code: public IntermediateResultPartition getPartitionById(IntermediateResultPartitionID resultPartitionId) {
  		// Looks ups the partition number via the helper map and returns the
  		// partition. Currently, this happens infrequently enough that we could
  		// consider removing the map and scanning the partitions on every lookup.
  		// The lookup (currently) only happen when the producer of an intermediate
  		// result cannot be found via its registered execution.
  		Integer partitionNumber = partitionLookupHelper.get(checkNotNull(resultPartitionId, "IntermediateResultPartitionID"));
  		if (partitionNumber != null) {
  			return partitions[partitionNumber];
  		} else {
  			throw new IllegalArgumentException("Unknown intermediate result partition ID " + resultPartitionId);
  		}
  	}
docs: Returns the partition with the given ID.
  
  @param resultPartitionId ID of the partition to look up
  @throws NullPointerException If partition ID <code>null</code>
  @throws IllegalArgumentException Thrown if unknown partition ID
  @return Intermediate result partition with the given ID