Question

The question might be pretty vague I know. But the reason I ask this is because the class must have been made with some thought in mind.

This question came into my mind while browsing through a few questions here on SO.

Consider the following code:

class A
{

    private int myVar;

    A(int varAsArg)
    {
          myVar = varAsArg;
    }

   public static void main(String args[])
   {

      List<A> myList = new LinkedList<A>();
      myList.add(new A(1));
      myList.add(new A(2));
      myList.add(new A(3));

      //I can iterate manually like this:
      for(A obj : myList)
                 System.out.println(obj.myVar);

      //Or I can use an Iterator as well:
      for(Iterator<A> i = myList.iterator(); i.hasNext();)
      {
         A obj = i.next();
         System.out.println(obj.myVar);
      }
   }
}

So as you can see from the above code, I have a substitute for iterating using a for loop, whereas, I could do the same using the Iterator class’ hasNext() and next() method. Similarly there can be an example for the remove() method. And the experienced users had commented on the other answers to use the Iterator class instead of using the for loop to iterate through the List. Why?

What confuses me even more is that the Iterator class has only three methods. And the functionality of those can be achieved with writing a little different code as well.

Some people might argue that the functionality of many classes can be achieved by writing one’s own code instead of using the class made for the purpose. Yes,true. But as I said, Iterator class has only three methods. So why go through the hassle of creating an extra class when the same job can be done with a simple block of code which is not way too complicated to understand either.

---

EDIT:

While I’m at it, since many of the answers say that I can’t achieve the remove functionality without using Iterator,I would just like to know if the following is wrong, or will it have some undesirable result.

for(A obj : myList)
{
           if(obj.myVar == 1)
                 myList.remove(obj);
}

Doesn’t the above code snippet do the same thing as remove() ?

Answer 1

First of all, the for-each construct actually uses the Iterator interface under the covers. It does not, however, expose the underlying Iterator instance to user code, so you can’t call methods on it.

This means that there are some things that require explicit use of the Iterator interface, and cannot be achieved by using a for-each loop.

Removing the current element is one such use case.

For other ideas, see the ListIterator interface. It is a bidirectional iterator that supports inserting elements and changing the element under the cursor. None of this can be done with a for-each loop.

for(A obj : myList)
{
           if(obj.myVar == 1)
                 myList.remove(obj);
}

Doesn’t the above code snippet do the same thing as remove() ?

No, it does not. All standard containers that I know of will throw ConcurrentModificationException when you try to do this. Even if it were allowed to work, it is ambiguous (what if obj appears in the list twice?) and inefficient (for linked lists, it would require linear instead of constant time).