7 Reuse

7.1 The advantages of reuseability

Reuse is the process of building new software from existing software assets, rather than starting from scratch. Reuse is an important factor in building flexible products that can be changed quickly in response to changes in requirements.

One of the advantages claimed of object technology is that it encourages a disciplined approach that facilitates reuse. Encapsulation encourages better designs that can be reused in a more reliable way, as there is exact knowledge of which operations access which data. It restricts the assumptions made of an object to a well-defined protocol. Inheritance allows a developer to reuse a class whose behaviour provides some of the behaviour required of the new class, so that only the different parts need to be designed.

Large-scale reuse, however, requires more than just discipline. It also demands mechanisms to describe the behaviour and interfaces of each replaceable part, and easy ways to connect such parts together. Originally, reuse in an object-oriented context was centred around the concept of class. In time, however, this perspective has widened, and other units of reuse have been defined. In the remainder of this section we will review some of them, including the concepts of framework, component and pattern.