code: public <K> DataStream<T> partitionCustom(Partitioner<K> partitioner, int field) {
  		Keys.ExpressionKeys<T> outExpressionKeys = new Keys.ExpressionKeys<>(new int[]{field}, getType());
  		return partitionCustom(partitioner, outExpressionKeys);
  	}
docs: Partitions a tuple DataStream on the specified key fields using a custom partitioner.
  This method takes the key position to partition on, and a partitioner that accepts the key type.
  
  <p>Note: This method works only on single field keys.
  
  @param partitioner The partitioner to assign partitions to keys.
  @param field The field index on which the DataStream is partitioned.
  @return The partitioned DataStream.