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GNU Smalltalk User's Guide

4.3.2 Animals

Imagine that we have three kinds of objects, representing Animals, Parrots, and Pigs. Our messages will be eat, sing, and snort. Our first pass at inserting these objects into the Smalltalk hierarchy would organize them like:

Object Animals Parrots Pigs

This means that Animals, Parrots, and Pigs are all direct descendants of Object, and are not descendants of each other.

Now we must define how each animal responds to each kind of message.

Animals eat --> Say ``I have now eaten'' sing --> Error snort --> Error Parrots eat --> Say ``I have now eaten'' sing --> Say ``Tweet'' snort --> Error Pigs eat --> Say ``I have now eaten"'' sing --> Error snort --> Say ``Oink''

Notice how we kept having to indicate an action for eat. An experienced object designer would immediately recognize this as a clue that we haven't set up our hierarchy correctly. Let's try a different organization:

Object Animals Parrots Pigs

That is, Parrots inherit from Animals, and Pigs from Parrots. Now Parrots inherit all of the actions from Animals, and Pigs from both Parrots and Animals. Because of this inheritance, we may now define a new set of actions which spares us the redundancy of the previous set:

Animals eat --> Say ``I have now eaten'' sing --> Error snort --> Error Parrots sing --> Say ``Tweet'' Pigs snort --> Say ``Oink''

Because Parrots and Pigs both inherit from Animals, we have only had to define the eat action once. However, we have made one mistake in our class setup--what happens when we tell a Pig to sing? It says "Tweet", because we have put Pigs as an inheritor of Parrots. Let's try one final organization:

Object Animals Parrots Pigs

Now Parrots and Pigs inherit from Animals, but not from each other. Let's also define one final pithy set of actions:

Animals eat --> Say ``I have eaten'' Parrots sing --> Say ``Tweet'' Pigs snort --> Say ``Oink''

The change is just to leave out messages which are inappropriate. If Smalltalk detects that a message is not known by an object or any of its ancestors, it will automatically give an error--so you don't have to do this sort of thing yourself. Notice that now sending sing to a Pig does indeed not say "Tweet"---it will cause a Smalltalk error instead.