Inheritance over composition

Question

The benefits of using composition over inheritance are quite well known;

What are the cases in which the opposite is preferable?

Practically, I can see the advantage of forcing a base constructor, but I would like to know other people's opinion about other cases/domains.

Answer 1

I believe the famous recommendation of "favor composition over inheritance" was coined in the GoF Design Patterns book.

It says (p.20):

Favor object composition over class inheritance.

Ideally, you shouldn't have to create new components to achieve reuse. You should be able to get all the functionality you need just by assembling existing components through object composition. But this is rarely the case, because the set of available components is never quite rich enough in practice. Reuse by inheritance makes it easier to make new components that can be composed with old ones. Inheritance and object composition thus work together.

Nevertheless, our experience is that designers overuse inheritance as a reuse technique, and designs are often made more reusable (and simpler) by depending more on object composition. You'll see object composition applied again and again in the design patterns.

Notice that this statement refers to class inheritance, and must be distinguished from interface inheritance which is fine.

Dynamism

Both are ways to achieve reusability, but the advantage of composition over inheritance is dynamism. Since the composition can be changed dynamically at runtime this represents a great advantage, whereas inheritance is statically defined at compile time.

Encapsulation

Also, composition is based on using the public interfaces of the composed objects, therefore objects respect each other's public interfaces and therefore this fosters encapsulation. On the other hand, inheritance breaks encapsulation since child components typically consume a protected interface from the parent. It is a well known problem that changes in the parent class can break the child classes, the famous base class problem. Also in inheritance parent classes define the physical representation of subclasses, therefore child clases depend on parent classes to evolve.

Cohesion

Another advantage of composition is that it keeps classes focused on one task and this foster cohesion as well.

Liabilities

Evidently a problem with composition is that you will have more objects and fewer classes. That makes a little more difficult to visualize your design and how it achieves its goals. When debugging code it is harder to know what is going on unless you know what exact instance of a given composite is currently being used by an object. So composition makes designs a bit harder to understand in my opinion.

Since the advantages of composition are multiple that's why it is suggested to favor it over inheritance, but that does not mean inheritance is always bad. You can achieve a great deal when inheritance is properly used.

Interesting References

I would suggest a study of GoF Design Patterns to see good examples of both types of reusability, for instance a Strategy Pattern that uses composition vs a Template Method that uses inheritance.

Most of the patterns make a great use of interface inheritance and then object composition to achieve their goals and only a few use class inheritance as a reusability mechanism.

If you want to delve more the book Holub on Patterns, on chapter 2 has a section called Why extends is Evil that delve much more on the liabilities of class inheritance.

The book mentions three specific aspects

• Losing Flexibility: The first problem is that explicit use of a concrete-class name locks you into a specific implementation, making down-the-line changes unnecessarily difficult.
• Coupling: A more important problem with implementation inheritance is coupling, the undesirable reliance of one part of a program on another part. Global variables are the classic example of why strong coupling is bad. If you change the type of a global variable, for example, all the code that uses that variable—that is coupled to the variable—can be affected, so all this code must be examined, modified, and retested. Moreover, all the methods that use the variable are coupled to each other through the variable. That is, one method may incorrectly affect the behavior of another method simply by changing the variable’s value at an awkward time. This problem is particularly hideous in multithreaded programs.
• Fragile-Base-Class Problem: In an implementation-inheritance system (one that uses extends), the derived classes are tightly coupled to the base classes, and this close connection is undesirable. Designers have applied the moniker “the fragile-base-class problem” to describe this behavior. Base classes are considered “fragile” because you can modify a base class in a seemingly safe way, but this new behavior, when inherited by the derived classes, may cause the derived classes to malfunction.

Answer 2

The only advantage of inheritance over composition that I can think of is that it can potentially save you from a lot of boiler plate method delegation.

If you truly have an is-a relationship and you simply want all the methods from a base class in your subclass, then inheritance gives you all those methods for free.