Iterator Interface.A question please......respond

Question

I am unable to grasp the meaning of the following below :
As we all know,Interfaces have abstract methods,(ie methods that
have no body) and any class that implements the Interface has to
implement the methods defined in the interface.That is,write the body for those methods.
So far so good.
Now coming to Collections called Vector.
We have the 'Iterator' interface implemented by the vector class.
The Iterator interface has the methods hasNext(),next().
and we have the following code in the our class which iterates thru'
the vector:
MyClass item;
while(iter.hasNext())
item = (MyClass)iter.next();
// Do something here.....
Now we are using the methods hasNext() and next() above.
But as they belong to the Iterator interface,(which means they
are basically methods with no implementations) then how is iteration
possible? We havent defined anything in the hasNext() method or the next()
so how does Java automatically start iteration
Has any one understood my question?Arent hasNext() and next() supposed
to be methods with no body???
If I am saying iter.hasNext(),that means hasNext() has been defined
somewhere to iterate thru the vector?
I am just confused regarding this issue as to how is hasNext() and next()
being implemented as I as the programmer have not implemented them
in my code
Please can some one answer my question?
Regards
Ajay

Answer 1

Thanks for yr reply.
So you mean to say the the Iterator methods hasNext()
and next()
are being implemented by the Vector class.right?Not exactly. The Vector class defines an inner class, and it is this class that implements the Iterator interface, and so these two methods. When you write code like
Vector myVector = new Vector();
// code to fill myVector...
Iterator it = myVector.iterator();the call to the iterator() method tells myVector to give you an instance of this inner class. The Vector class itself doesn't implement next() or hasNext().
Then why define an interface with these methods at
all?
Why not give concrete bodies to these methods and put
them in
the Object Class.If you did that, then all classes would inherit them - all classes would be iterators. That wouldn't make sense, though, as clearly not all classes are meant to be used to iterate over collections of objects.
Iterator is an interface so that a consistent API can be defined for iteration. As iterators are used in lots of different places, it is very helpful to have all of them guaranteed to have next() and hasNext() methods (amongst others). It is also very helpful to be able to call the iterator() method on any Collection class and know that the object that is returned, regardless of its implementation, will be of type Iterator.
Then we can call them polymorphically.???But that's what you're doing - Vector and ArrayList (for example) have different implementations of Iterator, but code that iterates over them doesn't need to know that. As both return objects that implement the Iterator interface, they are interchangeable; that's polymorphism at work.