Object-Oriented

Understanding Object-Oriented Programming

This chapter is a basic introduction to object-oriented programming. It introduces you to some of the basic concepts and terms you need to know as you get a handle on the specific details of how object-oriented programming works in Java.

Tip

If you're more of a hands-on type, you may want to just skip this chapter and go straight to Book III, Chapter 2, where you find out how to create your own classes in Java. You can always return to this chapter later to review the basic concepts that drive object-oriented programming. Either way is okay by me. I get paid the same whether you read this chapter now or skip it and come back to it later.

What Is Object-Oriented Programming?

The term object-oriented programming means many different things. But at its heart, object-oriented programming is a type of computer programming based on the premise that all programs are essentially computer-based simulations of real-world objects or abstract concepts. For example:

• Flight-simulator programs attempt to mimic the behavior of real airplanes. Some do an amazingly good job; military and commercial pilots train on them. In the 1960s, the Apollo astronauts used a computer-controlled simulator to practice for their moon landings.

• Many computer games are simulations of actual games humans play-such as baseball, NASCAR racing, and chess. But even abstract games such as Pac Man or Final Fantasy 4 attempt to model the behavior of creatures and objects that could exist somewhere. Those programs simulate a conceptual game-one that can't actually be played anywhere in the real world but can by simulated by a computer.

• Business programs can be thought of as simulations of business processes-such as order taking, customer service, shipping, and billing. For example, an invoice isn't just a piece of paper; it's a paper that represents a transaction that has occurred between a company and one of its customers. Thus a computer-based invoice is really just a simulation of that transaction.

TECHNICAL STAUFF

The notion of a programming language having a premise of this sort isn't new. Traditional programming languages such as C (and its predecessors, including even COBOL) are based on the premise that computer programs are computerized implementations of actual procedures-the electronic equivalent of "Step 1: Insert Tab A into Slot B." The LISP programming language is based on the idea that all programming problems can be looked at as different ways of manipulating lists. And the ever-popular database-manipulation language SQL views programming problems as ways to manipulate mathematical sets.

Here are some additional thoughts about the notion of computer programs being simulations of real-world objects or abstract concepts:

• Sometimes the simulation is better than the real thing. Word-processing programs started out as simulations of typewriters, but a modern word-processing program is far superior to any typewriter.

• The idea that all computer programs are simulations of one type or another isn't a new one. In fact, the first object-oriented programming language (Simula) was developed in the 1960s. By 1967, this language had many of the features we now consider fundamental to object-oriented programming-including classes, objects, inheritance, and virtual methods.

• Come to think of it, manual business recordkeeping systems are simulations too. A file cabinet full of printed invoices doesn't hold actual orders. It holds written representations of those orders. A computer is a better simulation device than a file cabinet, but both are simulations.