public void send() {  
        if (!isStart) 
        {  
            JOptionPane.showMessageDialog(null, "司服器尚未啟動，請不要猴急！", "Error",  
                    JOptionPane.ERROR_MESSAGE);  
        }  
        else if (clients.size() == 0) 
        {  
            JOptionPane.showMessageDialog(null, "沒有用戶連線，只能和空氣說話！", "Error",  
                    JOptionPane.ERROR_MESSAGE);  
        }  
        else {
                String message = txt_message.getText().trim();  
                if (message == null || message.equals("")) 
                {  
                    JOptionPane.showMessageDialog(null, "請不要亂發廢文！", "Error",  
                        JOptionPane.ERROR_MESSAGE);   
                }  
                else
                {
                    sendServerMessage(txt_message.getText());// 群发服务器消息  
                    contentArea.append("伺服器公告：" + txt_message.getText() + "\n");  
                    txt_message.setText(null);  
                }
            }
    }