in a dilemma about which to use decorator vs bridge pattern

Question

I am confused about which pattern to use to design the following scenario,

Interface GearBox {
   int upshift();
   int downshift();
   int reverse();
}

AutoGearBox implements GearBox{...}
ManualGearBox implements GearBox{...}

Now I want to add DualClutchGearBox to the hierarchy.All previous gearboxes are single clutch. How do I go about doing it?

With Decorator -->

DualClutchDecorator implements GearBox{
     DualClutchDecorator(GearBox box){...}
}

With Bridge -->

GearBox{
   GearBoxImpl impl;
   ....
}

AutoGearBox implements GearBox{...}
ManualGearBox implements GearBox{...}

abstract class GearBoxImpl{}
SingleClutchImpl extends GearBoxImpl{...}
DualClutchImpl extends GearBoxImpl{...}

Which is one is better and why?

Answer 1

I'm not sure I'd use either of these patterns. Is there a reason why you don't want to just create a 3rd concrete class?

You use Decorator when you need to dynamically change behavior. One of the main examples I can think of is Java's InputStreamReader. I can compose a decorated reader for whatever case I need and they conform to the same interface

// I need to read lines from a file
Reader r = new BufferedReader(new InputStreamReader(new FileInputStream(file)));

// Or I want to read lines from a byte array and track the line numbers
Reader r = new LineNumberReader(new InputStreamReader(new ByteArrayInputStream(bytes)));

So the idea of decorator is that I can change the behavior at runtime by adding decorators. This is not what you are trying to do from what I understand. A DualClutch will exhibit a specific behavior, there will be no need to change it on the fly.

I don't see a great case from Bridge either but I guess it depends on your specific case. Like I said, it seems like DualClutch will just have static behavior and an automobile will either have it or not. Seems like a simple concrete class would do the trick.

Answer 2

Decorator has to match with the interface from the object that want to decorate. With this in mind you can add addtitional behaviour to your decorated object without violating the interface. Keep in mind that the interface of the decorator can deliver extra functionality to your object.

The bridge on the other hand doesn't have this limitation. The interface that is facing the client can be different from the component that is underlying and is providing the implementation. It forms a bridge between the interface of the client and the actual implementation.

Answer 3

It depends.

Is your Interface defining the interface from the driver's point of view? Are you trying to abstract out common features of gearboxes (the ability to change gear!)?

Bear in mind that auto and manual gearboxes don't have precisely the same features in reality, so might not share the same interface anyway (For example, you can block-shift directly from 5th to 3rd in a manual gearbox, but not on most autos - and autos may have a kickdown feature that isn't needed on a manual gearbox).

Conversely, dual-clutch gearboxes may be indistinguishable from single clutch gearboxes in terms of their interface, differing only in implementation.