Vector2DIterator.java

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class Vector2DIterator implements Iterator<Integer> {
    Iterator<List<Integer>> parent = null;
    Iterator<Integer> child = null;

    void updateChild() {
        while (!child.hasNext() && parent.hasNext()) {
            child = parent.next().iterator();
        }
    }

    public Vector2DIterator(List<List<Integer>> vec2d) {
        if (vec2d == null || vec2d.size() == 0 ) { return; }
        parent = vec2d.iterator();
        if (parent.hasNext()) {
            child = parent.next().iterator();
            updateChild();
        }
    }

    @Override
    public boolean hasNext() {
        return child != null && child.hasNext();
    }

    @Override
    public Integer next() {
        Integer temp = child.next();
        updateChild();
        return temp;
    }

    static void iterate(Vector2DIterator v2DIter) {
        while (v2DIter.hasNext()) {
            System.out.print(v2DIter.next() + ", ");
        }
        System.out.println();
    }
    
    public static void main(String[] args) {
        List<Integer> inner0 = Arrays.asList(1, 2);
        List<Integer> inner1 = Arrays.asList(3);
        List<Integer> inner2 = Arrays.asList(4, 5, 6);
        List<List<Integer>> input = Arrays.asList(inner0, inner1, inner2);
        Vector2DIterator v2DIter = new Vector2DIterator(input);
        iterate(v2DIter);
    }
}