System, out, println, String, toString, JFrame, File, Scanner, next, nextInt, Exception, close, ArrayList, stream, JTextField, Math, Random, MenuItem, Month, parseInt, Query, Rectangle, Color, Oval, paint, Robot, SQLData, Stack, Queue, TimeZone, URL
, and so many others.You can reuse any of these names for any purpose in your code. But if you do, you might have trouble using a name with its normal meaning from the Java API. For example, you can write
int System = 7;
java.lang.System.out.println(System);
But you can't write
int System = 7;
System.out.println(System);