Why the following java code is giving error?

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Java

I recently learned about iterating a collection in Java using the Iterator. I thought I completely understood the concept until I tested my understanding. It turns out I didn't get it right.

Can someone please explain why the 1st code runs but the 2nd code results in an infinite while loop.

1st:

ArrayList<Integer> al = new ArrayList<>(Arrays.asList(1,2,3));
Iterator<Integer> iterator = al.iterator();
while(iterator.hasNext()){
    int i = iterator.next();
    System.out.println(i);
}//This works

2nd:

ArrayList<Integer> al = new ArrayList<>(Arrays.asList(1,2,3));
while (al.iterator().hasNext()) {
    System.out.println(al.iterator().next());
}//This doesn't work

As you can tell, I'm totally new to java and programming. Any help will be greatly appreciated.

What I have tried:

I tried searching this on google but couldn't find a solution specific to my problem, I guess I'm the only dumbest person around here.