What is System, out, println in System.out.println

问题:

Possible Duplicate:
What's the meaning of System.out.println in Java?

I was looking for the answer of what System, out and println are in System.out.println() in the Java. I searched and found a different answer like these:

• System is a built-in class present in java.lang package. This class has a final modifier, which means that, it cannot be inherited by other classes. It contains pre-defined methods and fields, which provides facilities like standard input, output, etc.

• out is a static final field (ie, variable)in System class which is of the type PrintStream (a built-in class, contains methods to print the different data values). static fields and methods must be accessed by using the class name, so ( System.out ).

• out here denotes the reference variable of the type PrintStream class.

• println() is a public method in PrintStream class to print the data values. Hence to access a method in PrintStream class, we use out.println() (as non static methods and fields can only be accessed by using the refrence varialble)

In another page i find another contrasting definition as

System.out.print is a standard output function used in java. where System specifies the package name, out specifies the class name and print is a function in that class.

I am confused by these. Could anybody please exactly tell me what they are?

回答1:

The first answer you posted (System is a built-in class...) is pretty spot on.

You can add that the System class contains large portions which are native and that is set up by the JVM during startup, like connecting the System.out printstream to the native output stream associated with the "standard out" (console).

回答2:

System is a final class from the java.lang package.

out is a class variable of type PrintStream declared in the System class.

println is a method of the PrintStream class.

回答3:

Whenever you're confused, I would suggest consulting the Javadoc as the first place for your clarification.

From the javadoc about System, here's what the doc says:

public final class System
extends Object

The System class contains several useful class fields and methods. It cannot be instantiated.
Among the facilities provided by the System class are standard input, standard output, and error output streams; access to externally defined properties and environment variables; a means of loading files and libraries; and a utility method for quickly copying a portion of an array.

Since:
JDK1.0

Regarding System.out

public static final PrintStream out
The "standard" output stream. This stream is already open and ready to accept output data. Typically this stream corresponds to display output or another output destination specified by the host environment or user.
For simple stand-alone Java applications, a typical way to write a line of output data is:

     System.out.println(data)