        private double getArea() {
            return pi * radius * radius;
        }

        private double getDiameter() {
            return 2 * radius;
        }

        private double getCircumference() {
            return 2 * pi * radius;
        }
    }
}

Answer 3

没有从构造函数传递半径变量，因为您已经在CircleCalculator 类中定义了构造函数，但是使用半径变量值计算面积、周长等的方法在不同的类中。

CircleCalculator setRadius = new CircleCalculator(r); 在这里，虽然setRadius 有用户输入的值，但它没有被使用。

CircleData circledata = new CircleData(); 在这里， circledata 没有引用任何内容。方法调用 circledata.getRadius() , circledata.getArea() , circledata.getDiameter() , circledata.getCircumference() 实际上返回 double 的默认值 0.0，因为 radius 变量的默认值是 0.0

此外，类文件中应该只有一个公共类，其名称应与类文件名匹配，并且该公共类应具有public static void main(String[] args) 方法。