How does Iterator ties into Collection Interface

I am new to Java.

Java has Collection interface

public interface Collection<E> extends Iterable<E>
{ // a lot of othe开发者_Go百科r stuff. 
Iterator<E> iterator();  }

What I don't understand is how does Iterator Interface is tying into Collection Interface ? When I look at the Collection Interface, I see that it has a method that returns Iterator. When Iterator for Collection is created, where does JVM looks to create an object that IS-An Iterator ?

Thanks !

The JVM doesn't do it; the code that ultimately implements the abstract iterator() method just creates an instance of an appropriate class and returns it. For example, in the class ArrayList(), which implements List which extends Collection, there is a method that implements iterator() by returning an instance of a class that understand how ArrayList is implemented internally. This Iterator class is private to the java.util package -- normally you'll never see its name.

Ok so this is all pretty advanced java stuff and it will be pretty tough to explain in one go here but I will do my best.

BACKGROUND: If you don't know about those funny <E> things, you should do a bit of looking into Java Generics. Also, if you don't already, you really need to know what an interface is. One really basic way to think of it is as a promised bit of functionality a class promises to provide.

Now to answer your question: There are three interfaces in the above code snippet, and if you want to create your own collection class you will need to provide implementations of all three:

The first is Collection. This is a simple concept that maps to the real world, it is literally a "collection" of objects. I think you get this...

The next one is Iterable this defines a singe type of behavior that all collections need to provide: the ability to traverse all of the elements of a collection, while accessing them one by one ie "iterate" over them. But it doesn't stop there. As you pointed out the Iterable functionality is provided by objects that implement the last interface:

Iterator: objects that implement this interface, actually know how to traverse the elements of a collection class, they hide all the details of how its actually done from thier clients and proved a few clean easy methods for actually doing it like hasNext() which checks to see if there are more things in the collection to visit and next() which actually visits the next thing.

phew...

The code for the iterator() method determines which concrete implementation of Iterator to return.

All non-abstract classes that implement the Collection interface are required to provide an implementation for the iterator() method.