Should I be using inheritance?

Question

This is more of a subjective question, so I'm going to preemptively mark it as community wiki.

Basically, I've found that in most of my code, there are many classes, many of which use each other, but few of which are directly related to each other. I look back at my college days, and think of the traditional class Cat : Animal type examples, where you have huge inheritance trees, but I see none of this in my code. My class diagrams look like giant spiderwebs, not like nice pretty trees.

I feel I've done a good job of separating information logically, and recently I've done a good job of isolating dependencies between classes via DI/IoC techniques, but I'm worried I might be missing something. I do tend to clump behavior in interfaces, but I simply don't subclass.

I can easily understand subclassing in terms of the traditional examples such as class Dog : Animal or class Employee : Person, but I simply don't have anything that obvious I'm dealing with. And things are rarely as clear-cut as class Label : Control. But when it comes to actually modeling real entities in my code as a hierarchy, I have no clue where to begin.

So, I guess my questions boil down to this:

1. Is it ok to simply not subclass or inherit? Should I be concerned at all?
2. What are some strategies you have to determine objects that could benefit from inheritance?
3. 始终基于行为(接口)而不是实际类型继承是否可接受？

Answer 1

Inheritance should always represent an "is-a" relationship. You should be able to say "A is a B" if A derives from B. If not, prefer composition. It's perfectly fine to not subclass when it is not necessary.

For example, saying that FileOpenDialog "is-a" Window makes sense, but saying that an Engine "is-a" Car is nonsense. In that case, an instance of Engine inside a Car instance is more appropriate (It can be said that Car "is-implemented-in-terms-of" Engine).

For a good discussion of inheritance, see Part 1 and Part 2 of "Uses and Abuses of Inheritance" on gotw.ca.