Software Engineering for Students A Programming Approach

Summary
Exercises 217
Writing a class means that strongly related elements of data and actions are
grouped together. A class presents an interface to its users and hides information
about its internal workings. It means that the user of a class need not worry about
its implementation. This promotes abstraction in thinking about the structure of
software. It also means that a class can be changed without any effect on the rest
of the program (provided that it continues to present the same interface). Thus
classes promote modularity.
Extending (inheriting from) a class is another way of making use of existing components
(classes). A subclass inherits the facilities of its immediate superclass and
all the superclasses. Most languages support single inheritance. A class can extend
the facilities of an existing class by providing one or more of:
■ additional methods
■ additional variables
■ methods that override (act instead of) methods in the superclass.
Polymorphism means that similarities between objects can be exploited in the
code that uses objects. This means that software is more concise and more easily
adapted.
Altogether encapsulation, inheritance and polymorphism mean that software is
modular, concise and adaptable. It also means that greater use can be made of
libraries of useful components. The programming language must explicitly support
these features for OOP to be viable.
Generics enable tailor-made collections to be constructed. This makes programs
more concise and assists with compile-time type checking, and consequently software
reliability.
There are a number of approaches to garbage collection for software that uses
dynamic allocation of memory. Some schemes are automatic but may create timing
problems. Some schemes rely on the programmer to make explicit requests,
but this can lead to subtle memory problems.
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Exercises
15.1 Explain how classes, inheritance and polymorphism support software development.
15.2 Explain how classes, inheritance and polymorphism promote reusable software.