What is the difference between System.out.println() and System.err.println() in java?

Answer 1

System.out is a PrintStream, you can call printf() on System.out. Thus, printf() can be used in place of println() when writing to the console whenever formatted output is desired. System.err is also an object of type PrintStream (byte stream), it can also be used instead of System.out. Where System.out is a standard output stream, System.out is a standard error-route stream, in which there's a system of error-message routing.

Answer 2

Historically, during the running of an application, any normal output would be routed to "Standard Out" while error messages were routed to "Standard Error." Depending upon the requirements of the application, each could output to separate files, or one could point to a file while the other pointed to a printer, or one could go to a file while the other outputs to the console, for example. It just depends on how you want to set up the output streams.

Answer 3

Out goes to the screen, by default.
Error MIGHT go to the screen. Error destination is typically defined in the OS.

Unix commands in particular tend to send error messages to places like COM ports, where they can be gathered and collated. This is how one PC can monitor the potential errors from a whole "farm" of PC's.

Answer 4

System.err.println