Iterator vs for

The main difference between Iterator and the classic for loop, apart from the obvious one of having or not having access to the index of the item you're iterating, is that using Iterator abstracts the client code from the underlying collection implementation, allow me to elaborate.

When your code uses an iterator, either in this form

for(Item element : myCollection) { ... }

this form

Iterator<Item> iterator = myCollection.iterator();
while(iterator.hasNext()) {    
    Item element = iterator.next();    
    ... 
}

or this form

for(Iterator iterator = myCollection.iterator(); iterator.hasNext(); ) {
   Item element = iterator.next();
   ...
}

What your code is saying is "I don't care about the type of collection and its implementation, I just care that I can iterate through its elements". Which is usually the better approach, since it makes your code more decoupled.

On the other hand, if you're using the classic for loop, as in

for(int i = 0; i < myCollection.size(); i++) {
   Item element = myCollection.get(i);
   ...
}

Your code is saying, I need to know the type of collection, because I need to iterate through its elements in a specific way, I'm also possibly going to check for nulls or compute some result based on the order of iteration. Which makes your code more fragile, because if at any point the type of collection you receive changes, it will impact the way your code works.

Summing it up, the difference is not so much about speed, or memory usage, is more about decoupling your code so that is more flexible to cope with change.

First of all, there are 2 kinds of for loops, which behave very differently. One uses indices:

for (int i = 0; i < list.size(); i++) {
    Thing t = list.get(i);
    ...
}

This kind of loop isn't always possible. For example, Lists have indices, but Sets don't, because they're unordered collections.

The other one, the foreach loop uses an Iterator behind the scenes:

for (Thing thing : list) {
    ...
}

This works with every kind of Iterable collection (or array)

And finally, you can use an Iterator, which also works with any Iterable:

for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) {
    Thing t = it.next();
    ...
} 

So you in fact have 3 loops to compare.

You can compare them in different terms: performance, readability, error-proneness, capability.

An Iterator can do things that a foreach loop can't. For example, you can remove elements while you're iterating, if the iterator supports it:

for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) {
    Thing t = it.next();
    if (shouldBeDeleted(thing) {
        it.remove();
    }
} 

Lists also offer iterators that can iterate in both directions. A foreach loop only iterates from the beginning to an end.

But an Iterator is more dangerous and less readable. When a foreach loop is all you need, it's the most readable solution. With an iterator, you could do the following, which would be a bug:

for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) {
    System.out.println(it.next().getFoo());
    System.out.println(it.next().getBar());
} 

A foreach loop doesn't allow for such a bug to happen.

Using indices to access elements is slightly more efficient with collections backed by an array. But if you change your mind and use a LinkedList instead of an ArrayList, suddenly the performance will be awful, because each time you access list.get(i), the linked list will have to loop though all its elements until the ith one. An Iterator (and thus the foreach loop) doesn't have this problem. It always uses the best possible way to iterate through elements of the given collection, because the collection itself has its own Iterator implementation.

My general rule of thumb is: use the foreach loop, unless you really need capabilities of an Iterator. I would only use for loop with indices with arrays, when I need access to the index inside the loop.

Iterator Advantage:

• Ability to remove elements from Collections.
• Ability to move forward and backward using next() and previous().
• Ability to check if there more elements or not by using hasNext().

Loop was designed only to iterate over a Collection, so if you want just to iterate over a Collection, its better to use loop such as for-Each, but if you want more that that you could use Iterator.

if you access to data by number (e.g. "i"), it is fast when you use array. because it goes to element directly

But, other data structure (e.g. tree, list), it needs more time, because it start from first element to target element. when you use list. It needs time O(n). so, it is to be slow.

if you use iterator, compiler knows that where you are. so It needs O(1) (because, it start from current position)

finally, if you use only array or data structure that support direct access(e.g. arraylist at java). "a[i]" is good. but, when you use other data structure, iterator is more efficient