Java Classes Question:

Well it depends upon you for what purpose do use system class.
Say, System.in >> belongs to inputstream interface helps in inputting from the console.
whereas, System.out and System.err is used to print something on the console. when out is used with System class then you generally call print() and println() methods to print the stuff you want to. whereas when you use err with System class then you generally try to print error message.