Java is an object-oriented (OO) language. So before beginning our discussion of Java, it is important that we introduce some of the underlying concepts involved in object-oriented programming. We need to talk about what an object is, how objects are grouped into classes, how classes are related to each other, and how objects use messages to interact with and communicate with each other.

What Is an Object?

Just as in the real world, an object is any thing whatsoever. An object can be a physical thing, such as a Car, or a mental thing, such as an Idea. It can be a natural thing, such as an Animal, or an artificial, human-made thing, such as an ATM. A program that manages an ATM would involve BankAccounts and Customer objects. A chess program would involve a Board object and ChessPiece objects.

What Is a Class?

A class is a template for an object. A class encapsulates the attributes and actions that characterize a certain type of object. In an object-oriented program, classes serve as blueprints or templates for the objects that the program uses. We say that an object is an instance of a class. A good analogy here is to think of a class as a cookie cutter and its objects, or instances, as individual cookies. Just as we use the cookie cutter to stamp out cookies of a certain type, in an object-oriented program, we use a definition of a class to create objects of a certain type.

Variables and Methods

However, when talking about a programming language, the more common way to describe an object's features is to talk about its variables and methods.

A variable, which corresponds to an attribute, is a named memory location that can store a certain type of value. You can think of a variable as a special container that can only hold objects of a certain type. For example, an int value is a whole number, such as 25 or -4.

A method, which corresponds to an action or a behavior, is a named chunk of code that can be called upon, or invoked, to perform a certain predefined set of actions. For example, the calculateArea() method can be called upon to calculate the area.

World's Best Language

Originally named "Oak" after a tree outside the office of its developer, James Goslin, Java is a relatively young programming language. It was initially designed by Sun Microsystems in 1991 as a language for embedding programs into electronic consumer devices, such as microwave ovens and home security systems. Java as a language for embedding programs into Web-based applications. Java has become one of the most important languages for Web and Internet applications.

The greatest challenges and most exciting opportunities for software developers today lie in harnessing the power of networks. Applications created today, whatever their intended scope or audience, will almost certainly be run on machines linked by a global network of computing resources. The increasing importance of networks is placing new demands on existing tools and fueling the demand for a rapidly growing list of completely new kinds of applications.

We want software that works consistently, anywhere, on any platform and that plays well with other applications. We want dynamic applications that take advantage of a connected world, capable of accessing disparate and distributed information sources. We want truly distributed software that can be extended and upgraded seamlessly. We want intelligent applications such as autonomous agents that can roam the Net for us, ferreting out information and serving as electronic emissaries. We know, to some extent, what we want. So why don't we have it?

The problem, historically, has been that the tools for building these applications have fallen short. The requirements of speed and portability have been, for the most part, mutually exclusive, and security has been largely ignored or misunderstood. In the past, truly portable languages were bulky, interpreted, and slow. These languages were popular as much for their high-level functionality as for their portability. Fast languages usually provided speed by binding themselves to particular platforms, so they met the portability issue only halfway. There were even a few safe languages, but they were primarily offshoots of the portable languages and suffered from the same problems. Java is a modern language that addresses all three of these fronts: portability, speed, and security. This is why it has become dominant in the world of programming today.