The JavaTM Tutorial

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Trail: Essential Java Classes
Lesson: Accessing System Resources

## Using the System Class

> Unlike most other classes, you don't instantiate the `System` class to use it. To be more precise, you cannot 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 complete discussion about class variables and class methods and how they differ from instance variables and instance methods, refer to Understanding Instance and Class Members in the Objects, Classes, and Interfaces lesson.

> 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 to the class name separated by a period, like this:

> >     System.out

> You call class methods in a similar fashion. For example, to call `System`'s `getProperty` method, you append the method name to the end of the class name separated by a period:

> >     System.getProperty(argument);

> The following 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 instantiates a `System` object. It just references the `getProperty` method and the `out` variable directly from the class.

> The code sample use `System`'s `getProperty` method to search the properties database for the property called "`user.name`." System Properties later in this lesson talks more about system properties and the `getProperty` method.

> The code sample also uses `System.out`, a `PrintStream` that implements the standard output stream. The `println` method prints its argument to the standard output stream. The next page of this lesson discusses the standard output stream and the other two standard I/O streams provided by the `System` class.

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