Johnson and Foote 1988
Designing reusable classes , by RalphJohnson & BrianFoote, Journal of Object-Oriented Programming, June/July 1988, Volume 1, Number 2, pages 22-35.
The text is available from the links below.
Analysis
This early paper has been very influential. It is long, and uses Smalltalk examples and terminology, but it is worth working through.
It presents one of the earliest collections of OOD heuristics: Johnson and Foote's heuristics.
It is also the original (as far as I know) paper to Favor composition over inheritance.
White box and black box reuse
One of the central themes of this paper is that there are two main ways of designing classes for reuse:
- White box reuse, in which new classes should reuse existing classes by inheriting from them.
- Black box reuse, in which new classes should reuse existing classes by composing them.
Black box reuse is preferred to white box, because the classes are less exposed to each other (i.e. they have weaker coupling). However, white box reuse is still recognised as a valuable technique.
With black box reuse we inherit all implementation details from the super class. This means the programmer must be aware of all these details. Black box reuse only requires the programmer to understand the protocol defined by the public interface of that class.
In general, black box reuse is easier but less flexible than white box reuse. As the design is improved, white box reuse should be substituted for black box reuse.
In terms of statically typed languages like Java:
- Black box reuse allows use only of the public interface of a server class;
- White box reuse allows use of both the public and protected interfaces of the server class.
