Java Derby中引发空指针异常的简单删除查询

我正在使用NetBeans IDE 8.1并尝试使用数据库中的产品id删除产品。但是我得到了一个运行时错误

java.lang.NullPointerException
at ambeysofas.Function.deleteProduct(Function.java:220)
at ambeysofas.AmbeySofas.btnDeleteProductActionPerformed(AmbeySofas.java:866)
at ambeysofas.AmbeySofas.access$500(AmbeySofas.java:19)
at ambeysofas.AmbeySofas$7.actionPerformed(AmbeySofas.java:550)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
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:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

行号220是删除查询本身。以下是函数deleteProduct的代码

 public boolean deleteProduct(int id){
    try {

        conn.createStatement();
        try{
        stmt.execute("DELETE FORM tbl_products WHERE product_id="+id);

           b=true;
        }catch(Exception e){
            System.out.print(e);
            e.printStackTrace();
               b=false;
        }

    } catch (SQLException ex) {
        System.out.println(ex);
        Logger.getLogger(Function.class.getName()).log(Level.SEVERE, null, ex);
        b=false;
    }
    return b;
}

该函数是从另一个文件调用的。零件代码如下所示：

private void btnDeleteProductActionPerformed(java.awt.event.ActionEvent evt) {                                                 
    // TODO add your handling code here:
     String str = JOptionPane.showInputDialog(this, "Enter Product ID", "Prompt", JOptionPane.OK_CANCEL_OPTION);
    try {
        ResultSet rs1 = new Function().fetchProductByID(Integer.parseInt(str));

        while (rs1.next()) {

            int r = JOptionPane.showConfirmDialog(this, rs1.getString("product_name") + " | " + rs1.getString("product_type"), "Confirm Deletion of Product:" + str, JOptionPane.OK_CANCEL_OPTION);

            if (r == 0) {
                if (new Function().deleteProduct(Integer.parseInt(str))) {
                    JOptionPane.showMessageDialog(this, "Product Deleted Successfully!", "Success!", JOptionPane.INFORMATION_MESSAGE);
                } else {
                    JOptionPane.showMessageDialog(this, "Something Went Wrong!", "Oops!", JOptionPane.ERROR_MESSAGE);
                }
            }
        }


    } catch (Exception e) {

    }
}                                                

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代码在执行删除查询时生成异常。如果有人能提出一些解决方案，我将不胜感激。

也许只是你的疑问中的一个失误

形式尽管来自

---

享受

如果您说错误在

stmt上。执行

行，则

stmt

必须为空-您在哪里给它赋值？我尚未初始化stmt。它在类中声明，换句话说，

stmt

为空，当然它不起作用。声明某些内容不会初始化它，至少您应该将

con.createStatement（）

替换为

stmt=con.createStatement（）

。它起作用了：）您能解释一下发生了什么吗。我在数据库方面有点弱。不，我已经改正了。它不起作用：（你有空指针，因为你的对象没有实例化你忘了实例化stmt:stmt=conn.createStatement（）；祝你玩得开心；）