List of Lists iterator.

ListOfLists.java

package collections;

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

/**
 * Created by amit.saurav on 11/23/16.
 */
public class ListOfLists <T> {
    private final List<List<T>> listOfLists;
    private int currentRow;
    private int currentCol;

    public ListOfLists(List<List<T>> lists) {
        listOfLists = lists;
        resetIterator();
    }

    public void resetIterator() {
        currentCol = currentRow = 0;
    }

    public boolean hasNext() {
        // Not enough elements left
        if (currentRow == listOfLists.size() - 1) {
            return false;
        }

        // Definitely more elements left in the current list
        if (listOfLists.get(currentRow).size() > currentCol + 1) {
            return true;
        }

        return getNextNonEmptyList() == -1 ? false : true;
    }

    public T next() {
        if (!hasNext()) {
            return null;
        }

        // Definitely more elements left in the current list
        if (listOfLists.get(currentRow).size() > currentCol) {
            return listOfLists.get(currentRow).get(currentCol++);
        }

        currentRow = getNextNonEmptyList();
        if (currentRow == -1) {
            resetIterator();
            return null;
        } else {
            currentCol = 0;
            return listOfLists.get(currentRow).get(currentCol++);
        }
    }

    private int getNextNonEmptyList() {
        int tempRow = currentRow;
        while (tempRow < listOfLists.size() - 1 && listOfLists.get(++tempRow).size() == 0);

        if (tempRow == listOfLists.size() - 1) {
            return -1;
        } else {
            return tempRow;
        }
    }

    public static void main(String[] args) {
        List<Integer> list1 = Arrays.asList( 1,  2,  3, 4, 5);
        List<Integer> list2 = Arrays.asList( 6,  7,  8, 9);
        List<Integer> list3 = Arrays.asList(10, 11, 12);
        List<Integer> list4 = Arrays.asList(13, 14);
        List<Integer> list5 = Arrays.asList(15);
        List<Integer> list6 = Arrays.asList();
        List<Integer> list7 = Arrays.asList();
        List<Integer> list8 = Arrays.asList(16);
        List<Integer> list9 = Arrays.asList();
        List<List<Integer>> lists = Arrays.asList(list1, list2, list3, list4, list5, list6, list7, list8, list9);

        ListOfLists<Integer> subject = new ListOfLists<>(lists);
        while (subject.hasNext()) {
            System.out.print(subject.next() + ", ");
        }
        System.out.println();
    }
}