Using the System Class

Unlike most other classes, you don't instantiate the System class to use it. (To be more precise, you can't instantiate the System class--it's a final class and all of its constructors are private.)
All of System's variables and methods are class variables and class methods--they are declared static. For a brief discussion about class variables and class methods and how they differ from instance variables and instance methods, refer to Static vs. Instance in the The Anatomy of a Java Application.
To use a class variable, you use it directly from the name of the class using Java's dot ('.') notation. For example, to reference the System's class variable out, you append the variable name (out) to the end of the class name (System) separated by a period ('.') like this:
System.out
You call class methods in a similar fashion. For example, to call System's class method getProperty(), you append the method name to the end of the class name separated by a period ('.'). Any arguments to the method go between the two parentheses; if there are no arguments, nothing appears between the parentheses.
System.getProperty(argument);
This small Java program uses the System class (twice), first to retrieve the current user's name and then to display it.
class UserNameTest {
    public static void main(String args[]) {
	String name;
	name = System.getProperty("user.name");
	System.out.println(name);
    }
}
You'll notice that the program never instantiated a System object; it just referenced the getProperty() method and the out variable directly from the class.
System's getProperty() method used in the code sample searches the properties database for the property called user.name. System Properties later in this lesson talks more about system properties and the getProperty() method.
System.out is a PrintStream that implements the standard output stream. The System.out.println() method prints it argument to the standard output stream. The Using System Resources