Adapter design pattern java

Our goal is to put a roung peg in a 5 cm radius round hole, two Square holes with widths 2 cm and 20 cm. To our orject we add three packages: adapters, round and square. We start with our RoundPeg and we place it in the round package. RoundPegs are compatible with RoundHoles.

public class RoundPeg {
    private double radius;

    public RoundPeg() {}

    public RoundPeg(double radius) {
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }
}

RoundHoles are compatible with RoundPegs. To the round package we also add the class RoundHole

public class RoundHole {
    private double radius;

    public RoundHole(double radius) {
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

    public boolean fits(RoundPeg peg) {
        boolean result;
        result = (this.getRadius() >= peg.getRadius());
        return result;
    }
}

The SquarePegs are not compatible with RoundHoles. But we have to integrate them into our program. To the square package we add the following:

public class SquarePeg {
    private double width;

    public SquarePeg(double width) {
        this.width = width;
    }

    public double getWidth() {
        return width;
    }

    public double getSquare() {
        double result;
        result = Math.pow(this.width, 2);
        return result;
    }
}

Now let's create a SquarePegAdapter class and put it in the adapters package.

import TheRayCode.adapter.example.round.RoundPeg;
import TheRayCode.adapter.example.square.SquarePeg;

public class SquarePegAdapter extends RoundPeg {
    private SquarePeg peg;

    public SquarePegAdapter(SquarePeg peg) {
        this.peg = peg;
    }

    @Override
    public double getRadius() {
        double result;
        // Calculate a minimum circle radius, which can fit this peg.
        result = (Math.sqrt(Math.pow((peg.getWidth() / 2), 2) * 2));
        return result;
    }
}

We now put this all together in the main method in our Demo class.

import TheRayCode.adapter.example.adapters.SquarePegAdapter;
import TheRayCode.adapter.example.round.RoundHole;
import TheRayCode.adapter.example.round.RoundPeg;
import TheRayCode.adapter.example.square.SquarePeg;

public class Demo {
    public static void main(String[] args) {
        // Round fits round, no surprise.
        RoundHole hole = new RoundHole(5);
        RoundPeg rpeg = new RoundPeg(5);
        if (hole.fits(rpeg)) {
            System.out.println("Round peg r5 fits round hole r5.");
        }

        SquarePeg smallSqPeg = new SquarePeg(2);
        SquarePeg largeSqPeg = new SquarePeg(20);
        // hole.fits(smallSqPeg); // Won't compile.

        // Adapter solves the problem.
        SquarePegAdapter smallSqPegAdapter = new SquarePegAdapter(smallSqPeg);
        SquarePegAdapter largeSqPegAdapter = new SquarePegAdapter(largeSqPeg);
        if (hole.fits(smallSqPegAdapter)) {
            System.out.println("Square peg w2 fits round hole r5.");
        }
        if (!hole.fits(largeSqPegAdapter)) {
            System.out.println("Square peg w20 does not fit into round hole r5.");
        }
    }
}

Let's compile and run we should get:

Round peg r5 fits round hole r5.
Square peg w2 fits round hole r5.
Square peg w20 does not fit into round hole r5.

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