Decorator design pattern with Composite Design pattern

Question

I understand the composite design pattern and the decorator design pattern and their uses. Can we have a case where we have to use them together? How would the class diagram for such a scenario look like?

Will we have the decorator(from Decorator design pattern), leaf nodes, and composite(from Composite design pattern) inherited from the Component? Here is what I am referring to for the decorator pattern:wiki link for decorator pattern and for composite pattern: composite wiki link

Answer 1

Yes, the decorator pattern is often used together with a composite pattern. Here is what GoF says:

Decorator is often used with Composite. When decorators and composites are used together, they will usually have a common parent class. So decorators will have to support the Component interface (...)

The reason is that the composite pattern's goal is to let:

clients treat individual objects and compositions of objects uniformely.

Combining the two allows to add features and responsibilities to composites and leafs with decorators, thus using composition over inheritance. The main benefits are:

• avoiding the drawbacks of the less flexible inheritance
• avoiding the drawbacks of repeating implementing the common interface
• enabling reuse of the same decorator for both composites and leaves, if the goal is to add common responsibilities

The result would look like:

Interestingly, the decorator pattern looks graphically similar to the composite, which may feel troubling. GoF explains this feeling:

A decorator can be viewed as a degenerate composite with only one component. However a decorator adds additional responsibilities (...)