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Defining a Class

The first bold line in the following listing begins a class definition block.
 * The HelloWorldApp class implements an application that
 * simply displays "Hello World!" to the standard output.
class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); //Display the string.

A class--the basic building block of an object-oriented language such as Java--is a template that describes the data and behavior associated with instances of that class. When you instantiate a class you create an object that looks and feels like other instances of the same class. The data associated with a class or object is stored in variables; the behavior associated with a class or object is implemented with methods. Methods are similar to the functions or procedures in procedural languages such as C.

Julia Child's recipe for rack of lamb is a real-world example of a class. Her rendition of the rack of lamb is one instance of the recipe, and mine is quite another. (While both racks of lamb may "look and feel" the same, I imagine that they "smell and taste" different.)

A more traditional example from the world of programming is a class that represents a rectangle. The class would contain variables for the origin of the rectangle, its width, and its height. The class might also contain a method that calculates the area of the rectangle. An instance of the rectangle class would contain the information for a specific rectangle, such as the dimensions of the floor of your office, or the dimensions of this page.

In the Java language, the simplest form of a class definition is

class name {
    . . .

The keyword class begins the class definition for a class named name. The variables and methods of the class are embraced by the curly brackets that begin and end the class definition block. The "Hello World" application has no variables and has a single method named main.

For more information about object-oriented concepts, see Object-Oriented Programming Concepts: A Primer(in the Writing Java Programs trail). To learn how object-oriented concepts are implemented in the Java language, see Objects, Classes, and Interfaces(in the Writing Java Programs trail).

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