Inheritance is a natural way to model the world or a domain of discourse, and so provides a natural model for OOA and OOD (and even OOP). This is common in the AI domain, where semantic nets use inheritance to understand the world by using classes and concepts for generalization and categorization, by reducing the real-world's inherent complexity.
Inheritance also provides for code and structural reuse. In the above Computer class diagram, all routines and structure available in class Computer are available to all subclasses throughout the diagram. All attributes available in Personal computers are also available to all of its subclasses. This kind of reuse takes advantage of the is-a-kind-of relationship. Class libraries also allow reuse between applications, potentially allowing order-of-magnitude increases in productivity and reductions in defect rates (program errors), as library classes have already been tested and further use provides further testing providing even greater reliability.
With differential programming, a class does not have to be modified if it is close to what's required; a derived class can be created to specialize it. This avoids code redundancy, since code would have to be copied and modified otherwise. See [Raj 89] for an alternative approach as found in Jade.
Polymorphism is often explicitly available in many OO languages (such as C++, CLOS, Eiffel, etc.) based on inheritance when type and class are bound together (typing based on subclassing, or subclass polymorphism), since only an object which is a member of (inherits from) a class is polymorphically assignment compatible with (can be used in place of) instances or references of that class. Such assignment can result in the loss of an object's dynamic type in favor of a static type (or even loss of an object's representation to that of the static class, as in C++ slicing). Maintaining the dynamic type of objects can be provided (and preferred); however, C++ provides both sliced and non- sliced replacement in a statically typed environment (see section 2.1).
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