src/main/java/io/reactivex/internal/util/ExceptionHelper.java
public static List<Throwable> flatten(Throwable t) {
        List<Throwable> list = new ArrayList<Throwable>();
        ArrayDeque<Throwable> deque = new ArrayDeque<Throwable>();
        deque.offer(t);

        while (!deque.isEmpty()) {
            Throwable e = deque.removeFirst();
            if (e instanceof CompositeException) {
                CompositeException ce = (CompositeException) e;
                List<Throwable> exceptions = ce.getExceptions();
                for (int i = exceptions.size() - 1; i >= 0; i--) {
                    deque.offerFirst(exceptions.get(i));
                }
            } else {
                list.add(e);
            }
        }

        return list;
    }
public static List<Throwable> flatten(Throwable t) {
        List<Throwable> list = new ArrayList<Throwable>();
        ArrayDeque<Throwable> deque = new ArrayDeque<Throwable>();
        deque.offer(t);

        while (!deque.isEmpty()) {
            Throwable e = deque.removeFirst();
            if (e instanceof CompositeException) {
                CompositeException ce = (CompositeException) e;
                List<Throwable> exceptions = ce.getExceptions();
                for (int i = exceptions.size() - 1; i >= 0; i--) {
                    deque.offerFirst(exceptions.get(i));
                }
            } else {
                list.add(e);
            }
        }

        return list;
    }
Returns a flattened list of Throwables from tree-like CompositeException chain.
@param t the starting throwable
@return the list of Throwables flattened in a depth-first manner