将记录添加到表中，直到计数器为真

Question

我正在使用Java + MySql开发一个图书馆管理系统，并且我试图根据计数器添加图书记录。我只需要允许用户添加图书，直到计数器为＆gt; = 10.当它为10时，它应该给出错误消息。如果它低于10，程序应允许用户添加记录。到目前为止，我一直想写这个，但结果不是我对系统的期望。关于改进这一点的任何想法。

    private void btn_addActionPerformed(java.awt.event.ActionEvent evt) {                                        
    try {             
        rs = st.executeQuery("SELECT COUNT(*) FROM `read_with_us`.`book_inventory` WHERE book_title='" + txt_booktitle.getText() + "'");
        rs.next();
        int count = rs.getInt(1);

        if (count >=10) {
            JOptionPane.showMessageDialog(null, "Sorry Maximum allowable quantity (10) has been reached!!");
        }
        else {
            String sql2 = "INSERT INTO `read_with_us`.`book_inventory` (`book_id`, `book_title`, `isbn_no`, `book_author`, `book_publisher`, `category_id`, `status`) VALUES (?,?,?,?,?,?,?);";

            pst = con.prepareStatement(sql2);
            pst.setInt(1, Integer.parseInt(txt_bookid.getText()));
            pst.setString(2, txt_booktitle.getText());
            pst.setString(3, txt_isbnno.getText());
            pst.setString(4, txt_author.getText());
            pst.setString(5, txt_pubisher.getText());
            String category = (String) com_category.getSelectedItem();
            pst.setString(6, category);
            String status = (String) com_status.getSelectedItem();
            pst.setString(7, status);
            pst.execute();
            //Clearing the fields
            txt_booktitle.setText("");
            txt_isbnno.setText("");
            txt_author.setText("");
            txt_pubisher.setText("");
            com_category.setSelectedItem("1");
            com_status.setSelectedItem("Available");
            JOptionPane.showMessageDialog(null, "Successfully Added");
            pst.close();
            rs.close();
            con.close();
        }
    } catch (Exception e) {

        JOptionPane.showMessageDialog(null, "Error:" + " " + e.getMessage());
        e.printStackTrace();
    }
}

＆＃13;

＆＃13;

run:
java.lang.NullPointerException
	at Books.btn_addActionPerformed(Books.java:656)
	at Books.access$200(Books.java:19)
	at Books$3.actionPerformed(Books.java:161)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
	at java.awt.Component.processMouseEvent(Component.java:6533)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
	at java.awt.Component.processEvent(Component.java:6298)
	at java.awt.Container.processEvent(Container.java:2236)
	at java.awt.Component.dispatchEventImpl(Component.java:4889)
	at java.awt.Container.dispatchEventImpl(Container.java:2294)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
	at java.awt.Container.dispatchEventImpl(Container.java:2280)
	at java.awt.Window.dispatchEventImpl(Window.java:2746)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
	at java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.awt.EventQueue$3.run(EventQueue.java:709)
	at java.awt.EventQueue$3.run(EventQueue.java:703)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
	at java.awt.EventQueue$4.run(EventQueue.java:731)
	at java.awt.EventQueue$4.run(EventQueue.java:729)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

＆＃13;

＆＃13;

＆＃13;

Answer 1

不要像这样使用rs.next()，而是使用if：

PreparedStatement ps = con.prepareStatement("SELECT COUNT(*) FROM `read_with_us`.`book_inventory` WHERE book_title = ?");
ps.setString(1, txt_booktitle.getText());
rs = ps.executeQuery();
int count = 0;
if(rs.next()){
   count = rs.getInt(1);
}