public boolean isEmpty() {
    if (subtaskSplitMap.isEmpty()) {
      return true;
    }
    for (Map.Entry<Integer, PriorityBlockingQueue<MixedFormatSplit>> entry :
        subtaskSplitMap.entrySet()) {
      if (!entry.getValue().isEmpty()) {
        return false;
      }
    }
    return true;
  }
Key is subtaskId, Value is the queue of unAssigned mixed-format splits.
amoro-format-mixed/amoro-mixed-flink/amoro-mixed-flink-common/src/main/java/org/apache/amoro/flink/read/hybrid/assigner/ShuffleSplitAssigner.java