The simple rule is interfaces can extend interfaces, but not classes. classes can extend classes, but not interfaces.
Donald R. Cossitt
Joined: Jan 31, 2003
An interface is essentially a collection of constants and abstract methods. To make use of an interface, you implement the interface in a class - that is, you declare that the class implements the interface and you write the code for each of the methods declared in the interface as part of the class definition. Check Learning the Java Language What is an Interface? Implementing an Interface