Question

I have to iterate through an arraylist in this manner.

ArrayList<Integer> li = new ArrayList<Integer>();
li.add(20);
li.add(30);
li.add(40);
li.add(50);
li.add(70);

for (int i = 0; i < li.size() - 1; i++)
    System.out.println(li.get(i) + " " + li.get(i + 1));

Output:

20 30
30 40
40 50
50 70

How to do the same using an iterator?

Answer 1

Use two iterators. I tested this and it worked for me.

    Iterator<Integer> first = li.listIterator();

    // Will raise IndexOutOfBoundsException if list is empty.
    Iterator<Integer> second = li.listIterator(1);

    while (second.hasNext()) {
        System.out.println(first.next() + " " + second.next());
    }

Edit: No need for inner if. Silly me.

Explanation: the listIterator(index) method returns an iterator that starts at the specified position in the list where as listIterator() returns an iterator that starts at position zero.

The first iterator therefore starts at 0 and the second starts at 1. Then it is merely a question of printing the next() on both. This will work irrespective of whether the number of elements in the list is odd or even.

Edit 2

My logic fu is very weak today. Thanks @barjak for pointing out the case about the empty list and the unnecessary first.hasNext().