Any good examples of inheriting from a concrete class?

By | July 30, 2019

Question

Background:

As a Java programmer, I extensively inherit (rather: implement) from interfaces, and sometimes I design abstract base classes. However, I have never really felt the need to subclass a concrete (non-abstract) class (in the cases where I did it, it later turned out that another solution, such as delegation would have been better).

So now I’m beginning to feel that there is almost no situation where inheriting from a concrete class is appropriate. For one thing, the Liskov substitution principle (LSP) seems almost impossible to satisfy for non-trivial classes; also many other questions here seem to echo a similar opinion.

So my question:

In which situation (if any) does it actually make sense to inherit from a concrete class?
Can you give a concrete, real-world example of a class that inherits from another concrete class, where you feel this is the best design given the constraints? I’b be particularly interested in examples that satisfy the LSP (or examples where satisfying LSP seems unimportant).

I mainly have a Java background, but I’m interested in examples from any language.

Solution

You often have a skeletal implementations for an interface I. If you can offer extensibility without abstract methods (e.g. via hooks), it is preferable to have a non-abstract skeletal class because you can instantiate it.

An example would be a forwarding wrapper classes, to be able to forward to another object of a concrete class C implementing I, e.g. enabling decoration or simple code-reuse of C without having to inherit from C. You can find such an example in Effective Java item 16, favor composition over inheritance. (I do not want to post it here because of copyrights, but it is really simply forwarding all method calls of I to the wrapped implementation).

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