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javadesign-patternsdecorator
Atrus
Atrus

Reputation: 798

Decorator Pattern with decorator specific methods in Java

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?

Upvotes: 6

Views: 1709

Answers (3)

Dave Newton
Dave Newton

Reputation: 160191

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.

Upvotes: 9

AlexR
AlexR

Reputation: 115328

Decorator pattern by definition does not allow adding methods other than those that are defined in the interface. Actually you can always add methods to any class but once these methods are not defined in the implementing interface client cannot call them using this interface.

Simple solution to your problem is to define several interfaces, e.g. Coffee and Milk. Then you can define class Capuchino that implements both interfaces and probably holds 2 instances: SimpleCofee and FoamedMilk. But this solution looks more as a combination of Decorator and Facade.

Upvotes: 3

Thomas
Thomas

Reputation: 88707

The decorators can have any number of specific methods. You might introduce a decorator specific interface that decorators implement in addition to implementing the decorated interface.

However, someone has to know those methods exist, thus either the decorator has to call the method internally, the caller has to know it uses a decorator instead of the decorated object (you could test using instanceof) or use reflection to check for those methods.

Upvotes: 4

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