It's just a normal warning, because you should not print to the standard output(console) from production code.
Everybody uses System.out.println and printStacktrace because they are good tools while developing, but the warnings are good too, because they remind you to get rid of these calls before releasing your code.
The replacement which netbeans gives you uses the standard Java logging classes. They are quite basic and work, but especially when you are creating libs use slf4j. When you don't know what logging is for, google yourself some nice tutorials
Or maybe log4J
But, yes , you should pay attention to netbeans warning, they teach you some stuff, like , for example, i didnt know about diamond operator.