ZeroFieldOperator is a base class for operators that take no input. The main use case is the CountOperator, which counts the occurrences of each unique key. Other uses are possible, for example, weighted random number assignment.
The primary rationale for ZeroFieldOperator and ZeroFieldCalculator is to clarify the information available to such a routine. In particular, the split fields passed to processHeaderLine and processNextLine don't include all fields in the input, something that might not be obvious when implementing an operator. (Only fields required by operators acting on specific fields are included.)