10.1 I/O

The System class defines three static variables for the three default I/O stream objects that are used by Java programs:

in

This variable refers to an InputStream that is associated with the process's standard input.

out

This variable refers to a PrintStream object that is associated with the process's standard output. In an applet environment, the PrintStream is likely to be associated with a separate window or a file, although this is not guaranteed.

This stream is the most commonly used of the three I/O streams provided by the System class. Even in GUI-based applications, sending output to this stream can be useful for debugging purposes. The usual idiom for sending output to this stream is:

System.out.println("some string");

err

This variable refers to a PrintStream object that is associated with the process's standard error output. In an applet environment, the PrintStream is likely to be associated with a separate window or a file, although this is not guaranteed.