Lecture of 8 January 1996

• How many (who are not CISE majors) have yet to apply for a CISE computer account? Beat the rush, apply for one now!
• On-line syllabus is now updated to include Honor Code information.

Thinking Object-Oriented (cont.)

• Review properties of object-oriented view of computation discussed in last class.
• OOP programs support a recursive approach to program design because of the aggregate nature of objects.
• Flower-sending to illustrate message passing to elicit appropriate object behavior:
  • Call Flo the florist and ask her to send flowers to grandmother Elsie. (Initiator, Message, Receiver, Responsibility, Method)
  • I don't care how Flo gets the job done, that's her business. (Information hiding)
  • What if I get someone else to send flowers to Elsie? (Receiver-dependent behavior).
  • What if I'm given the name of someone I don't know (by some reliable source) and I ask them to send flowers? In a program supporting variables to designate receivers, we may not know until run-time what specific method of delivery will be used. (Late-binding) Flo.sendFlowersTo (Elsie); // Florist method is used Terry.sendFlowersTo (Elsie); // Wife Terry's method is used X.sendFlowersTo (Elsie); // X's method (if it exists) is used
  • What kind of person is Flo? To what category does she belong? What does she have in common with other florists? What does she have in common with other shopkeepers What about with other humans? What about other animals? What about other material objects?

    Florist->Shopkeeper->Human->Mammal->Animal->MaterialObject

    There are many ways one can construct such a hierarchy of classes of objects.

    Potter->Artist->Human
    Dog->Mammal
    Dentist->Human
    Platypus->Mammal
    Flower->Plant->MaterialObject

  • An object is an instance of the classes to which it belongs. A class is a subclass or derived class of a more general class in a class hierarchy. A subclass inherits attributes from each superclass higher in the tree.

    A class is abstract if there are no entities belonging to that class that do not belong to a subclass as well.

  • Objects Belonging to these classes are distinct (and maintain their own state representations) however, the underlying form of objects belonging to the same class and their behaviors are similar (if not identical).

    Thus, in general, the objects is a repository for state and the class is a repository for behavior.

  • Platypus represents a case in which a subclass must override properties of a superclass. (Egg-laying vs. live birth for the Mammal category.)
• Review Properties of OOP concerning flower-sending example.