Classes of `animal-shelter' application

The classes in figure fig:shelter can be briefly described as follows:

• base-object-class
  An object of the 'base-object-class' is used as an error handler. If no object, neither from a derived class nor from a super class is capable of handling a message sent to an object, then the message will be passed on to an instance of the 'base-object-class' which would have to handle the error.

• animal-shelter
  This is the main class of the application describing objects that represent a home for animals, being able to accept animals and to dismiss them. An instance of this class also responds to the message find, which can be used to search for an object of type animal, and to the object display, which makes the object display itself, i.e. to display all inhabitants of the shelter.

  As attributes we find a container object for the animals implemented as a list and a slot for a base-object needed to answer unknown messages sent to the object.

• animal
  This class is a super class or base class, which means a class, from which other classes are derived, like `cat' and `dog' in our example.

  The method `who-are-you' is something like a virtual method, functioning as a reminder that a real `who-are-you' method has to be implemented in the derived classes. If a derived class would not provide a real `who-are-you' method, the virtual method would issue an error message, if the `who-are-you' message was sent to the instance of a derived class.

  The `get-name' method is a real method inherited by all derived classes. Thus `get-name' does not have to be explicitely defined in the classes derived from class `animal'.

• dog
  The class `dog' is derived from class `animal' and has to implement the method `who-are-you'. The attribute `super-class' contains an instance of class `animal' providing the upward link in the chain for delegation purposes.
• cat
  The class `cat' is identical to class `dog' with one exception: the `who-are-you' method is implemented differently than in class `dog'.

Georg P. Loczewski 2004-03-05

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