Different Types of Subclass Objects with One Superclass

Question

So I have a superclass GeoFig class, and two subclasses Cylinder and Sphere. I want to include one method in the superclass to calculate the volume(called setVolume), but each subclass has a different formula for calculating the volume. Through the subclass' constructors I will setVolume and will also getVolume. How do I do this?

Answer 1

My Java is very rusty, so consider any code here to be pseudo-code just to demonstrate the concepts...

You're not modeling the objects in code correctly. The code should match the real-world concepts being modeled. So let's consider those real-world concepts.

What is a GeoFig?

What does one look like? If you held one in your hand, what shape would it be? There's no answer to that, because it's not a concrete object. It's a conceptual or abstract object. So it should be an abstract class:

abstract class GeoFig { }

What attributes describe a GeoFig?

Does it have a length? A width? A radius? Not really, no. But for the purposes of being an object in 3-dimensional space we can assume that it has a volume. We just don't know how to calculate that volume:

abstract class GeoFig {
    abstract double getVolume();
}

Now we have our parent class.

What is a Cylinder?

It's a geometric object with a volume, so we can inherit from the parent class:

class Cylinder inherits GeoFig {
    public double getVolume() {
        return 0;
    }
}

How do we calculate the volume of a Cylinder?

π * r^2 * h

But we don't have r or h yet...

What attributes describe a Cylinder?

It has a height and a radius. In fact, it must have these to exist at all. So it requires them for object construction:

class Cylinder inherits GeoFig {

    private final double height;
    private final double radius;

    public Cylinder(double height, double radius) {
        this.height = height;
        this.radius = radius;
    }

    public double getHeight() {
        return this.height;
    }

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

    double getVolume() {
        return 0;
    }
}

(This assumes immutability. Make appropriate changes if you want to be able to change the dimensions of a Cylinder.)

Now we can also calculate the volume:

double getVolume() {
    return Math.PI * this.radius * this.radius * this.height;
}

Repeat the same logical process for any other shapes.