Interview: Design an iterator for a collection of collections

Question

Design an iterator for a collection of collections in java. The iterator should hide the nesting, allowing you to iterate all of the elements belonging to all of the collect

Answer 1

Here is another implementation:

import java.util.Iterator;
import java.util.NoSuchElementException;

import static java.util.Collections.emptyIterator;

public class Multiterator implements Iterator {
    private Iterator> root;
    private Iterator current;

    public Multiterator(Iterator> root) {
        this.root = root;
        current = null;
    }

    @Override
    public boolean hasNext() {
        if (current == null || !current.hasNext()) {
            current = getNextNonNullOrEmpty(root);
        }
        return current.hasNext();
    }

    private Iterator getNextNonNullOrEmpty(Iterator> root) {
        while (root.hasNext()) {
            Iterator next = root.next();
            if (next != null && next.hasNext()) {
                return next;
            }
        }
        return emptyIterator();
    }

    @Override
    public E next() {
        if (current == null) {
            throw new NoSuchElementException();
        }
        return current.next();
    }
}