Chapter14 - 14 Introduction to inheritance I nteresting...

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14 Introduction to inheritance I nteresting systems are seldom born into an empty world. Almost always, new software expands on previous developments; the best way to create it is by imitation, refinement and combination. Traditional design methods largely ignored this aspect of system development. In object technology it is an essential concern. The techniques studied so far are not enough. Classes do provide a good modular decomposition technique and possess many of the qualities expected of reusable components: they are homogeneous, coherent modules; you may clearly separate their interface from their implementation according to the principle of information hiding; genericity gives them some flexibility; and you may specify their semantics precisely thanks to assertions. But more is needed to achieve the full goals of reusability and extendibility. For reusability , any comprehensive approach must face the problem of repetition and variation, analyzed in an earlier chapter. To avoid rewriting the same code over and over again, wasting time, introducing inconsistencies and risking errors, we need techniques to capture the striking commonalities that exist within groups of similar structures — all text editors, all tables, all file handlers — while accounting for the many differences that characterize individual cases. For extendibility , the type system described so far has the advantage of guaranteeing type consistency at compile time, but prohibits combination of elements of diverse forms even in legitimate cases. For example, we cannot yet define an array containing geometrical objects of different but compatible types such as POINT and SEGMENT . Progress in either reusability or extendibility demands that we take advantage of the strong conceptual relations that hold between classes: a class may be an extension, specialization or combination of others. We need support from the method and the language to record and use these relations. Inheritance provides this support. A central and fascinating component of object technology, inheritance will require several chapters. In the present one we discover the fundamental concepts. The next three chapters will describe more advanced consequences: multiple inheritance, renaming, subcontracting, influence on the type system. Chapter 24 complements these technical presentations by providing the methodological perspective: how to use inheritance, and avoid misusing it.
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INTRODUCTION TO INHERITANCE § 14.1 460 14.1 POLYGONS AND RECTANGLES To master the basic concepts we will use a simple example. The example is sketched rather than complete, but it shows the essential ideas well. Polygons Assume we want to build a graphics library. Classes in this library will describe geometrical abstractions: points, segments, vectors, circles, ellipses, general polygons, triangles, rectangles, squares and so on.
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