public function getField($fieldName)
    {
        if (!isset($this->fields[$fieldName])) {
            throw new FieldNotDefined($this, $fieldName);
        }
        return $this->fields[$fieldName];
    }
public function getField($fieldName)
    {
        if (!isset($this->fields[$fieldName])) {
            throw new FieldNotDefined($this, $fieldName);
        }
        return $this->fields[$fieldName];
    }
@param string $fieldName

@return Field
@throws FieldNotDefined