These are Java programs to show you how to use it.

$javac xxxxx.java e.g. $ javac HelloJava.java

After javac, it will create a file name like xxxxx.class

$ java xxxxx e.g. $ java HelloJava

is only to show you how to create a java program and print something on the screen.

is to show you how to get data from what you enter to a java program, and how to use scanner function. Beside, checking input values.