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.

: A model of subobjects and subobject selection gives us a concise expression of key semantic relationships in a variety of inheritance-based languages. Subobjects and their selection have been difficult to reason about explicitly because they are not explicit in the languages that support them. The goal of this paper is to present a relatively simple calculus to describe subobjects and subobject selection explicitly. Rather than present any deep theorems here, the goal is to present a general calculus that can be used to explore the design of inheritance systems. . Modeling Subobject-based Inheritance Jonathan G. Rossie Jr. Daniel P. Friedman Department of Computer Science, Indiana University 215 Lindley Hall, Bloomington, Indiana 47405 fjrossie,dfriedg@cs.indiana.edu Telephone: 812.855.4885, Fax: 812.855.4829 Mitchell Wand College of Computer Science, Northeastern University 360 Huntington Avenue #161CN, Boston, MA 02115 wand@ccs.neu.edu Telephone: 617.373.2072, Fax: 617....