Classes represent the fundamental structur of an object-oriented system. In spite of the fact that they are
superbly helpful, you need something more to structure enormous frameworks, which may have hundreds
of classes. Package is a grouping construct that enables you to take any construct in the UML and
gathering its components. Its most common use is to group classes, but you can use packages for every other bit of the UML.
as well.
Packages can also be members of other packages, so you are left with a hierarchic structure in which top-level packages.
A package can contain both subpackages and classes. In C++ programming terms, packages correspond to namespaces.
Each class must have a unique name within its owning package. If you two class with same name in the system use fully qualified name to distinguish them, for instance system::disk::image and system::graphic::image
Contest diagra min box
Nested pakages:
Fully qualified package name:
Fully qualified class name:
A useful technique here is to reduce the interface of the package by exporting only a small subset of the operations associated with the package's public classes. You can do this by giving all classes private visibility, so that they can be seen only by other classes in the same package, and by adding extra public classes for the public behavior. These extra classes, called Facades [Gang of Four], then delegate public operations to their shyer companions in the package.
Refs: 1