code: public SingleOutputStreamOperator<T> sum(String field) {
  		return aggregate(new SumAggregator<>(field, input.getType(), input.getExecutionConfig()));
  	}
docs: Applies an aggregation that sums every window of the pojo data stream at
  the given field for every window.
  
  <p>A field expression is either the name of a public field or a getter method with
  parentheses of the stream's underlying type. A dot can be used to drill down into objects,
  as in {@code "field1.getInnerField2()" }.
  
  @param field The field to sum
  @return The transformed DataStream.