在装饰器对象上调用未装饰的方法

Question

Suppose we have implemented the Decorator Pattern as in the image below. With the CondimentDecorator class, the Beverage class can be decorated. The methods cost() and getDescription() will take care for that.

When creating a decorated Beverage like below, calling the method getName() won't work on the decorated object. In order to make this work, this method should also be placed in the class CondimentDecorator and delegate the call to the composed beverage variable.

Beverage b = new Milk(new Espresso("A very nice espresso"));
b.getName() // returns null

When having a lot of methods which don't need decoration, all of these should also be placed in CondimentDecorator to simply delegate. So the question is: What is a neat (generally accepted) way to solve this 'problem'?

Answer 1

The generally accepted approach to this 'problem' in a statically typed language like Java or C# (yes, C# is not quite statically typed, I know about DLR, and dynamic) is to have the decorated methods delegate to the decorated object explicitly . This solution has the obvious benefits of simplicity and explicitness .

In your example, if you don't override getName it means that you want to change the behaviour of the decorated object, which is exactly the purpose of the decorator pattern.

If you would choose to delegate to the decorated object by default then it would appear as if inheritance was broken, since CondimentDecorator inherits from Beverage but does not use the methods of base class.

Then, to change this behaviour, you would somehow need to define the methods that are not delegated to decorated object, but should work "as usual inheritance". That would be not obvious and very messy.

That being said, what you want to achieve should be possible in Java.

For instance in .NET you could do this in the following ways, and I believe Java has something similar

1. By generating dynamic runtime proxies that use inheritance to dynamically modify the behaviour of the object, see Castle Dynamic Proxy
2. By using a MSIL rewriting technique that rewrites your itermediate code at compile time to modify it, for example see Fody - Method decorator

Additionally, check out how prototypical inheritance works in JavaScript - that behaviour is very similar to what you're trying to achieve

Answer 2

In your case if only few operation1() it is ok to add a delegate call to CondimentDecorator. If only few call() needs decoration then could make a decorator when needed, like: DecoratorFactory.make(new Milk(new Espresso("A very nice espresso"))).getDescription();

or

CondimentDecorator.getDescription(new Milk(new Espresso("A very nice espresso")));

Key: patterns serve you.