In this Java 101 tutorial, I’ve explained how interfaces differ from classes, and showed you how to declare, implement, and extend interfaces in your Java programs.

The Java 2 SDK organizes its vast collection of classes and interfaces into a tree-like hierarchy of packages within packages, which is equivalent to directories within directories.

A public method should never be called by another public method within the same class hierarchy. It should only be called by other classes via the classes' public interface. Class calls might not be ...