UML Distilled: A Brief Guide to the Standard Object Modeling Language, Second Edition

Interfaces and Abstract Classes

One of the great qualities of object-oriented development is that you can vary the interfaces of classes independent of the implementation. Much of the power of object development comes from this property. However, few people make good use of it.

Programming languages use a single construct, the class, which contains both interface and implementation. When you subclass, you inherit both. Using the interface as a separate construct is rarely used, which is a shame.

A pure interface, as in Java, is a class with no implementation and, therefore, has operation declarations but no method bodies and no fields. Interfaces are often declared through abstract classes. Such classes may provide some implementation, but often ...