Smalltalk and Object Orientation: an Introduction - Free

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Traffic Light
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Controller
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Figure 17.16: Example composites
A class may define a pattern of objects and links that are part of it and that exist whenever the class
is instantiated. Such a class is called a composite. It is a class that contains an object diagram. It may be
thought of as an extended form of aggre gation where the relationships among the parts are valid only
within the composite. A composite is a kid of pattern or macro that represents a conceptual clustering
for a given purpose. Composition is shown by drawing a class box around its embedded components (as
illustrated in Figure 17.16) which are prototypical objects and links. That is, a composite defines a
context in which references to classes and associations, defined elsewhere, can be used.
17.5 Packages
Packages a re used to group associated modeling elements such as classes in the object model (or
subsystems in component diagrams). They are drawn as tabbed folders as illustrated in Figure 17.17.
Figure 17.17 actually illustrates four packages called clients, Business model, Persistent store, Bank
and Network. In this particular diagram the contents of Clients, Persistent Store, Bank and Network have
been suppressed (by convention these packag es have their names placed in their body) with only
Business Model being shown in detail (with its name in the top tab). This package possesses two classes
Customer and Account as well as a nested package Bank. The dashed lines between the packages
illustrate dependencies between packages. For example, the package Clients directly depends on the
packages Business Model and Network (this actually means that at least one element in the package
Clients relies on at least one element in the other two packages).
Clients
Business Model
Customer
Bank
Account
Network
Persistent Store
Figure 17.17: Packages with Dependencies
Classes may belong to exactly one package but references may be made to classes in other packages.
Such references have the package name foll owed by the class name (separated by two colons), for
example: Business Model :: Customer.
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