Question

I need a single collection of “processor” classes (I happen to be using Java) that can operate on a concrete shape, say a circle. Some of the classes can operate on the abstract shape, while others require the concrete circle class.

The problem is that, I don’t know of any language or design mechanism I can use to avoid adapter classes like CircleSaver. If I can’t avoid this, I’ll be creating an adapter method for each processor class that requires a concrete shape.

Is there a name for this problem / pattern? Is there a solution to it?

List<CircleProcessor> circleProcessors = new ArrayList<CircleProcessor>
 (Arrays.asList(new CircleDrawer(), new CircleSaver()));

interface ShapeProcessor {
    void execute(Shape circle);
}

interface CircleProcessor {
    void execute(Circle circle);
}

class CircleDrawer implements CircleProcessor {
    public void execute(Circle circle) {
        // Draw circle
    }
}

class ShapeSaver implements ShapeProcessor {
    public void execute(Shape shape) {
        shape.save();
    }
}

// Adapter method that I would love to avoid
class CircleSaver implements CircleProcessor {
    public void execute(Circle circle) {
        new ShapeSaver().execute(circle);
    }
}

Answer 1

After asking the question, I think I’ve come up with a much better version that uses generics. If there’s a better solution and/or a name for this problem, please let me know!

    List<ShapeProcessor<Circle>> circleProcessors = new ArrayList<ShapeProcessor<Circle>>(
 Arrays.asList(new CircleDrawer(), new ShapeSaver<Circle>()));

public interface ShapeProcessor<T extends Shape> {
    void execute(T t);
}

public class CircleDrawer implements ShapeProcessor<Circle> {
    @Override
    public void execute(Circle circle) {
        // Draw circle
    }
}

public class ShapeSaver<T extends Shape> implements ShapeProcessor<T> {
    @Override
    public void execute(T t) {
        t.save();
    }
}