Interfaces

An interface is an abstract class where all the methods are abstract (and public) and they define the interface (i.e. the methods) to all the classes in the hierarchy.
In this type of hierarchy, the interface is higher in the hierarchy while the implementation details are lower in the hierarchy.
Interfaces allow you to design a hierarchy that can be extended to classes that haven't been created yet.
Syntactically, your new class implements the interface.
An interface is used in place of multiple inheritance.
In Java, a class can only have a single parent (it can only extend a single class).
A class can implement any number of interfaces.
Since the class that implements the interface must write the code for each method, there is no ambiguity as to which method will be called.
Note that "isa" holds - your new class is an instance of its parent class and of all interfaces it implements.
For instance, you may have noticed that Java calls many methods for you. This is because it is calling "known" methods - i.e. the signature for the method is known.
One way to allow this in a class hierarchy is to define the known methods as an interface. Then any class that implements the interface, must implement the methods in the interface.
Multi-threading and event handling in Java use interfaces (more on these advanced topics soon!).