Our classification of objects in everyday life is naturally hierarchical. We know that all cats are mammals , and all mammals are animals . Smaller classes inherit characteristics from the larger classes to which they belong. If all mammals breathe, then all cats breathe.
We can express this concept in ruby:
ruby> class Mammal | def breathe | print "inhale and exhale\n" | end | end nil ruby> class Cat<Mammal | def speak | print "Meow\n" | end | end nil
Though we didn't specify how a
should breathe, every cat will inherit that behavior from the
was defined as a subclass of
. (In OO terminology, the smaller class is a
and the larger class is a
.) Hence from a programmer's standpoint, cats get the ability to breathe for free; after we add a
method, our cats can both breathe and speak.
ruby> tama = Cat.new #<Cat:0xbd80e8> ruby> tama.breathe inhale and exhale nil ruby> tama.speak Meow nil
There will be situations where certain properties of the superclass should not be inherited by a particular subclass. Though birds generally know how to fly, penguins are a flightless subclass of birds.
ruby> class Bird | def preen | print "I am cleaning my feathers." | end | def fly | print "I am flying." | end | end nil ruby> class Penguin<Bird | def fly | fail "Sorry. I'd rather swim." | end | end nil
Rather than exhaustively define every characteristic of every new class, we need only to append or to redefine the differences between each subclass and its superclass. This use of inheritance is sometimes called differential programming . It is one of the benefits of object-oriented programming.