Choosing between composition and inheritance
So how do all these comparisons between composition and inheritance help you in your designs? Here are a few guidelines that reflect how I tend to select between composition and inheritance.
Make sure inheritance models the is-a relationship
My main guiding philosophy is that inheritance should be used only when a subclass is-a superclass. In the example above, an
An important question to ask yourself when you think you have an is-a relationship is whether that is-a relationship will be constant throughout the lifetime of the application and, with luck, the lifecycle of the code. For example, you might think that an
Don't use inheritance just to get code reuse
If all you really want is to reuse code and there is no is-a relationship in sight, use composition.
Don't use inheritance just to get at polymorphism
If all you really want is polymorphism, but there is no natural is-a relationship, use composition with interfaces.
So how do all these comparisons between composition and inheritance help you in your designs? Here are a few guidelines that reflect how I tend to select between composition and inheritance.
Make sure inheritance models the is-a relationship
My main guiding philosophy is that inheritance should be used only when a subclass is-a superclass. In the example above, an
Apple
likely is-a Fruit
, so I would be inclined to use inheritance. An important question to ask yourself when you think you have an is-a relationship is whether that is-a relationship will be constant throughout the lifetime of the application and, with luck, the lifecycle of the code. For example, you might think that an
Employee
is-a Person
, when really Employee
represents a role that a Person
plays part of the time. What if the person becomes unemployed? What if the person is both an Employee
and a Supervisor
? Such impermanent is-a relationships should usually be modelled with composition. Don't use inheritance just to get code reuse
If all you really want is to reuse code and there is no is-a relationship in sight, use composition.
Don't use inheritance just to get at polymorphism
If all you really want is polymorphism, but there is no natural is-a relationship, use composition with interfaces.
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