code: protected String getFieldName(String propertyName) {
      return ObjectUtils.defaultIfNull(propertyNameToFieldNameMapping.get(propertyName), propertyName);
    }
docs: Gets the name of the field mapped to the given property.  If no such mapping exists, then the given property name
  is returned.
  
  @param propertyName a String value specifying the name of the property to retrieve the corresponding field name.
  @return a String value indicating the field name corresponding to the specified property.