public void talk()throws IOException
    {
        datagramChannel.register(selector , SelectionKey.OP_READ | SelectionKey.OP_WRITE) ;
        while(selector.select() > 0)
        {
            Set readyKeys = selector.selectedKeys() ;
            Iterator it = readyKeys.iterator() ;
            while(it.hasNext())
            {
                SelectionKey key = null ;
                try
                {
                    key = (SelectionKey) it.next() ;
                    //为什么要进行这一步??                   
                    it.remove() ;

                    if(key.isReadable())
                        receive(key) ;
                    if(key.isWritable())
                        send(key) ;
                }
                catch (IOException e)
                {
                    e.printStackTrace() ;
                    try
                    {
                        if(key != null)
                        {
                            key.cancel() ;
                            key.channel().close() ;
                        }
                    }
                    catch (Exception ex)
                    {
                        ex.printStackTrace() ;
                    }
                }
            }
        }
    }

为什么在每次获取到Selectionkey后都要用迭代器在集合中删除掉这一项呢?