我构建了一个具有此方法的程序,我也想要 计算我创建的对象数量。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?它应该说那里 正在创建4个对象。 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.");
}
}
答案 0 :(得分:6)
只有在没有参数的情况下调用numCircles
构造函数时,才会增加Circle
。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。
在所有构造函数上增加numCircles
。
顺便说一下,因为numCircles
是static
,返回此数字的方法getNumCircles
也应该是static
,您可以按如下方式调用它:{{1} }。
答案 1 :(得分:3)
您正在调用以创建对象(带参数的对象)的构造函数不会调用增加计数器的no-arg构造函数。尝试:
public Circle(double radius)
{
this();
this.radius = radius;
}
public Circle(Circle c3)
{
this();
radius = c3.radius;
}
您必须显式调用this()
否则构造函数将隐式调用super()
,在您的情况下是Object
构造函数。
答案 2 :(得分:2)
正如其他答案所提到的,您应该首先通过每个构造函数中的this()
调用零参数构造函数,以确保它被执行。目前,您的代码不会调用零参数构造函数中的代码,因为它从不直接调用,也不会在调用其他构造函数时调用它。
或者,您可以在声明下面添加一个内联初始化块,例如:
private static int numCircles = 0;
// Increment numCircles for every object constructed.
{
numCircles++;
}
这不太标准,可能更糟糕的做法,但它会自动确保执行,而不需要在任何构造函数中增加numCircles。
答案 3 :(得分:0)
你应该用0初始化numCircles
并在每次构造函数调用时增加它,特别是因为你有多个构造函数你必须跟踪它们都在递增numCircles
。你可以从另一个内部调用一个构造函数,所以你可能不会为每个构造函数添加相同的语句。
答案 4 :(得分:0)
移动你的代码'numCircles ++;'圆(双半径)构造函数。创建圆形对象时不会调用Circle()
public Circle(double radius)
{
this.radius = radius;
numCircles++;
}