Developers of application software must often work with "legacy systems." These are systems that have evolved over many years and are considered irreplaceable, either because it is thought that duplicating their function would be too expensive, or because they are trusted by users. Because of their age, such systems are likely to have been implemented in a conventional language with limited use of data abstraction or encapsulation.The lack of abstraction complicates adding new applications to such systems and the lack of encapsulation impedes modifying the system because applications depend on system internals. We describe our experience providing and using an object-oriented interface to a legacy system.

This paper describes an experimental system for the original generation and subsequent modification of volume models of complex physical objects, using interactive computer graphics. The models are built up from primitive volumes, e.g., cuboids, cylinders, swept sutfaces, etc., entered by a mechanical engineer interacting with a two dimensional projection of the model on a graphic display screen. The primitives may be entered at any orientation in 3-space and combined to form a single polyhedral model. The central issue is the provision of an eficient, natural means for generating these models. It was in this environment that the research reported here was undertaken. The work was intended to answer some fundamental questions regarding mechanical CAD Copyright 1981 by International Business Machines Corporation. Copying is permitted without payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract may be used without further permission in computer-based and other information-service systems. Permission to republish other excerpts should be obtained from the Editor.

Developers of application software must often work with "legacy systems." These are systems that have evolved over many years and are considered irreplaceable, either because it is thought that duplicating their function would be too expensive, or because they are trusted by users. Because of their age, such systems are likely to have been implemented in a conventional language with limited use of data abstraction or encapsulation.The lack of abstraction complicates adding new applications to such systems and the lack of encapsulation impedes modifying the system because applications depend on system internals. We describe our experience providing and using an object-oriented interface to a legacy system.

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