List<List<Integer>> list = new ArrayList<>(); list.add(Arrays.asList(new Integer[]{1,2})); list.add(Arrays.asList(new Integer[]{3,4})); list.add(null); list.add(Arrays.asList(new Integer[]{4,3,5})); List<Integer> result = list.stream() .filter(item-> item != null) .reduce(new ArrayList<>(), (all, item ) -> {all.addAll(item); return all;}); System.out.println(result);