code: List<PartitionInstance> getAbsent() {
      List<PartitionInstance> absentPartitions = new ArrayList<>();
      for (Map.Entry<Integer, List<PartitionInstance>> e : nodeIndexToHostedPartitions.entrySet()) {
        if (e.getKey() < 0) {
          absentPartitions.addAll(e.getValue());
        }
      }
      return absentPartitions;
    }
docs: Returns the partition instances whose node indexes are < 0.