在java中,Map里的put方法,如果key值不存在,则返回值是null,但是key值如果存在,则会返回原先被替换掉的value值.(当然,map中的key和value都允许是null).

		Map map = new HashMap();Object obj = null;obj = map.put(null, null);System.out.println(obj); //nullobj = map.put(null, "123"); System.out.println(obj); //nullobj = map.put(null, null);System.out.println(obj); //123obj = map.put(1, 123);System.out.println(obj); //nullobj = map.put(1, "123");System.out.println(obj); //123obj = map.put(1, 123);System.out.println(obj); //123