Question

我正在编写代码来从MySQL数据库中检索数据我想将一列查询添加到JList。

这是我的代码：

    try {
        ArrayList<String> names = new ArrayList();
        db = new Database();
        db.connect();
        Statement st = db.getConnection().createStatement();
        ResultSet rs = st.executeQuery("SELECT nombre FROM alumnos");

        while (rs.next()) {
            names.add(rs.getString("nombre"));
        }

        DefaultListModel listModel = new DefaultListModel();
        for(String nms : names) {
            listModel.addElement(nms);
        }

        //This is the line 140
        // and lstInicio is the JList
        lstInicio.setModel(listModel);

    } catch (SQLException sql) {
        System.err.println(sql.getMessage());
    }

我认为一切正常，但当我到达这部分代码时，我得到一个例外：

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at proyecto.ausentismo.MainWindow.loadList(MainWindow.java:140)
at proyecto.ausentismo.MainWindow.<init>(MainWindow.java:21)
at proyecto.ausentismo.Login.btnLoginActionPerformed(Login.java:121)
at proyecto.ausentismo.Login.access$000(Login.java:11)
at proyecto.ausentismo.Login$1.actionPerformed(Login.java:49)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$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$Actions.actionPerformed(BasicButtonListener.java:303)
at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1664)
at javax.swing.JComponent.processKeyBinding(JComponent.java:2879)
at javax.swing.JComponent.processKeyBindings(JComponent.java:2926)
at javax.swing.JComponent.processKeyEvent(JComponent.java:2842)
at java.awt.Component.processEvent(Component.java:6282)
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.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1895)
at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:762)
at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1027)
at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:899)
at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:727)
at java.awt.Component.dispatchEventImpl(Component.java:4731)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:688)
at java.awt.EventQueue$3.run(EventQueue.java:686)
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:702)
at java.awt.EventQueue$4.run(EventQueue.java:700)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:699)
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)

有人知道造成这种异常的原因吗？我怎么能找到？ 编辑：我评论了错误行。并且stil得到了例外

Answer 1

我怀疑，您的lstInicio变量为空。解决方案：回顾您的代码，找出您认为初始化它的位置，并了解它实际上未初始化的原因。根据你到目前为止发布的内容，我们可以说这一切。

编辑：@ jesuscc29：很高兴你能让它运转起来。这里的关键教训不是你所遇到的具体问题的答案，而是调试NullPointerException（NPE）的一般过程，因为我可以保证你会一次又一次地遇到这些bug。始终首先找出哪一行，然后找到该行上的哪个变量导致NPE发生，然后尝试追溯到您的代码以查看变量为空的原因。这样做，90％的时间你会发现问题，这是解决方案。

异常向JList添加元素

1 个答案: