Object-oriented programming languages support encapsulation, thereby improving the ability of software to be reused, refined, tested, maintained, and extended. The full benefit of this support can only be realized if encapsulation is maximized during the design process.We argue that design practices which take a data-driven approach fail to maximize encapsulation because they focus too quickly on the implementation of objects. We propose an alternative object-oriented design method which takes a responsibility-driven approach. We show how such an approach can increase the encapsulation by deferring implementation issues until a later stage.

This paper introduces the programming language Modular Smalltalk, a descendant of the Smalltalk-80 programming language. Modular Smalltalk was designed to support teams of software engineers developing production application programs that can run independently of the environment in which they are developed. We first discuss our motivation for designing Modular Smalltalk. Specifically, we examine the properties of Smalltalk-80 that make it inappropriate for our purposes. We then present an overview of the design of Modular Smalltalk, with an emphasis on how it overcomes these weaknesses.