code: public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
      if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
        invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
        return tasks;
      }
      List<? extends ForkJoinTask<?>> ts = (List<? extends ForkJoinTask<?>>) tasks;
      Throwable ex = null;
      int last = ts.size() - 1;
      for (int i = last; i >= 0; --i) {
        ForkJoinTask<?> t = ts.get(i);
        if (t == null) {
          if (ex == null)
            ex = new NullPointerException();
        } else if (i != 0)
          t.fork();
        else if (t.doInvoke() < NORMAL && ex == null)
          ex = t.getException();
      }
      for (int i = 1; i <= last; ++i) {
        ForkJoinTask<?> t = ts.get(i);
        if (t != null) {
          if (ex != null)
            t.cancel(false);
          else if (t.doJoin() < NORMAL)
            ex = t.getException();
        }
      }
      if (ex != null)
        rethrow(ex);
      return tasks;
    }
docs: Forks all tasks in the specified collection, returning when {@code isDone} holds for each task
  or an (unchecked) exception is encountered, in which case the exception is rethrown. If more
  than one task encounters an exception, then this method throws any one of these exceptions. If
  any task encounters an exception, others may be cancelled. However, the execution status of
  individual tasks is not guaranteed upon exceptional return. The status of each task may be
  obtained using {@link #getException()} and related methods to check if they have been
  cancelled, completed normally or exceptionally, or left unprocessed.
  
  @param tasks the collection of tasks
  @param <T> the type of the values returned from the tasks
  @return the tasks argument, to simplify usage
  @throws NullPointerException if tasks or any element are null