hashMap和hashtable的的区别 hashable get值 如果为空为报空指针异常

public static void main(String[] args) {
Map<Object, Object> map = new HashMap<Object, Object>();
map.put("a", null);
map.get("b");

Hashtable hashtable = new Hashtable();
hashtable.put("a", "qewer");
hashtable.get("b");
hashtable.put("c", hashtable.get("sds"));



}