Decorator Pattern with decorator specific methods in Java

Question

Using the coffee decorator example shown on Wikipedia ( http://en.wikipedia.org/wiki/Decorator_pattern) how would it be possible for someone to be able to have methods that only the decorators have, for example, the milk decorator could have a method called "fatContent". Is this even possible with this type of design pattern? If not, what kind of pattern could I use to accomplish this?

Answer 1

You could, but you'd need to know the type in order to actually call the method (assuming no reflection etc.) if it doesn't match the type you're passing around.

Types determine what's known at compile time: if CoffeeDecorater doesn't include a fatContent signature, nothing receiving a CoffeeDecoractor knows the method exists.

You could create an additional interface, but you'd need to either know that it existed so you could check for it (instanceof), or interrogate the class to check for a specific signature (reflection).

Java either knows a method exists at compile time, or checks for it at runtime.