System.err与System.out的区别

System.out leads the output to the standard output stream (normally mapped to the console screen). System.err leads the output to the standard error stream (and, by default to the console, as well). The idea behind having these two is that the standard output should be used for regular program output, and standard error should be used for error messages.

Both the streams can be redirected to different destinations.(这就是所谓的重定向) If it is desired to redirect the output to an output file and error messages to a different log file, than on UNIX it can be done as follows:

 
java MyClass > output.log 2>error.log

This causes the regular output (using System.out) to be stored in output.log and error messages (using System.err) to be stored in error.log.

If you have put error messages in your program, but you don't want to see them, the technique given below can be used to do it:

 
Java MyClass 2> /null

This redirects the System.err messages to /null, so no error messages appear on the screen, but the normal program output can still be seen.