Characterizing Object-Oriented Languages
Object-oriented design is based on three major principles: encapsulation, inheritance, and polymorphism. A programming language is said to support OO (Object-Oriented) design if it supports these three concepts in its syntax. Such a language should provide you with tools to easily define and use these paradigms. Encapsulation refers to the concept of making an object a "black box." When you use an object, you should not know its internal workings. You don't need to understand how an object works. An object should expose only the absolute necessary information needed to interface with it. It should give you a friendly interface to those limited set of methods and properties that the designer thought might be useful for other users. Encapsulation also means that an object includes everything it needs: both the data and the operations on it (methods). The encapsulation concept is very powerful because it allows an efficient division of labor in large software projects. Each team member can work on his or her object without interfacing too much with other members of the group. Overhead in development projects grows up exponentially with the number of interfaces between the group members. Encapsulation is a major contributor to OO design being a solution to the famous "Software Crisis."
Software reuse is another characteristic of OO design. One of the major ways to achieve software reuse is by inheritance. A class is a function that defines an object. A superclass is a class from which new classes, subclasses, are created. A subclass inherits all its methods and properties from its superclass. Practically, all subclasses are generated automatically, and hence the enormous savings. You don't have to define these subclasses one by one. Of course, you can override inherited methods and properties. In fact, there is no point to create a subclass which is a 100% duplication of its superclass, unless you override at least one property or one method.
Polymorphism is probably the most complicated component of the three must-haves. The essence of this concept is that every class should handle different data types. You shouldn't create different classes to handle different data types. The classic example is the drawing class. You should not write different classes to draw circles, rectangles, and ovals. It should be one class that is smart enough to call the proper method to operate on the appropriate shape.
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