Iterator instantiation

[Scott Appleton]

My question concerns code like the following fragment:

TreeSet ts = new TreeSet();
// Populate ts
for (Iterator it = ts.iterator(); it.hasNext(); ) {
System.out.println("TreeSet element: " + it.next());
} // for Iterator it
Iterator is an interface and can't be instantiated, so "Iterator it = new Iterator();" doesn't compile. I fail to see the real distinction between this line and the code above, where it does appear that an Iterator object is being instantiated. Why does it make a difference whether you use the new keyword or call a method that returns an Iterator? If Iterator can't be instantiated, how is it that *any* method can return an Iterator?
[This message has been edited by Jim Yingst (edited May 08, 2001).]

[Ashwin Desai]

Hi,
Iterator it = ts.iterator();
The above statement returns you an implementation of an iterator. i.e. a class that implements the Iterator interface. We need not be concerned with the actual class, since we are only concerned with the methods of the Iterator interface that the class has implemented.

Hope this helps.
Ashwin

[Laojar Chuger]

From the design of Collection, iterator should be created by classes implementing the collection interface.

[Dave Brookes]

Hi,
Iterator it simply declares a typed reference variable which may then be associated with an instance of any class that implements the Iterator interface. Polymorphism in action and very powerful too!
Regards,
Dave

[Anonymous]

In the sentence:
Iterator it = ts.iterator();
You declare a reference it to an object that implements that Iterator interface. Since:
ts.iterator()
does it, there's no problem because there isn't an instance.

[Thomas Paul]

Look at this code:

<font size="+0">
public class RealIterator implements Iterator {
   // code to actually implement the Iterator methods
}
public class RealList implements Collection, List {
    // code to actually implement Collection and List methods
    public Iterator iterator() {
        return realIterator;
    }
}

So now when I instantiate a RealList object and ask for an Iterator, it actually gives me a RealIterator.
List list = new RealList();
Iterator i = list.iterator();
i is actually a pointer to a RealIterator object.

[This message has been edited by Thomas Paul (edited May 09, 2001).]

[Cindy Glass]

The point is that the Iterator interface does not know how to make a REAL iterator. It is just a specification for the methods that an iterator must have.
However the TreeSet class that implements Iterator DOES know how to make a REAL iterator for itself, and provides all the methods that an iterator must have to work as an iterator.
The variable that is holding the iterator has the "type" iterator, which is a nice feature of polymorphism. This iterator type variable can hold real iterators from any class that implements interator, even though none of the objects that it holds is strictly an instance of iterator. In this case it is a TreeSet which "has an" iterator.