code: public Evaluator getEvaluator( ValueType type,
                                     String operatorId,
                                     boolean isNegated,
                                     String parameterText ) {
          return this.getEvaluatorDefinition( operatorId ).getEvaluator( type,
                                                                         operatorId,
                                                                         isNegated,
                                                                         parameterText );
      }
docs: Returns the evaluator instance for the given type and the
  defined parameterText
  
  @param type the type of the attributes this evaluator will
  operate on. This is important because the evaluator
  may do optimizations and type coercion based on the
  types it is evaluating. It is also possible that
  this evaluator does not support a given type.
  
  @param operatorId the string identifier of the evaluator
  
  @param isNegated true if the evaluator instance to be returned is
  the negated version of the evaluator.
  
  @param parameterText some evaluators support parameters and these
  parameters are defined as a String that is
  parsed by the evaluator itself.
  
  @return an Evaluator instance capable of evaluating expressions
  between values of the given type, or null in case the type
  is not supported.