code: public String getParameterValue(final String _key)
      {
          final String[] paramValues = getParameterValues(_key);
          return paramValues != null && paramValues.length > 0 ? paramValues[0] : null;
      }
docs: Evaluates with given key in the list of all parameters for given key and
  returns them (if found) as string with index 0 in the string array of the
  parameter values. If not found a <code>null</code> is returned.
  
  @param _key name of parameter value which should returned
  @return value for given key or <code>null</code> if not exists
  @see #getParameterValues to get the string array for given key