When to use composition and when to use inheritance

Question

Is there a way of telling when to use composition and when to use inheritance in your code.

Example: Let's say we have 4 classes called Computer, MotherBoard, Monitor, Keyboard. I can make computer inherit from the other three classes and I can also declare MotherBoard, Monitor, Keyboard in the field of computer, thus for the second case using composition. Which way is better? Which way is preferred by most developers?

Answer 1

I prefer the inheritance in the following cases:

• Both classes are in the same logical domain
• The subclass is a proper subtype of the superclass
• The superclass’s implementation is necessary or appropriate for the subclass
• The enhancements made by the subclass are primarily additive.

Otherwise, the composition is appropriate.

Note that in this case, the computer is composed of the components you have listed. As a shortcut, I sometimes ask myself the following:

• Does Computer have a Mouse? If so, then use composition. Notice a computer can have no mouse.
• Is MacBook a Computer? If so, then use inheritance.

... but in general which way is preferred

In general, the decision should be based on the situation :)