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Object-oriented
techniques achieve further reusability through the encapsulation of programs and
data. The techniques and mechanisms we shall discuss here are primarily concerned
with paradigms for packaging objects in such a way that they can be conveniently
reused without modification to solve new problems. Instantiation
and Object Classes
Instantiation is perhaps the most basic object-oriented
reusability mechanism. Every programming language provides some built-in data
types (like integers and floating-point numbers) that can be instantiated as needed.
Objects may either be statically or dynamically instantiated. Statically instantiated
objects are allocated at compile-time and exist for the duration that the program
executes. Dynamically instantiated objects require run-time support for allocation
and for either explicit deallocation or some form of garbage collection.
The next step is to provide a way for programmers to define and instantiate their
own objects. This can be done by providing the programmer with a facility to define
object classes, as is the case in Smalltalk. An object class specifies a set of
visible operations, a set of hidden instance variables and a set of hidden methods
which implement the operations. The instance variables can only be modified indirectly
by invoking the operations. When a new instance of an object class is created,
it has its own set of instance variables, and it shares the operations' methods
with other instances of its class. A simple example is the class ComplexNumber.
The programmer would define an interface consisting of the arithmetic operations
that complex numbers support, and provide the implementation of these operations
and the internal data structures. It would be up to the programmer to decide,
for example, whether to use a representation based on Cartesian or polar coordinates.
An alternative approach to instantiation is to use prototypical objects rather
than object classes as the "template" from which new instances are forged.
This is exactly what we do when we make a copy of a text file containing a document
composed in a formatting language like TeX ~ or troff:
we reuse the structure
of the old document, altering its contents, and possibly refining the layout.
This approach is useful to avoid a proliferation of object classes in systems
where objects evolve rapidly and display more differences than similarities.
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