我正在编写从MySQL数据库检索数据的代码。
我想将查询的一列添加到JList中。
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)
有人知道是什么导致了这个异常吗?我怎么才能知道呢?编辑:我注释了错误行。仍然会得到异常
正如我所怀疑的，您的lstInicio变量为空。解决方案:回头看看你的代码，找出你认为你在哪里初始化了它，并看看为什么它实际上没有被初始化。根据你的帖子，我们只能说这么多了。
编辑:@jesuscc29:很高兴你让它工作。这里的关键教训与其说是您遇到的特定问题的答案，不如说是调试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bug。总是首先找出哪一行，然后找出该行上的哪个变量导致了NPE的发生，然后尝试跟踪到代码中，看看变量为空的原因。这样做，90%的时间你会找到问题和解决方案。