无法计算对象数量

Question

我构建了一个具有此方法的程序,我还想计算我创建的对象数.但是在运行程序之后.它说有0个对象被创建.谁能知道为什么这不正确？应该说有4个对象正在创建.这是我的代码:

    /**
     This program implements code for a Circle class,
     which has a radius field, set and
     get methods, and a getArea method.
     Author: Michael Wu.
    */

    public class Circle
    {
        private double radius;
        private static int numCircles;

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

        //SetRadius method,sets radius.
        public void setRadius(double radius)
        {
            this.radius = radius;
        }

        //GetRadius method; returns radius.
        public double getRadius()
        {
            return radius;
        }

        //Constructor increments numbers of circles.
        public Circle()
        {
            numCircles++;
        }

        //Copy constuctor.
        public Circle(Circle c3)
        {
            radius = c3.radius;
        }

        //GetNumbercircles method; get number of circles.
        public int getNumCircles()
        {
            return numCircles;
        }

        //Copy method, copy objects.
        public Circle copy()
        {
            Circle copyObject = new Circle(radius);

            return copyObject;
        }

        //Call the getArea method.
        public double getArea()
        {
            return radius*radius*Math.PI;
        }
    }

    /**
     This program created several circle objects and then their areas and radius will be displayed on the screen.
     Author: Michael Wu.
    */

    public class CircleDemo
    {
        public static void main(String[] args)
        {
            int numCircles;

            //Create two circle objects.
            Circle c1 = new Circle(3.7);
            Circle c2 = new Circle(5.9);
            Circle c3 = new Circle(c1);

            //Declare a circle object. 
            Circle c4;

            //Make c3 reference a copy of a object refferenced by c1.
            c4 = c2.copy();

            //Display outputs.
            System.out.println("The radius for circle1 is "+ c1.getRadius());
            System.out.println("The area for circle1 is "+ c1.getArea());

            System.out.println("The radius for circle2 is "+ c2.getRadius());
            System.out.println("The area for circle2 is "+ c2.getArea());

            System.out.println("The radius for circle3 is "+ c3.getRadius());
            System.out.println("The area for circle3 is "+ c3.getArea());

            System.out.println("The radius for circle4 is "+ c4.getRadius());
            System.out.println("The area for circle4 is "+ c4.getArea());

            //Get the number of circles.
            numCircles = c1.getNumCircles();

            System.out.println("There are "+numCircles+" objects being created.");
        }
    }

Answer 1

只有numCircles在Circle没有参数的情况下调用构造函数时才会增加.但你永远不会称那个特定的构造函数; 你打电话给其他建设者.

numCircles所有构造函数的增量.

顺便说一下,因为numCircles是static,getNumCircles返回这个数字的方法也应该是static,你可以调用它,如下所示:Circle.getNumCircles().