constructors and inheritance

[dawnMist]

Hello,

I have a class called GeneralRegularPolygon with my own constructor.

public class GeneralRegularPolygon implements RegularPolygon, Colored
{
    private Vertex2D center;
    private double radius;

    private int numEdges;
    private double edgeLength;
    private String color = "white";
    
    public int getNumEdges() {
        return numEdges;
    }
    
    public double getEdgeLength() {
        return edgeLength;
    }

    public Vertex2D getCenter() {
        return center;
    }
     
    public String getColor() {
        return color;
    }
    
    public void setColor(String color) {
        this.color = color;
    }
    
    public GeneralRegularPolygon(Vertex2D center, int numEdges, double edgeLength) {
        this.center = center;
        this.numEdges = numEdges;
        this.edgeLength = edgeLength;
}

Now I want to create RegularOctagon class extending the upper one

public class RegularOctagon extends GeneralRegularPolygon {

    public RegularOctagon(Vertex2D center, int numEdges, double edgeLength) {
        super.getCenter() = center;
        super.getNumEdges() = numEdges;
        super.getEdgeLength() = edgeLength;
    }
}

But this brings an error: constructor cannot be applied to given types; required Vertex2D,int, double - found: no arguments; reason: actual and foral argument lists differ in length

Where is the problem here? And how should I get the class RegularOctagon to create a special instance of GeneralRegularPolygon?

Thank you for any tips.

[eRaaaa]

public RegularOctagon(Vertex2D center, int numEdges, double edgeLength) {
super(center, numEdges, edgeLength);
}

Please read Inheritance (The Java™ Tutorials > Learning the Java Language > Interfaces and Inheritance) again

[dawnMist]

And what if I need to inherit only center and edgeLength? Because clearly the numEdges = 8 for octagon.
When I try to write another constructor with two parameters:

private Vertex2D center;
private double edgeLength;

public RegularOctagon(Vertex2D center, double edgeLength) {
     this.center = center;
     this.edgeLength = edgeLength;
}

then I get the above error.

[SJF]

By default super(); is called even if you don't tell it to!

Call the super constructor you want as the first line of your constructor.

super(center, 8, edgeLength);

Enumerated (a bit)

public class Sandbox {
	
	public static class Joe{
		public int myX;
		public int myY;
		
		public Joe(int x, int y){		// Joe's only constructor
			myX = x;
			myY = y;
		}
	}
	
	public static class Jeff extends Joe{
		public Jeff(int x){
			super(x, 3);		//basically, Jeff = new Joe(x, 3)
		}
	}

	public static void main(String[] args) {

		Jeff j1 = new Jeff(1);			//new Jeff

		System.out.print(j1.myY);		//new Jeff's myY = 3!
	}
}

[dawnMist]

Ok, thanks for this, now I get it.
My question is - is there any way of writing the constructor with only two parameters?
Because I am supposed to (the test class used to check my work tests it like this "RegularOctagon s = new RegularOctagon(new Vertex2D(1.0, -1.0), 2.0);" // Vertex2D is my class representing a point via X and Y coordinates) and I cannot see a way how...

[SJF]

Yes.

Look at the above example, Jeff only has one parameter in its only constructor, whereas Joe has two. ....

Jeff extends Joe

super(x, 3) in Jeff calls the Joe(x, y) constructor!

[dawnMist]

Of course! Thank you very much, you´ve been very helpful!

The last thing for now - how can I inherit private fields?

public class GeneralRegularPolygon implements RegularPolygon, Colored
{
    private Vertex2D center;
    private double radius;
 
    private int numEdges;
    private double edgeLength;
    private String color = "white";   
    .
    .
    .
}

public class Circle extends GeneralRegularPolygon {

   public static final int NUM_EDGES = Integer.MAX_VALUE;
 
   public Circle(Vertex2D center, double radius) {
        super(center, NUM_EDGES, radius);
    }
    
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

The error here says that radius has private access in GeneralRegularPolygon.
How so? Why can´t I use these private fields?

[SJF]

You're going to need setters and getters for private variables.

   public double getRadius(){
        return radius;
   }

etc.

(You may also want to look at your super() call from Circle constructor, you're setting the edgeLength in GeneralRegularPolygon to the input radius.)

[dawnMist]

Thanks for that, I corrected it.

Now I have this piece of code:

public class GeneralRegularPolygon {
    private Vertex2D center;
    private double radius;
    private int numEdges;
    private double edgeLength;

    public double getRadius() {
        return (edgeLength)/(2 * Math.sin(Math.PI/numEdges));
    }

    public double getHeight() {
        return 2 * getRadius();
    }
    
    public double getWidth() {
        return 2 * getRadius();
    }
}

public class Circle extends GeneralRegularPolygon {
    private double radius;

    public double getRadius() {
        return this.radius;
    }
}

and I want the Circle class to use the getHeight() and getWidth() methods from class GeneralRegularPolygon, but with the radius value of the class Circle. I cannot write these two methods again in the Circle class.
Is there anyway how to do this?

[eRaaaa]

?? You don`t need more methods? You don`t have to write methods, you can just call the getHeight and getWidth methods on a circle instance ?!

[SJF]

Your circle class should contain the radius, and @Override the getHeight() and getWidth() from it's parent class.

The Polygon class should not have a radius! For that matter, is a circle a polygon? It has one side! (Two if you say inside and outside, but that's a different discussion).