Lab 5: Exercise 2

Delegation

solidCuboid.mass()
      ↓
SolidCuboid receives the call
      ↓
SolidCuboid forwards the request
      ↓
SolidImpl.mass()
      ↓
result returned back

SolidCuboid, SolidSphere, and SolidImpl all implements Solid. It’s just that SolidCuboid and SolidSphere both have a mass() method. So they outsource that to SolidImpl.

SolidImpl IS-A Shape3D SolidImpl HAS-A Shape3D This is a common pattern for delegation.

More on delegation

Solid (Interface)

interface Solid {
    double mass();
}

SolidImpl (Class) (Shared Implementation with SolidSphere to Solid)

class SolidImpl implements Solid {
    private final Shape3D shape;
    private final double density;

    SolidImpl(Shape3D shape, double density) {
        this.shape = shape;
        this.density = density;
    }

    @Override
    public double mass() {
        return shape.volume() * density;
    }
}

SolidSphere (Class) (Shared Implementation with SolidImpl to Solid)

class SolidSphere extends Sphere implements Solid {
    private final SolidImpl solid;

    SolidSphere(double radius, double density) {
        super(radius);
        this.solid = new SolidImpl(this, density);
    }

    @Override
    public double mass() {
        return solid.mass();
    }

    @Override
    public String toString() {
        return "solid-" + super.toString() +
               " with a mass of " +
               String.format("%.2f", mass());
    }
}

Explanation So i needed to outsource the work to SolidImpl class, That’s why my mass() is just a simple solid.mass() I still defined mass() in solidcuboid because I implemented Solid (an interface with the mass() function) I implemented solid because I wanted to implement the same thing that the SolidImpl class implemented.