Programmer's Guide to Java™ Certification, A: A Comprehensive Primer, Second Edition

6.4. Interfaces

Extending classes using single implementation inheritance creates new class types. A superclass reference can denote objects of its own type and its subclasses strictly according to the inheritance hierarchy. Because this relationship is linear, it rules out multiple implementation inheritance, that is, a subclass inheriting from more than one superclass. Instead Java provides interfaces, which not only allow new named reference types to be introduced, but also permit multiple interface inheritance.

Defining Interfaces

A top-level interface has the following general syntax:

<accessibility modifier> interface <interface name>                              <extends interface clause> // Interface header{ // Interface body     <constant declarations> ...