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Implementing Inheritance 373

12

The Animal Kingdom ....................................................................................

To facilitate the discussion of derivation and inheritance, this chapter focuses on the rela-

tionships among a number of classes representing animals. You can imagine that you

have been asked to design a children’s game—a simulation of a farm.

In time, you will develop a whole set of farm animals, including horses, cows, dogs, cats,

sheep, and so forth. You will create methods for these classes so that they can act in the

ways the child might expect, but for now you’ll stub-out each method with a simple print

statement.

Stubbing-out a function means you’ll write only enough to show that the function was

called, leaving the details for later when you have more time. Please feel free to

extend the minimal code provided in today’s lesson to enable the animals to act more

realistically.

You should find that the examples using animals are easy to follow. You also find it easy

to apply the concepts to other areas. For example, if you were building an ATM bank

machine program, then you might have a checking account, which is a type of bank

account, which is a type of account. This parallels the idea of a dog being an mammal,

which in turn is an animal.

FIGURE12.1
Hierarchy of animals.

Animal

Mammal Reptile

Horse Dog

Retrievers Spaniels

Labradors Goldens

Working Sporting Terriers