A better way to use Java 8

I convert the following code into Java 8 code I wonder if I'm doing it right or if there are other good ways

Java 7

for (final Category category : categories) {
      final List<Category> subCategories = getCategories(category);
      if (subCategories != null) {
        currentLevel.addAll(subCategories);
      }
    }

Java8

categories.stream().map(category -> getCategories(category)).filter(list->list!=null).flatMap(cat -> cat.parallelStream()).collect(Collectors.toList())

Any Java 8 way parses the following code into a compact form

while (CollectionUtils.isNotEmpty(currentLevel)) {
  for (final Iterator<Category> iterator = currentLevel.iterator(); iterator.hasNext();) {
    final Category category = iterator.next();
    if (result == null) {
      result = new HashSet<Category>();
    }
    if (!result.add(category)) {
      // avoid cycles by removing all which are already found
      iterator.remove();
    }
  }

  if (currentLevel.isEmpty()) {
    break;
  }
  final Collection<Category> nextLevel = getAllSubcategories(currentLevel);
  currentLevel = nextLevel;
}

Solution

Your solution is OK, but it's useless to map parallel to parallel streams If you look at the implementation of flatmap in openjdk / Oracle JDK, you can see that the flow created through the lambda passed to flatmap will immediately change to sequential mode So you won't have any parallelism. It's best to replace parallelstream () with stream () to avoid confusion If you really want to parallelize work, it is usually best to parallelize only the outermost streams