Union design pattern: inheritance and polymorphism  (Page 2/4)

Cats, monkeys and whales, while diverse creatures, are all mammals. Hence to model such a system in the computer, it makes sense to make Cat , Monkey and Whale all subclasses of an abstract Mammal superclass. Each species has many behaviors (methods) but I will only concentrate on 3 in particular:

• boolean givesMilk() : returns true if the animal can give milk to feed its young, false otherwise
• String makeSound() : returns a String represenation of a common sound the animal makes.
• boolean givesLiveBirth() : returns true if the animal bears live young.

In the table below are the methods and what happens when each species executes that method:

We could start out with the following class implemenation ( Mammal0.java ):

Let's start our analysis:

• A mammal is defined by the fact that it gives milk to feed its young. It is thus not surprising that all the givesMilk() methods in the subclasses return true. The givesMilk() method is a prime candidate for "hoisting" up into the Mammal superclass ("hoisting" = moving the method upwards from the subclass to the superclass).
• makeSound() returns a different value for each species, but intrisically, we expect any animal, which includes mammals, to be able to make some sort of sound. Thus Mammals should have a makeSound() method, but since, at the Mammals level, we don't know exactly how that sound will be made, the method at that level must be abstract. The makeSound() method at the concrete Cat , Monkey and Whale level however, would be concrete because each animal makes its own unique sound.
• givesLiveBirth() returns exactly the same value for all of our rather diverse selection of animals here. It seems like a fine candidate for hoisting. Or is it....? Let's go ahead an hoist it anyway.

This is what we have so far ( Mammal1.java ):

Before we go charging ahead, let's stop for a moment and review what we've done: Cats, monkeys, and whales do represent a wide spectrum of mammals, but remember, the abstract Mammal class is a representation of ALL mammals, not just the ones we have so far. The correlation of like behavior with all our represented animals does not imply its inclusion in their abstract representation!

For instance, one day, in our wanderings through Australia, we encounter a Duckbilled Platypus . Let's see how it behaves with respect to our 3 methods:

Duckbilled platypus lay eggs!!

Giving live birth is not part of the definition of a mammal. On the other hand, the question of whether or not the animal gives live birth can always be asked of any animal, including all mammals. The result may be true or false however, so the method must be abstract at the Mammal level.