0.3 Xna0104-what is oop and why should you care?  (Page 4/7)

Expose the interface and hide the implementation

As in the steering mechanism for a car, a common approach in OOP is to "hide the implementation" and to "expose the interface" through encapsulation.

Inheritance example

Another important aspect of OOP is inheritance. Let's form an analogy with the teenager who is building a hotrod. That teenager doesn't normally start witha large chunk of steel and carve an engine out of it. Rather, the teenager will usually start with an existing engine and make improvements to it.

OOP lingo

In OOP lingo, that teenager extends the existing engine, derives from the existing engine, inherits from the existing engine, or subclasses the existing engine (depending on which author is describing the process) .

Just like in "souping up" an engine for a hotrod, a very common practice in OOP is to create new improved classes and objects by extending existing classdefinitions.

Extending the Game class

When you use Visual C# to create a new XNA game project, the IDE creates skeleton code for a new class. As you will learn later in this course, it isthen up to you to put some meat on the skeleton and turn it into a game. The first executable statement in the skeleton code is shown in Listing 1 .

The code in Listing 1 is the first executable statement in the definition of a new class named Game1 (or whatever name you give the new project) . The definition of the new class extends an existing class named Game , which resides in the Microsoft.Xna.Framework namespace. The existing class named Game , is the main controller for your new game. I will have much more to say about the Game class in a future module.

Reuse, don't reinvent

One of the major arguments in favor of OOP is that it provides a formal mechanism that encourages the reuse of existing programming elements. One of themottos of OOP is "reuse, don't reinvent."

Polymorphism example

A third important aspect of OOP is polymorphism. This is a Greek word meaning something like one name, many forms . This is a little more difficult to explain in non-programming terminology. However, we will stretch our imaginationa little and say that polymorphism is somewhat akin to the automatic transmission in your car. In my Honda, for example, the automatic transmissionhas four different methods or functions known collectively as Drive (in addition to the functions of Reverse, Park, and Neutral) .

Select Drive to go forward

As an operator of the automobile, I simply select Drive (meaning go forward) . Depending on various conditions at runtime, the automatic transmission systemdecides which version of the Drive function to use in every specific situation. The specific version of the function that is used is based on the currentconditions (speed, incline, etc.) . This is somewhat analogous to what we will refer to in a subsequent tutorial module as runtime polymorphism.

Object-oriented programming vocabulary

OOP involves a whole new vocabulary (or jargon) which is different from or supplemental to the vocabulary of procedural programming.