list2.java

package com.company;

import java.util.*;

public class Main {

    public static void main(String[] args) {

        ArrayList<ArrayList<Object>> masterList = new ArrayList<>();

        ArrayList<Object> output = new ArrayList<>();

        ArrayList<Object> brianList = new ArrayList<>();
        brianList.addAll(Arrays.asList('b', 'r', 'i', 'a', 'n'));

        ArrayList<Object> jeffList = new ArrayList<>();
        jeffList.addAll(Arrays.asList('j', 'e', 'f', 'f'));

        ArrayList<Object> numberList = new ArrayList<>();
        numberList.addAll(Arrays.asList('1', '2', '3', '4', '5', '6', '7', '8', '9', '0'));

        masterList.add(brianList);
        masterList.add(jeffList);
        masterList.add(numberList);

        ArrayList<Iterator<Object>> iterators = new ArrayList<>();

        for(ArrayList<Object> sub : masterList) {
            iterators.add(sub.iterator());
        }

        int i = 0;
        while(!iterators.isEmpty()) {
            Iterator it = iterators.get(i);

            if(it.hasNext())
                output.add(it.next());
             else 
                iterators.remove(it);
            
            ++i;

            if(i > iterators.size() - 1)
                i = 0;
        }

        StringBuilder outputString = new StringBuilder();

        for(Object o : output) {
            outputString.append(o);
        }

        System.out.println(outputString.toString());
    }
}