void subscribeNext() { Observable<? extends T> t; synchronized (guard) { t = queue.peek(); if (t == null || active >= maxConcurrency) { return; } active++; queue.poll(); } MergeItemSubscriber itemSub = new MergeItemSubscriber(SOURCE_INDEX.getAndIncrement(this)); subscribers.add(itemSub); csub.add(itemSub); WIP.incrementAndGet(this); t.unsafeSubscribe(itemSub); request(1); }