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Unlike most other classes, you don't instantiate theSystem
class to use it. To be more precise, you cannot instantiate theSystem
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 declaredstatic
. 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 Membersin 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 theSystem
's class variableout
, you append the variable name to the class name separated by a period, like this:You call class methods in a similar fashion. For example, to callSystem.outSystem
'sgetProperty
method, you append the method name to the end of the class name separated by a period:The following small Java program uses theSystem.getProperty(argument);System
class twice, first to retrieve the current user's name and then to display it.You'll notice that the program never instantiates aclass UserNameTest { public static void main(String[] args) { String name; name = System.getProperty("user.name"); System.out.println(name); } }System
object. It just references thegetProperty
method and theout
variable directly from the class.The code sample use
System
'sgetProperty
method to search the properties database for the property called "user.name
." System Properties later in this lesson talks more about system properties and thegetProperty
method.The code sample also uses
System.out
, aPrintStream
that implements the standard output stream. Theprintln
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 theSystem
class.
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