Trying to Understand Interfaces

Question

Hi all,
As you might expect, I am new to JAVA. I have read the basics tutorial on interfaces and also the object oriented programming basics tutorial. I understand that you can't implement methods or variables in interfaces, etc. Here is what I am struggling with:
A friend has written a class that uses the JNDI classes to query an LDAP service for person information and update database information. I notice that the query returns an object of type NamingEnumeration. But this is an interface, not a class. When he gets the items/elements from the Enumeration, he uses a (Attributes) to cast the results to their proper type.
So, if I am writing a class and want to implement the NamingEnumeration interface over my class... and if I want to retrieve that class by its NamingEnumeration interface, rather than its actual class...how would I create a method that does this.
My first attempt at creating this class went like this:
public class DocumentEnumeration extends DocumentCollection implements NamingEnumeration
If I called the .next method of my class, I would return a type Document. So far I don't know how to retrieve this DocumentEnumeration as a NamingEnumeration, and I received an error in my IDE about the constructor with arguments () is not visible. I did not understand this error either.
Any help would be appreciated,
Jeff

Answer 1

DrClap's responses are all correct and to the point. Let me add a preface. Forget everything you've read, close your eyes and say "interfaces are dead simple!" about a hundred times. They are.
An interface is a bundle of related capabilities. For example, I am a male homo sapiens (my class). I ski and play tennis - these are things I can do. As a Skier I have methods like buckleBoots(), getOnLift(), getOffLift() and skiDownHill(). All skiers implement these methods.
TennisPlayers implement a different set of methods: serve(), volley() and so on. Some TennisPlayers may also implement Skier and some Skiers may implement TennisPlayer, but if an object of class LiftAttendant is going to call our getOnLift() method, it doesn't care about serve() or volley(). It knows that we have the handful of methods that concern LiftAttendants.
Note that many different classes can implement an interface. Certainly some female homo sapiens implement Skier. I've also seen some apes and hot dogs implementing Skier.
The Runnable interface defines just one method: public void run(). A Runnable is any class that claims it implements Runnable. If the class declaration claims that it implements Runnable, the compiler won't generate a .class file unless the class defines a public void run() method. When you pass a Runnable to a Thread, the Thread knows exactly one thing about the Runnable, that it has a public void run() method which the Thread can call after it is started. The Thread has no clue what it might be running, but because the object is a Runnable it can correctly call whatever.run().
Why might a well-written method return an Interface type, not a class? Generality. The LiftAttendant need only know that an object has a getOnLift() method. So anything the implements Skier will be fine for the LiftAttendant.
So an interface defines a set of methods (as few as one) that, taken together, constitute a capability. A class that implements these methods can claim to implement the interface, and other objects that care about that capability can say they work with Skier, or Runnable or whatever, so that they can handle lots of different things, not worrying about all the other things their Objects might implement or be.
Very simple. Very powerful.