A class library is a set of classes, expressed in a object-oriented programming language, which is designed to be reused in several development projects.
Class libraries are developed within organisations developing software, but there is also a market for software components in the form of classes. In addition, some vendors of programming environments bundle class libraries with compilers, linkers, editors etc.
Examples of areas where class libraries have been developed:
- Graphical user interfaces: Classes for Windows, Buttons, Menus, EventHandlers, etc.
- Data structures: Classes for Linked Lists, Sets, Stacks, etc.
- Mathematics: Classes for Matrices, Statistics etc.
- Communication: Classes which offer communication protocols to external databases, applications, etc.
The difference from a programmers point of view is that a class is a packaged description of both data structures and behaviour/functions.
In addition, class library classes may be extended and adapted, using inheritance, dynamic binding and composition of classes.
Example 1:
If the Graphical Window class in the library does not match my requirements, I may create a subclass where I can specify the differences.
Example 2:
When developing a ClientRegistrationWindow class for my application, I compose this window class from existing Button and Menu classes and other classes in the library.
For a class to be reusable, it is important that it has high quality in e.g. design, testing and documentation. The demands on library classes are higher than on "ordinary" classes, something which is also reflected in development costs.
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