java.lang.NullPointerException 和结果集

Question

这是我的第一篇文章，请耐心等待。我正在开发一个 java 项目，它连接到一个 sql 数据库。我目前正在尝试做的是，当我单击一个按钮时，将在我的数据库的文本区域中显示具有指定 ID 的项目名称和项目价格。但是，当我单击按钮时，会出现如下错误:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at OrderFrame.waterButtonActionPerformed(OrderFrame.java:494)
at OrderFrame.access$000(OrderFrame.java:11)

这是我的 waterButtonActionPerformed 的代码:

private void waterButtonActionPerformed(java.awt.event.ActionEvent evt) {                                            
// TODO add your handling code here:
    try{

    String query;
    query = "SELECT itemName, itemPrice FROM item WHERE itemID = '11111'";
    String itemName = " ",itemPrice =" ";  

      ***ResultSet rs = st.executeQuery(query);***

       if(rs != null){
        while(rs.next())
        { 
         itemName = rs.getString(1);
         itemPrice = rs.getString(2);
        }
       orderTextArea.setText(itemName);
       orderTextArea.setText(itemPrice);
       }
         } catch (SQLException ex) {}
}        

第 494 行是 ResultSet 的声明。我希望有人能帮我解决这个问题。提前谢谢你。

Answer 1

像这样重写处理程序:

waterButton.addActionListener(new ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {                                            
// TODO add your handling code here:
    try{
       Statement st = connectionObject.createStatement();

       // might want to use placeholders here as well
       String query= "SELECT itemName, itemPrice FROM item WHERE itemID = '11111'"; 
       String itemName = " ",itemPrice =" ";  

       ResultSet rs = st.executeQuery(query);

       if(rs != null){
         do { 
           itemName = rs.getString(1);
           itemPrice = rs.getString(2);
        } while (rs.next());
        orderTextArea.setText(itemName);
        orderTextArea.setText(itemPrice);
        }
      } catch (SQLException ex) { 
         System.err.println(new java.util.Date()+" : "+ex.getMessage(); 
      }
});