a temp gist, this code is used in testng to create combination of parameter in dataProvider

ParamSetListBuilder.java

    public static class ParamSetListBuilder {

        private List<List> paramList = new ArrayList<>();
        private Logger logger = LoggerFactory.getLogger(getClass());

        public ParamSetListBuilder addParams(List paramListToCombine) {
            addParamsAtIndex(paramListToCombine, Helper.calcParamIndex(paramList));
            return this;
        }

        public ParamSetListBuilder addParamsAtIndex(List paramListToCombine, int index) {
            logger.info(paramListToCombine.toString());
            if(paramList.isEmpty()) {
                paramList = Helper.makeListOfListOfSignleElement(paramListToCombine);
            } else {
                paramList = Helper.combineListOfList(paramList, paramListToCombine, index);
            }
            return this;
        }

        public List<List> get() {
            return paramList;
        }

        @Override
        public String toString() {
            return MoreObjects.toStringHelper(this)
                    .add("paramList", paramList)
                    .toString();
        }

        /**
         * Helper to have pure function, which are easy to understand and test
         */
        public static class Helper {

            public static int calcParamIndex(List<List> listOfList) {
                if(listOfList.isEmpty()) {
                    return 0;
                } else {
                    return listOfList.get(0).size();
                }
            }

            public static List clone(List list) {
                return (List)((ArrayList)list).clone();
            }

            public static List<List> combine(List a, List b, int index) {
                List<List> listOfList = new ArrayList<>(b.size());
                for(Object o : b) {
                    List tempList = clone(a);
                    tempList.add(index, o);
                    listOfList.add(tempList);
                }
                return listOfList;
            }

            public static List<List> combineListOfList(List<List> listOfList, List listToCombine, int index) {
                List<List> combinedListOfList = new ArrayList<>();
                for(List l : listOfList) {
                    combinedListOfList.addAll(combine(l, listToCombine, index));
                }
                return combinedListOfList;
            }

            public static List<List> makeListOfListOfSignleElement(List list) {
                List<List> listOfLists = new ArrayList<>(list.size());
                for(Object o : list) {
                    List temp = new ArrayList();
                    temp.add(o);
                    listOfLists.add(temp);
                }
                return listOfLists;
            }
        }
    }


    @Test
    public void test() {
        List<String> xList = Stream
                .of("x", "y", "z")
                .collect(Collectors.toCollection(ArrayList::new));
        ParamSetListBuilder paramSetListBuilder = new ParamSetListBuilder();
        paramSetListBuilder.addParamsAtIndex(xList, 0);
        logger.info(paramSetListBuilder.get().toString());
        assertEquals(xList.size(), paramSetListBuilder.get().size());

        List<String> aList = Stream
                .of("a", "b", "c")
                .collect(Collectors.toCollection(ArrayList::new));
        paramSetListBuilder.addParams(aList);
        logger.info(paramSetListBuilder.get().toString());
        List<String> dList = Stream
                .of("1", "2", "3", "4")
                .collect(Collectors.toCollection(ArrayList::new));

        paramSetListBuilder.addParams(dList);
        logger.info(paramSetListBuilder.get().toString());
    }