1. Packages

2. Introduction
As the number of elements in a design grows it is often necessary to group related elements together into packages.
A package is a general-purpose mechanism for organizing elements into groups.
Grouping like elements into packages is a technique for managing complexity.
The items in a particular package should be semantically close and subject to change at the same time.

3. Introduction Package is synonymous with directory and folder.
Graphically, a package is drawn as a tabbed folder.
The visibility of individual elements within a package can vary.
Packages are loosely coupled and highly cohesive.
Packages, unlike classes, cannot be instantiated.
Packages have no meaning in the problem domain. They are used to organize the various elements of the problem domain.

4. Terms and Concepts Names Owned elements
Every package must have a name.
The name is a text string.
The name may be a simple name (Graphics.dll) or a path name (DllLibrary::Graphics.dll).
Owned elements
A package is a container that may contain other packages. (A directory may contain subdirectories).
Package icons may be drawn inside other package icons.
The elements contained directly within a package should have unique names.

5. Terms and Concepts Visibility
Just as the members of a class may me declared as public, protected or private, the members of a package may be declared as public, protected or private.
Public (+) - Visible to all. Provides the interface to the package.
Protected (#) - Visible only to descendants in inheritance relationships.
Private (-) - Not accessible outside the package.

6. Simple and Extended Packages

7. Owned Elements

8. Terms and Concepts Importing and Exporting
Packages form a wall around their contents.
Elements within a package cannot be arbitrarily accessed from outside the package, even if the element(s) are public.
The public elements are known as exports.
To access elements in a package, the package must be imported.
Importing grants one-way permission for the elements in the importing package to access elements in the imported package.
Imports are modeled as a dependency adorned with the stereotype <import>.

9. Importing and Exporting

10. Terms and Concepts Generalization Standard Elements
Based on the same principle as generalization among classes.
A package that is derived (inherits) from another package can access all of its public and protected members.
Graphically, generalization between packages is represented the same way as generalization between classes.
Standard Elements
There are 5 pre-defined stereotypes that apply to packages: facade, framework, stub, subsystem and system.

11. Generalization

12. Common Modeling Techniques
Package elements together that you wish to manipulate as a set. These elements will have similar semantics.
The larger the system, the greater the need for multiple packages.
Imagine how difficult it would be to comprehend a textbook that was not organized into chapters.
When in doubt about the visibility of a package element, make it private.
Look for relationships between packages.
Packages may be used for architectural modeling.