Circle类的多个构造函数

Question

I am trying modify the class Circle to include a third constructor for constructing a Circle instance with two arguments - a double for radius and a String for color. Also Modify the main class to construct an instance of Circle using this constructor. I am having trouble with this, i keep getting the message that constructor Circle is never used. Please have a look at the code.

 public class Circle {
        private double radius;
        private String color;


        public Circle() {
            radius = 1.0;
            color = "red";
        }


        public Circle(double r) {
            radius = r;
            color = "Blue";
        }

        public Circle(double r, String c) {
            radius = r;
            color =c;
        }



        public double getRadius() {
            return radius;
        }

        public void setRadius(double newRadius) {
            radius = newRadius;
        }


        public String getColor()
        {
            return color;
        }

        public void setColor(String newColor) {
            color=newColor;
        }



        public double getArea() {
            return radius*radius*Math.PI;
        }
    }

    public class Main {

        public static void main(String[] args) {

            Circle c1 = new Circle();
            System.out.println("The circle has radius of " +     c1.getRadius());
            System.out.println("and area of " + c1.getArea());

            Circle c2 = new Circle(2.5);
            System.out.println("The circle has radius of " + c2.getRadius());
            System.out.println("and area of " + c2.getArea());


            Circle c3 = new Circle(0.5);
            c3.setColor("Green");
            System.out.println("The circle has radius of " 
            + c3.getRadius());
            System.out.println("and area of " + c3.getArea());
            System.out.println("color is: " + c3.getColor());




            Circle c5 = new Circle();
            c5.setRadius(500.0);
            System.out.println("radius is: " + c5.getRadius());
            c5.setColor("Yellow");
            System.out.println("color is: " + c5.getColor());


        }
    }

Answer 1

Well, you aren't using it, so the message should hardly be a surprise. Just stick in a call to the two-argument constructor somewhere (eg, Circle c3 = new Circle(0.5, "Green"); ), and the message should go away. Of course, if you change all the instance creations to the two-argument constructor, you'll then get the warning for the zero- and one-argument versions.

Alternatively, you can change your constructor definitions:

public class Circle {
    public Circle() {
        this(1.0, "red");
    }


    public Circle(double r) {
        this(r, "Blue");
    }

    public Circle(double r, String c) {
        radius = r;
        color = c;
    }
    ...
}

I have to say, though, that it's weird to have the default value for the color be "red" when you use a default radius and "Blue" when you specify a radius. I would recommend against that, in which case I'd change the first constructor to:

public Circle() {
    this(1.0);
}

You might also want to look at using an enum for your colors, to avoid problems with case differences (like "red" vs. "Red"). You can always convert between an enum value and a String using the built-in enum methods name() and valueOf(String) .