Chapter three introduces the System class, packages, and Java libraries. It also teaches you how to use the Scanner class to read user’s input from standard input (System.in).
Note that the exercises presented here also introduce another common way to obtain user’s input through parameters passed to a class’s main method. Both techniques are equally valid; however, the Scanner class is for interactively capturing user input while command-line parameters are passed to a program at startup. This difference is demonstrated in the exercise on the Scanner class.