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Chapter 4. Object-Oriented Programming > The Three Pillars of Object-Oriented P...

4.4. The Three Pillars of Object-Oriented Programming

Object-oriented programming is built on three sturdy pillars: encapsulation , specialization, and polymorphism.

Each class should be fully encapsulated; that is, it should define the state and responsibilities of that type. For example, if you create an Employee object, that Employee object should fully define all there is to know, from the perspective of your program, about each Employee. You do not, typically, want to have one class that defines the Employee's work information, and a second, unrelated class that defines the Employee's contact information. Instead, you want to encapsulate all this information inside the Employee class, perhaps by aggregating the contact information as a member of the Employee class.

Specialization allows you to establish hierarchical relationships among your classes. For example, you can define a Manager to be a specialized type of an Employee and an Employee to be a specialized type of Person. This allows you to leverage the state and abilities of an Employee object in the more specialized form of the Manager.

Polymorphism allows you to treat a group of objects in a similar way and have the objects sort out how to implement the programming instructions. For instance, suppose you have a collection of Employee objects and you want to tell each Employee to give herself a raise. It turns out that Employees get a straight 5% raise, while raises for Managers are determined by how well they've fulfilled their annual objectives. With polymorphism, you can tell each object in the collection to give itself a raise, and the right thing happens regardless of the real type of the object. That is, each employee gets 5%, while each manager gets the appropriate raise based on objectives.

4.4.1. Encapsulation

The first pillar of object-oriented programming is encapsulation. The idea behind encapsulation is that you want to keep each type or class discreet and self-contained. This allows you to change the implementation of one class without affecting any other class.

A class that provides a method that other classes can use is called a server. A class that uses that method is called a client. The goal of encapsulation is that you can change the details of how a server does its work without breaking anything in the implementation of the client.

This is accomplished by drawing a bright and shining line between the public interface of a class and its private implementation. The public interface is a contract issued by your class that says, I promise to be able to do this work. Specifically, you'll see that a public interface says call this method, with these parameters, and I'll do this work and return this value. A client can rely on a public interface not to change. If the public interface does change, then the client must be recompiled and perhaps redesigned.

The private implementation, on the other hand, is private to the server. The designer of the server class is free to change how it does the work promised in the public interface, so long as it continues to fulfill the terms of its implicit contract: it must take the given parameters, do the promised work, and return the promised value.

For example, you might have a public method that promises as follows: Give me a dollar amount and a number of years, and I'll return the net present value. How you compute that amount is your business; if a client supplies a dollar amount and a number of years, you must return the net present value. You might implement that initially by keeping a table of values. You might change that at a later time to compute the value using the appropriate algebra. That is your business, and does not affect the client. As long as you don't change the public interface (e.g., as long as you don't change the number or type of parameters expected or change the type of the return value), your clients will not break while you change the implementation.

4.4.2. Specialization and Generalization

The second pillar, specialization, is implemented in VB.NET by declaring that a new class derives from an existing class. When you do so, the specialized class inherits the characteristics of the more general class. The specialized class is called a derived class, while the more general class is known as a base class.

The specialization relationship is referred to as the is-a relationship. A dog is a mammal, a car is a vehicle. (Dog would be derived from the base class Mammal, Car from the base class Vehicle.)

Specialization allows you to create a family of objects. In Windows a button is a control. A listbox is a control. Controls have certain characteristics (color, size, location) and certain abilities (can be drawn, can be selected). These characteristics and abilities are inherited by all of their derived types. This allows for a very powerful form of reuse. Rather than cutting and pasting code from one type to another, the shared fields and methods are inherited by the derived type. If you change how a shared ability is implemented, you do not have to update code in every derived type; they inherit the changes.

For example, a Manager is a special type of Employee. The Manager adds new capabilities (hiring, firing, rewarding, praising) and a new state (annual objectives, management level, etc.). The Manager, however, also inherits the characteristics and capabilities common to all Employees. Thus a Manager has an address, a name, an employee ID, and Managers can be given raises, can be laid off, and so forth. You'll see specialization at work in Chapter 6.

4.4.3. Polymorphism

Polymorphism, the third pillar of object-oriented programming, is closely related to inheritance. The prefix poly means many; morph means form. Thus, polymorphism refers to the ability of a single type or class to take many forms.

The essence of polymorphism is this: at times you will know you have a collection of a general type, for example a collection of Controls. You do not know (or care) what the specific subtype each of your controls is (one may be a button, another a listbox, etc.). The important thing is that you know they all inherit shared abilities (e.g., the Draw( ) method) and that you can treat them all as controls. If you write a programming instruction that tells each control to draw itself, this is implemented properly on a per-control basis (i.e., buttons draw as buttons, listboxes draw as listboxes, etc.). You do not need to know how each subtype accomplishes this; you only need to know that each type is defined to be able to draw.

Polymorphism allows you to treat a collection of disparate derived types (buttons, listboxes, etc.) as a group. You treat the general group of controls the same way, and each individual control does the right thing according to its specific type. Chapter 6 provides more concrete examples.

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