code: public String getFieldValue(String fieldName) {
          Utils.require(!Utils.isEmpty(fieldName), "fieldName");
          
          List<String> values = m_valueMap.get(fieldName);
          if (values == null || values.size() == 0) {
              return null;
          }
          Utils.require(values.size() == 1, "Field has more than 1 value: %s", fieldName);
          return values.get(0);
      }
docs: Get the single value of the field with the given name or null if no value is
  assigned to the given field. This method is intended to be used for fields expected
  to have a single value. An exception is thrown if it is called for a field with
  multiple values.
  
  @param fieldName Name of a field.
  @return          Value of field for this object or null if there is none.