Smalltalk and Object Orientation: an Introduction - Free

12. An Object Oriented Organizer
12.1 Introduction
This chapter presents a detailed example application constructed using the collection classes. The
Organizer is in tended as an electronic personal Organizer. It therefore possesses an address book, a
diary (or appointments section) and a section for notes. The remainder of this chapter describes one way
of implementing such an Organizer. At the end of this chapter is a programming exercise for you the
reader.
12.2 The Organizer
This example involves more than one class and has a more complex architecture than anything you have
seen so far. The architecture is illustrated in Figure 12.1. This example illustrates another important
concept in object orientation, that of an object within an object. These are often referred to as part-of
hierarchies, i.e. one object is part-of another. This should not be confused with the cla ss hierarchy
which is a kind-of hierarchy.
anOrganizer
addressBook
Dictionary
notes
Bag
appointments
Dictionary
Figure 12.1: The structure of anOrganizer Object
As illustrated in Figure 12.1 an instance of the Organizer class contains three other objects.
These objects are held in the instance variables addressBook, appointments and notes. The
instances within addressBook and appointments are dictionary objects, while the notes
instance variable holds a bag object.
12.3 The class definition
We shall put all Organizer class in a class category called Organizer Class. As illustrated in the
above figure the class definition possesses three instance variables, addressBook, appointments
and notes. Figure 12.2 illustrates the class definition.
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