外文资料译文
facilities for data abstraction.
THE OBJECT-ORIENTED PARADIGM. The object- oriented paradigm
retains much of the characteristics of the procedural paradigm, since procedures are
still the primary form for composing computations. However, rather than operate on
abstract values, programs in the object-oriented paradigm operate on objects. An
object is very similar to an abstract data type and contains data as well as procedures.
There are three primary characteristics of the object-oriented paradigm. We have
already described the first, ENCAPSULATION, the mechanism for enforcing data
abstraction. The second characteristic is INHERITANCE. Inheritance allows new
objects to be created from existing, more general ones. The new object becomes a
specialized version of the general object. New objects need only provide the methods
or data that differ because of the specialization. When an object is created (or derived)
from another object, it is said to inherit the methods and data of the parent object, and
includes any new representations and new or revised methods added to it.
The third and final characteristic of object-oriented programming is
POLYMORPHISM. Polymorphism allows many different types of objects to perform
the same operation by responding to the same message. For example, we may have a
collection of objects which can all perform a sort operation. However, we do not
know what types of objects will be created until run-time. Object-oriented languages
contain mechanisms for ensuring that each sort message is sent to the right object.
Encapsulation, inheritance, and polymorphism are considered the fundamental
characteristics of object-oriented programming and all object-oriented languages must
provide these characteristics in some way. Not surprisingly, languages support these
characteristics in very different ways. Smalltalk, C++, Objective-C, and Lisp with
CLOS (the Common Lisp Object System) are all examples of object-oriented
languages, and each provides support for encapsulation, inheritance, and
polymorphism.
Constructing an object-oriented program involves determining the objects that
are needed to solve the problem. The objects are then used to construct computations
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