java - 向 JList 添加元素时出现异常

Question

我正在编写代码来从 MySQL 数据库检索数据。
我想将查询的一列添加到 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());
    }

我认为一切都很好，但是当我到达这部分代码时，我得到一个异常:

    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)

有人知道是什么原因导致了这个异常吗？我怎么知道呢？编辑:我评论了错误行。仍然得到异常

Answer 1

正如我怀疑的那样，您的 lstInicio 变量为 null。解决方案:回顾您的代码，找出您认为在何处初始化了它，并了解为什么它实际上没有初始化。根据您迄今为止发布的内容，我们只能说这么多。

<小时/>

编辑:@jesuscc29:很高兴你已经成功了。这里的关键教训并不是对您所遇到的具体问题的答案，而是调试 NullPointerException (NPE) 的一般过程，因为我可以保证您会一次又一次地遇到这些错误。始终首先找出哪一行，然后找出该行上的哪个变量导致 NPE 发生，然后尝试追溯到代码以查看变量为何为空​​。这样做，90% 的情况下您都会找到问题并找到解决方案。