Iterator

There is a powerful and consistent way to traverse almost all data structures in Java including lists - Iterator.

The two important methods belonging to Iterator interface are:

1. hasNext(): returns true if there is an item in front of the current position of the iterator.
2. next(): returns the item in front of the current position of the iterator (if any), and moves the iterator forward. Throws NoSuchElementException if the iteration has no more elements.

A class that implements Iterator interface can add other methods too. ListIterator is one such class.

We will focus only on hasNext() and next().

The advantages of using an iterator are,

1. It’s consistent across any class that implements the Iterator interface.
2. It’s intuitive as in “while the collection has another item, access it”.

Creating and using a ListIterator on a List object

A ListIterator iter is created on a List object list as:

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ListIterator<Integer> iter = list.listIterator();

Assuming the list is [10, 70, 20, 90, 30, 80], the initial state of iter is represented by the red block.

You can check if there is an item in front of the current position of the iterator as:

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boolean itemExists = iter.hasNext();

Once you check an item exists, you can access it as:

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Object item = iter.next();

This (accessing an item) results in the iterator moving forward by one.

Complete example

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ArrayList<Integer> list = new ArrayList(Arrays.asList(10, 70, 20, 90, 30, 80));
ListIterator<Integer> iter = list.listIterator();
while(iter.hasNext()) {
  System.out.print(iter.next()+" ");
}
//displays 10 70 20 90 80 30

Initial state

After first iteration

After second iteration

After third iteration

After fourth iteration

After fifth iteration

After sixth iteration