Declarations, Initialization, and Scoping - SCJP Sun Certified Programmer for Java Platform SE6 - page 147

Extending Interfaces
An interface can extend another interface. In fact, an interface can extend multiple
interfaces. (Don't accuse Java of not allowing multiple inheritance!) Use the extends
keyword to declare that an interface extends another interface. For example, the following
interface extends the Drawable interface:
1. public interface Paintable extends Drawable {
2. public void paint();
3. }
A class that implements Paintable must override paint and also the three methods in
Drawable .
Let's look at an example of multiple interface inheritance. The following Image interface
extends both java.lang.Runnable and Drawable :
1. public interface Image extends Runnable, Drawable {
2. public String getFormat();
3. }
A class that implements Image must implement the getFormat method, as well as the run
method from Runnable and the three methods from Drawable .
Multiple Inheritance with Interfaces
While valid, writing an interface that extends multiple interfaces is not a common
occurrence in Java. There are situations where the multiple inheritance makes sense, but
this is not something you will do every day.
Declaring Enumerations
Java 5.0 introduced the concept of enumerations to the Java language, along with a new
keyword: enum . An enumeration is a fi xed set of constants. An enum is a Java class that
represents an enumeration. You use enumerations whenever you have a set of items whose
values are known at compile time. Common uses of enumerations include days of the
week, months of the year, the planets in the solar system, the directions on a compass, or
your favorite fl avors of ice cream. The possibilities for enums are endless, and you should
use them in your Java applications whenever applicable because they provide a type-safe
representation of constant data in your application.