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Section 6 :  Overloading, Overriding, Runtime Type, and Object Orientation  


State the benefits of encapsulation in object oriented design and write code that implements tightly encapsulated classes and the relationships "is a" and "has a".


Benefits of Object oriented implementation 

These benefits accrue from two particular features. The first of these and perhaps the most important is the notion of abstract data type. The second of these is the extensibility provided by inheritance. 

Abstract data type 

An abstract data type simply means a well encapsulated aggregate of data and behavior. The primitive data types defined in any programming language are in fact abstract data types, albeit not user defined. The state of an object should be represented only with variables of private accessibility. All behavior should be accessed only via methods. By insisting that the variables inside an object are not accessible outside the object, you ensure that the nature of those variables is irrelevant outside the object. This in turn means that you can change the nature of the storage, for maintenance purposes, for performance improvement, or for any other reason freely. This freedom is one of the greatest benefits of encapsulation.

Inheritance:  For this please see the next section on overloading and overriding.

"is a" and "has a"

An object can perform many functions and may contain different types of data. The source of these data and behavior may come from two sources. The first source is from inheritance from its parent classes and the other source is any object it may contain as its state.

Each class in Java, except Object class, is a subclass of another. In any class hierarchy, any class can be called by the name of any class which is above it in the hierarchy. Let us consider a class called Vehicle. One of its subclasses can be called Car. Cars can of different models like Ford or Mercedes. Now any Ford can be called a Car. It is also a vehicle. This relationship is known as "is a" relationship.

A car has many components, each of which can belong to a different hierarchy of classes. For example, let us consider brakes. Brakes can be of different types (classes). There can be a whole hierarchy of brakes. Since a car has brakes, this relationship is known as "has a" relationship.


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