异常将元素添加到JList

Question

我正在编写代码以从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()); 
    } 

我觉得一切都很好，但是当我到达这部分代码我得到一个异常：

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) 

有谁知道是什么原因造成这例外？我怎么知道？ 编辑：我评论了错误的行。并stil得到异常

Answer 1

正如我怀疑，您的lstInicio变量为空。解决方案：回顾你的代码，找出你认为你初始化它的地方，看看它为什么没有被初始化。这就是我们可以基于迄今为止发布的内容说的所有内容。

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编辑：@ jesuscc29：高兴你得到它的工作。这里的关键教训并不是解决具体问题的答案，而是调试NullPointerException（NPE）的一般过程，因为我可以保证你一次又一次地运行这些bug。总是先找出哪一行，然后该行上的哪个变量导致NPE发生，然后尝试追溯到代码中，以查看变量为什么为空。做到这一点，90％的时间你会发现问题和解决方案。