How to make my class iterable?

[Theodore David Williams]

   
public class SpecialCollection implements Iterable<Object> {
   private Object[] collection
.
.
.

   public Iterator<Object> iterator() {
      return new SpecialCollectionIterator(collection);
   }

   class SpecialCollectionIterator implements Iterator<Object> {
      private Object[] collection = null;
      private int index = 0;

      public boolean hasNext() {
         return (index < collection.length);
      }

      public Object next() {
         if (hasNext()) {
            return collection[index];
            index++;
         } else {
            // what to do here????
         }
      }

      public void remove() {
         throw new UnsupportedOperationException();
      }
   }
}

So my question is if the next() method determines that there is no next element what do I do? Do I return null, do I throw an exception??

Thanks in advance!

[Theodore David Williams]

Looking at the API maybe I am supposed to throw UnsupportedOperationException.

Problem is when I throw this exception it propagates all the way up the stack to my main. So what do I do to get my iterator class to stop iterating when it hits the last element?

next

public Object next()
Returns the next element in the iteration.
Returns:
the next element in the iteration.
Throws:
NoSuchElementException - iteration has no more elements.

[Joanne Neal]

Theodore David Williams wrote:So what do I do to get my iterator class to stop iterating when it hits the last element?

You call the hasNext method before calling the next method.

Or you catch the exception that is thrown - which should be a NoSuchElementException not an UnsupportedOperationException.

[Campbell Ritchie]

You are making things too difficult for yourself.
Well, actually, you will have no real problems. That array, being nicely set to null, will throw a load of nice Exceptions whenever you try to use it You should delete that array, and use the same array which is already a field in the surrounding class.
You will need an int called size which should be set in the surrounding class already. You need to use that in the hasNext method

@Override
public boolean hasNext()
{
    return index < size;
}

The remove() method should remove. If you are dealing with an array, you would have to reduce the rest of the array:

@Override
public void remove()
{
    System.arraycopy(collection, index + 1, collection, index, collection.length - index - 1);
    collection[--size] = null; // Note the use of pre-decrement. End value must be nulled.
}

In the else part of the next() method, you throw the NoSuchElementException.

At least I think that is what you do. Anybody finding any mistakes (especially Joanne and Rob), be sure to tell me. But I am off to cook tea now, so I shan't take any notice.

[Joanne Neal]

Campbell Ritchie wrote:But I am off to cook tea now, so I shan't take any notice.

Having something nice ?

[Garrett Rowe]

Campbell Ritchie wrote:
The remove() method should remove.

Or throw an UnsupportedOperationException as the OP has done. The remove method is documented as optional (and it can be argued should have never been part of the Iterator interface to begin with).

[Rob Spoor]

I'm with Garrett on that one. You can't remove anything from arrays.

This entire discussion could be prevented by using Arrays.asList(collection).iterator()

[Campbell Ritchie]

Joanne Neal wrote:Having something nice ?

It's Friday, and I am very conventional, so it's fish. Some salmon and some sea-bass, baked in the oven with onions tomatoes and herbs. And I would have added mushrooms if I'd remembered to buy some. And runner beans and new potatoes. The tomatoes were quite old; they had been off the plant for a good 10 minutes before they were cooked.