Cryptography Reference
In-Depth Information
possess the same interface. But once they have been created (computer scientists
say that classes are
instantiated
by objects), they lead independent lives. Their
internal states are changed independently of one another and they execute
different tasks corresponding to their respective roles in the program.
Object-oriented programming propagates the use of classes as the building
blocks of larger structures, which can again be classes or groups of classes,
into complete programs, just as houses or automobiles are constructed of
prefabricated modules. In the ideal case programs can be cobbled together
from libraries of preexisting classes without the necessity for the creation of a
significant amount of new code, at least not on the order of magnitude as is
typical in conventional program development. As a result it is easier to orient
program development to reflect the actual situation, to model directly the actual
processes, and thereby to achieve successive refinement until the result is a
collection of objects of particular classes and their interrelations, in which the
underlying real-world model can still be recognized.
Such a way of proceeding is well known to us from many aspects of our lives,
for we do not generally operate directly with raw materials if we wish to build
something, but we use, rather, completed modules about whose construction
or inner workings we have no detailed knowledge, nor the necessity of such
knowledge. By standing on the shoulders of those who built before us, it becomes
possible for us to create more and more complex structures with a manageable
amount of effort. In the creation of software this natural state of affairs has
not previously found its true expression, as software developers turn again and
again to the raw materials themselves: Programs are constructed out of atomic
elements of a programming language (this constructive process is commonly
called
coding
). The use of run-time libraries such as the C standard library does
not improve this situation to any great degree, since the functions contained in
such libraries are too primitive to permit a direct connection to a more complex
application.
Every programmer knows that data structures and functions that provide
acceptable solutions for particular problems only seldom can be used for similar
but different tasks without modification. The result is a reduction in the advantage
of being able to rely on fully tested and trusted components, since any alteration
contains the risk of new errors—as much in the design as in programming. (One
is reminded of the notification in manuals that accompany various consumer
products: “Any alteration by other than an authorized service provider voids the
warranty.”)
In order that the reusability of software in the form of prefabricated
building blocks not founder on the rocks of insufficient flexibility, the concept
of
inheritance
, among a number of other concepts, has been developed. This
makes it possible to modify classes to meet new requirements without actually
altering them. Instead, the necessary changes are packaged in an extension layer.
