UML Weekend Crash Course™ - To Parent Directory

246
Sunday Morning
Project A7
Phase 1
role 1
directory
Receiving
role 2
subsystem
Order
tracking
role 3
model
Receiving
Order
tracking
Alternative notations
Figure 24-1 Three uses for packages: directories, subsystems, and models
In the second role, the package can represent a subsystem, like the Receiving subsystem
in Figure 24-1. A subsystem is a UML-defined stereotype that identifies a cohesive subset of
the total system. For example, the Inventory Control System might be organized into a
Receiving subsystem and a Shipping subsystem, among others.
Elements placed in a subsystem type of package are, by default, visible only within the
package. However, the visibility of individual model elements within the package may be
defined as public, private, or protected. Every subsystem package must have at least one
public interface (that is, at least one class with a public interface).
The third use of packages is called a model. A model is also a UML-defined stereotype,
similar to a subsystem in that it contains a cohesive set of elements of the system. The difference
is that the model focuses on a topic or type of behavior within the system. For
example, information about the Order Tracking topic of the third package in Figure 24-1 will
very likely appear in most of the Inventory Control subsystems. Also, because the model is
focused on one topic, it will not contain any system elements that do not help explain the
topic.
Packages Provide a Namespace
All these package types provide a separate namespace for the model elements contained
within them, including other packages. Naming elements within a package requires two
pieces of information: the element name and the element type. For example, a package may
contain something called Product of type Class and something called Product of type
Statechart diagram. Names must be unique across elements of the same type within a package
but do not have to be unique across different types. A package could not contain two
items called Product that are both of type Class.
Model elements in different packages may have the same name. But whenever the two
elements are used together, they must be qualified with the owning package name. A fully
qualified element name uses the notation package :: element, for example Receiving ::
Product and Shipping :: Product.