カウンタが真になるまでテーブルにレコードを追加する

Question

Java + MySqlを使ってライブラリ管理システムを開発していて、カウンタに基づいてブックレコードを追加しようとしています。カウンターが10以上になるまでユーザーがブックを追加できるようにするだけです.10のときはエラーメッセージが表示されます。 10より小さい場合、プログラムはユーザーがレコードを追加できるようにする必要があります。これまで私はこれを書いていましたが、その結果は私がシステムから期待するものではありません。これを改善するためのアイデア。

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(); 
    } 
} 

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

Answer 1

使う代わりに、そのようなrs.next()を使用しない場合：

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); 
}