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Using Classes and Objects

This section explains how the "Hello World" application uses classes and objects. If you aren't familiar with object-oriented concepts, then you might find this section confusing. If so, feel free to skip ahead to the lesson Object-Oriented Programming Concepts

The "Hello World" application is about the simplest program you can write. It defines only one class. However, most programs you write will be more complex, requiring the creation of additional classes.

The "Hello World" application does use another class--the System class-- which is part of the API (application programming interface) provided with the Java platform. The System class provides system-independent access to system-dependent functionality. For information about the System class, see Accessing System Resources.

The bold code in the following listing illustrates the use of a class variable of the System class, and of an instance method.

/** 
 * 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.
    }
}

Using a Class Method or Variable

Let's take a look at the first segment of the statement:

System.out.println("Hello World!");

The construct System.out is the full name of the out variable in the System class. Notice how in this case, out is referred to directly from the class name. It is a class variable--a variable associated with the class rather than with an instance of the class-- and it is accessed directly. Like variables, methods can also be associated with the class. In both cases this is done with the static keyword, in the source code for the class where that variable or method is defined. For classes provided by the Java platform, you can determine whether or not a particular member is static by referring to the API specification for that class. If you see the static keyword, that variable or method belongs to the class. The following link shows the API specification for the System class.

To refer to class variables and methods, join the class name and the name of the class method or class variable together with a dot (".").

Using an Instance Method or Variable

Methods and variables that are not class methods or class variables are known as instance methods and instance variables. To refer to instance methods and variables, you must reference the methods and variables from an object.

While System's out variable is a class variable, it refers to an instance of the PrintStream class (a class provided with the Java development environment) that implements the standard output stream.

When the System class is loaded into the application, it instantiates PrintStream and assigns the new PrintStream object to the out class variable. Now that you have an instance of a class, you can call one of its instance methods:

System.out.println("Hello World!");

As you can see, you refer to instance methods and variables similarly to the way you refer to class methods and variables. You join an object reference (out) and the name of the instance method or variable (println) together with a period (".").

The Java compiler allows you to cascade references to class and instance methods and variables together, resulting in constructs like the one that appears in the sample program:

System.out.println("Hello World!");

This line of code displays "Hello World!" to the application's standard output stream.

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