Using ListIterator to move back and forth over a LinkedList in Java

Question

I have a LinkedList over which I need to iterate back and forth multiple times. I am using it to keep track of a series of pages in a workflow that will be created dynamically.

Answer 1

This should do your job:

public class Main {
    public static void main(String[] args) {
        final LinkedList<String> list = new LinkedList<String> ();

        list.add ("1"); list.add ("2"); list.add ("3"); list.add ("4");

        final MyIterator<String> it = new MyIterator (list.listIterator());

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

    public static class MyIterator<T> {

        private final ListIterator<T> listIterator;

        private boolean nextWasCalled = false;
        private boolean previousWasCalled = false;

        public MyIterator(ListIterator<T> listIterator) {
            this.listIterator = listIterator;
        }

        public T next() {
            nextWasCalled = true;
            if (previousWasCalled) {
                previousWasCalled = false;
                listIterator.next ();
            }
            return listIterator.next ();
        }

        public T previous() {
            if (nextWasCalled) {
                listIterator.previous();
                nextWasCalled = false;
            }
            previousWasCalled = true;
            return listIterator.previous();
        }

    }   
}

And a fiddle for it.

Answer 2

Do something like this (pseudocode) --

class SkipIterator extends ListIterator {

    public E previous(){
        E n = super.previous();
        return super.previous();
    }

    ...

}

then:

LinkedList<String> navigationCases;
navigationCases.add("page1");
navigationCases.add("page2");
navigationCases.add("page3");
navigationCases.add("page4");

SkipIterator navigationItr = (SkipIterator)navigationCases.listIterator();
navigationItr.next(); // Returns page1
navigationItr.next(); // Returns page2
navigationItr.previous(); // Returns page1

Cheers

Answer 3

ListIterator was designed to behave this way. See the conversation beneath ShyJ's answer for the rationale.

I find this behavior to be beyond idiotic, and have instead written a very simple alternative. Here's the Kotlin code with a extension function for ArrayLists:

class ListIterator<E>(var list: ArrayList<E>) : Iterator<E> {

    private var cursor: Int = 0

    fun replace(newList: ArrayList<E>) {
        list = newList
        cursor = 0
    }

    override fun hasNext(): Boolean {
        return cursor + 1 < list.size
    }

    override fun next(): E {
        cursor++
        return current()
    }

    fun hasPrevious(): Boolean {
        return 0 <= cursor - 1
    }

    fun previous(): E {
        cursor--
        return current()
    }

    fun current(): E {
        return list[cursor]
    }

}

fun <E> ArrayList<E>.listFlippingIterator() = ListIterator(this)

If you wish to include removal functionality, I highly recommend writing the API to explicitly instruct the iterator if it should remove left or right, e.g. by defining those methods as removeNext() and removePrevious().