System in.
Standard input of type InputStream
, usually connected to the keyboard in terminal/console oriented programs.
System out.
Standard output of type PrintStream
. Usually sends the data saved on it to the console/terminal. Often used by console-based programs as command line tools (grep
, find
).
System.err
Error output of type PrintStream
. Works like System.out, except it is used to send error messages.
In a terminal-oriented Java application, both outputs will be the same (the command line), but you can reconfigure them so that Sistem.out keeps printing on the terminal, and System.err writes to a file, for example.
This reconfiguration can be done directly at the terminal as per these examples.
As no one linked the documentation: https://docs.oracle.com/javase/7/docs/api/java/lang/System.html
– user28595
Thank you so much for the @diegofm edition, I forgot to quote the questions separately as topics here!
– Victor