From Java documentation

If a class has no modifier (the default, also known as package-private), it is visible only within its own package (packages are named groups of related classes — you will learn about them in a later lesson.)

At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning.

Full story you can read here (_{Which I wrote recently}):

http://codeinventions.blogspot.com/2014/09/default-access-modifier-in-java-or-no.html