java - Java 中的类变量和类方法，我的错

Question

类；

package com.eteration.model;

public class Circle {
    private static double PI;
    private int radius;
    private double area;

    public double calculateArea(){
        return radius*radius*PI;
    }

    public static double getPI() {
        return PI;
    }

    public static void setPI(double pI) {
        PI = pI;
    }

    public int getRadius() {
        return radius;
    }

    public void setRadius(int radius) {
        this.radius = radius;
    }

    public double getArea() {
        return area;
    }

    public void setArea(double area) {
        this.area = area;
    }

}

剪贴簿；

Circle.PI=3.14;
System.out.println(Circle.getPI());
Circle circle1=new Circle(7);
System.out.println(circle1.getPI());
circle1.PI=3.1415;
Circle circle2=new Circle(3);
System.out.println(circle1.getPI());
System.out.println(circle2.getPI());
circle2.setPI(3.141592);
System.out.println(circle1.getPI());
System.out.println(circle2.getPI());
System.out.println(Circle.getPI());

我正在尝试锻炼。在本练习中，我在将示例编码到剪贴簿后创建了一个类。但是，我看到了错误。这些错误； “Circle 无法重新分配给变量，无法解析 Circle，无法解析 Circle 的类型。”帮助我。

Answer 1

你不能这样做Circle Circle1=new Circle(7); 因为您没有为采用整数的类 Circle 定义构造函数......

我会修改类来改进它，如下所示:

• 将常量PI公开，并删除setter和getter，您将能够像Circle.PI一样访问它

  class Circle {
  public static double PI = 3.1415927;
  private int radius;
  private double area;
  
  public double calculateArea() {
      return radius * radius * PI;
  }
  
  public Circle(int radius) {
      this.radius = radius;
  }
  
  public int getRadius() {
      return radius;
  }
  
  public void setRadius(int radius) {
      this.radius = radius;
  }
  
  public double getArea() {
      return area;
  }
  
  public void setArea(double area) {
      this.area = area;
  }
  }
  

<小时/>

之后剪贴簿将如下所示:

类；

包 com.eteration.model;

公共(public)课圈子{ 私有(private)静态双PI； 私有(private) int 半径； 私有(private)双区域；

public double calculateArea(){
    return radius*radius*PI;
}

public static double getPI() {
    return PI;
}

public static void setPI(double pI) {
    PI = pI;
}

public int getRadius() {
    return radius;
}

public void setRadius(int radius) {
    this.radius = radius;
}

public double getArea() {
    return area;
}

public void setArea(double area) {
    this.area = area;
}

}剪贴簿；

//Circle.PI=3.14; // dont need this
System.out.println(Circle.PI ); // instead of getPI()
Circle circle1 = new Circle(7);  //the same
//System.out.println(circle1.getPI()); //dont need it since is a class constant
//circle1.PI=3.1415; // dont need it
Circle circle2=new Circle(3); //ok
System.out.println(circle1.getPI());//dont need it
System.out.println(circle2.getPI());//dont need it
circle2.setPI(3.141592);//dont need it
System.out.println(circle1.getPI());//dont need it
System.out.println(circle2.getPI());//dont need it
System.out.println(Circle.getPI());//dont need it