Interfaces and specifications for the Smalltalk-80 collection classes

Abstract

The hierarchy of interfaces implicit in the Smalltalk-80 collection class library is computed and analyzed. The interface hierarchy is independent of the inheritance hierarchy because methods are frequently deleted by subclasses, and because unrelated classes sometimes implement the same messages. Specifications of the interfaces are developed, revealing subtle relationships among messages and their methods. The specifications help identify several kinds of problems in the library: inherited methods that violate the subclass invariant; methods that have the same name but unrelated behaviors; methods that have the same (or related) behavior but different names. This exercise demonstrates the utility of interfaces and specifications, and suggests improvements to the collection class library structure.