參照源碼的相關說明

/**
?*?@return?the?previous?value?associated?with?<tt>key</tt>,?or
?*?????????<tt>null</tt>?if?there?was?no?mapping?for?<tt>key</tt>.
?*?????????(A?<tt>null</tt>?return?can?also?indicate?that?the?map
?*?????????previously?associated?<tt>null</tt>?with?<tt>key</tt>,
?*?????????if?the?implementation?supports?<tt>null</tt>?values.)
?*/