code: public SortedGrouping<T> sortGroup(int field, Order order) {
  		if (groupSortSelectorFunctionKey != null) {
  			throw new InvalidProgramException("Chaining sortGroup with KeySelector sorting is not supported");
  		}
  		if (!Keys.ExpressionKeys.isSortKey(field, inputDataSet.getType())) {
  			throw new InvalidProgramException("Selected sort key is not a sortable type");
  		}
  
  		ExpressionKeys<T> ek = new ExpressionKeys<>(field, inputDataSet.getType());
  
  		addSortGroupInternal(ek, order);
  		return this;
  	}
docs: Sorts {@link org.apache.flink.api.java.tuple.Tuple} elements within a group on the specified field in the specified {@link Order}.
  
  <p><b>Note: Only groups of Tuple or Pojo elements can be sorted.</b>
  
  <p>Groups can be sorted by multiple fields by chaining {@link #sortGroup(int, Order)} calls.
  
  @param field The Tuple field on which the group is sorted.
  @param order The Order in which the specified Tuple field is sorted.
  @return A SortedGrouping with specified order of group element.
  
  @see org.apache.flink.api.java.tuple.Tuple
  @see Order