Iterator has .next() - is there a way to get the previous element instead of the next one?

Question

I have an Iterator that I use on a HashMap, and I save and load the iterator. is there a way to get the previous key in the HashMap with Iterator? (java.util.Iterator)

Answer 1

using iterator, No you dont have an option to get a previous key value. it has only hasNext() and next() methods.

Answer 2

Make your own Iterator:

public class EnhancedIterator<E> implements Iterator<E>{
    private List<E> list;
    private int indexSelected=-1;
    public EnhancedIterator(List<E> list){
        this.list=list;
    }

    @Override
    public boolean hasNext() {
        return indexSelected<list.size()-1;
    }

    @Override
    public E next() {
        indexSelected++;
        return current();
    }

    @Override
    public void remove() {
        list.remove(indexSelected);
    }
    public void remove(int i){
        list.remove(i);
        if(i<indexSelected){
            indexSelected--;
        }
    }
    public E previous(){
        indexSelected--;
        return current();
    }
    public E current(){
        return list.get(indexSelected);
    }
    public E get(int i){
        return list.get(i);
    }
}

Answer 3

Not directly, as others pointed out, but if you e.g. need to access one previous element you could easily save that in a separate variable.

T previous = null;
for (Iterator<T> i = map.keySet().iterator(); i.hasNext();) {
    T element = i.next();

    // Do something with "element" and "previous" (if not null)

    previous = element;
}

Answer 4

Ultimately Iterators are not fully suited for your task.

Why not create a List from your Set (via, eg, List list = new LinkedList(set)) and iterate by using a standard indexed for-loop? That way you know the previous element is at i - 1.