Java - Endless loop with ListIterator .hasNext()

Question

ArrayList<String> list = new ArrayList<String>();
list.add("test");

while(list.listIterator().hasNext()) {
    System.out.println(list.listIterator().next());
}

This generates an endless loop of lines with "test". Why does this happen and how to fix it?

Fortran

You are generating a new iterator at each loop step so as it is always brand new and your list contains an item, hasNext() always returns true. – Desultory

you can alternatively do it in one line when you use a for loop: for (ListIterator<String> it = list.listIterator(); it.hasNext();) { /* looped code */ }, limits the scope of the iterator to just that loop. – Ashla

Answer 1

The iterator created for hasNext is not the same that is for next.

ArrayList<String> list = new ArrayList<String>();
list.add("test");
Iterator listIterator = list.listIterator()
while(listIterator.hasNext()) {
        System.out.println(listIterator.next());
}

Millford

Answer 2

Since you are creating a new iterator on each loop, the newly created iterator pointer will be positioned before the first element in the List. hasNext() method in that case will always return true and next() method will return the first element in the list.

Nixie

Answer 3

Here:

System.out.println(list.listIterator().next());

you create an new iterator each time you try to print out the next element.

Just try the following:

ArrayList<String> list = new ArrayList<String>();

list.add("test");

Iterator<String> it = list.iterator();

while(it.hasNext()) {

    System.out.println(it.next());

}

Nepean

Answer 4

Use this code,

List<String> list = new ArrayList<String>();
ListIterator<String> litr = null;
list.add("test");
litr=list.listIterator();
System.out.println("Elements in forward directiton");
while(litr.hasNext()){
   System.out.println(litr.next());
}

Printery

Answer 5

You create new instance of iterator in every list.listIterator() call.

Create list iterator once, before loop starts to solve your problem.

Surtout

Answer 6

Your calling listIterator everytime. Try assign to a variable and loop over that.

Involution

Answer 7

list.listIterator() returns new Iterator. Below is source of AbstractList

  public ListIterator<E> listIterator() {
        return listIterator(0);
    }
  public ListIterator<E> listIterator(int location) {
        return new FullListIterator(location);
    }

You should make loop like this,

ArrayList<String> list = new ArrayList<String>();
list.add("test");
Iterator<String> iterator = list.listIterator();
while(iterator .hasNext()) {
        System.out.println(iterator.next());
        }

Warr

Answer 8

Your problem is that you're creating new iterator at each list.listIterator() call. All the new instances will be at the beginning of the iterator. You can fix it if you use the same iterator:

    ArrayList<String> list = new ArrayList<String>();
    list.add("test");

    Iterator<String> iterator = list.listIterator();

    while(iterator.hasNext()) {
        System.out.println(iterator.next());
    }

Fail