Decorator design pattern using inheritance vs interfaces

Question

I'd like to implement the Decorator design pattern using inheritance (Decorator extends Component) because i need access to the protected fields/methods of the Component class.

The problem is that the Component class represents an algorithm, it performs some preprocessing upon construction and holds a significant amount of data. Now each time I'll be decorating a Component I'll create a new Decorator instance which will require the construction of a new (useless) Component instance performing unneeded computations and holding unneeded data.

I wanted to use interfaces instead of inheritance but then I wont be able to access Component's protected information.

Am i right to worry about the waste of resources when extending the Component class? And if so, how can i avoid it without losing access to the information i need?

One final note: I could create the Decorator instance supplying it with "dummy" data so that it will perform minimal amount of computation but this solution feels messy.

Thank you.

Answer 1

I'm not sure if this would count as the decorator pattern at all actually. Sounds more like plain old inheritance.

Am i right to worry about the waste of resources when extending the Component class?

Obviously depends on how much resources you're wasting.

And if so, how can i avoid it without losing access to the information i need?

You could perhaps "open up" Component by extending it and adding methods for accessing the protected parts you need. Then use interfaces and composition to implement a decorator for the this new class.