Hashtable使用误区一例

        Collection keySet = clients.keyset();
        Iterator it = keySet.iterator();
        while(it.hasNext())
        {
            Key key = (Key)it.next();
            Client client = (Client)clients.get(key);
            .......    
        }

The Iterators returned by the iterator and listIterator methods of the Collections returned by all of Hashtable's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and values methods are not fail-fast.

	Enumeration enu = clients.kes();
        while(enu.hasMoreElements())
        {
            Key key = (Key)enu.nextElement();
            Client client = (Client)clients.get(key);
            .......    
        }