when I declare a Map or Map <Object,Object> I can put anything in this Map
Map map = new HashMap(); map.put(""); but if I declare it as Map <?,?> I can put nothing in it
Map<?,?> map = new HashMap(); map.put(""); it will goes wrong why ?
- 1bayou.io/draft/Capturing_Wildcards.htmlZhongYu– ZhongYu2016-01-13 04:24:31 +00:00Commented Jan 13, 2016 at 4:24
Add a comment |
3 Answers
Map<?,?> map = new HashMap<Integer, Integer>(); // compiles just fine! ? represents some fixed but unknown type. You can't put "" in a Map<Integer, Integer>, and a Map<?, ?> is allowed to be any type of Map, including a Map<Integer, Integer>.
2 Comments
4862610
how can I use it , I declare a Map but I can`t put anything in it
Louis Wasserman
@Wangbo you shouldn't use it for Maps you are going to put stuff into. You can use it for Maps you're going to take things out of, though.
In java collection Frame, ? means unknown type. You can only read elements from that, but can not add elements except for NULL value.
So you can compile fine like below:
Map<?,?> map = new HashMap(); map.put(null, null); 
Comments
For this snippet code:
Map<?,?> map = new HashMap(); Map<?,?> means a Map typed to an unknown type.
The question mark (?), called the wildcard,the wildcard means "the value type parameter could be anything", it doesn't mean "you can use this as if it were anything you want it to be".
For more info go to link
