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:

class Mammal
   def breathe
     puts "Inhale and exhale"
   end
 end

class Cat<Mammal
   def speak
     puts "Meow"
   end
end

tama = Cat.new
tama.breathe
tama.speak

Output

Inhale and exhale
Meow

Though we didn't specify how a Cat should breathe, every cat will inherit that behavior from the Mammal class since Cat was defined as a subclass of Mammal. (In OO terminology, the smaller class is a subclass and the larger class is a superclass.) Hence from a programmer's standpoint, cats get the ability to breathe for free; after we add a speak method, our cats can both breathe and speak.

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.

class Bird
   def preen
     puts "I am cleaning my feathers."
   end
   def fly
     puts "I am flying."
   end
 end

class Penguin<Bird
   def fly
     fail "Sorry. I'd rather swim."
   end
end

bird = Bird.new
bird.preen
bird.fly
puts("\nPenguin time!")
bird.preen
penguin = Penguin.new
penguin.fly

Output

I am cleaning my feathers.
I am flying.

Penguin time!
I am cleaning my feathers.

Note that the message "I am cleaning my feathers." was generated twice (once by the call to bird.preen and once by the call to penguin.preen, but the message "Sorry. I'd rather swim." did not appear as part of the output. We will discuss exception processing later.

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.