我还是 Java 新手,单击按钮时无法更改 JLabel 中的图像。我没有收到任何编译器错误,但确实收到了运行时错误。
Line 107: l1.setIcon(icon);
“线程中的异常“AWT-EventQueue-0”java.lang.NullPointerException”
public class HomeController1 extends JPanel implements ActionListener
{
private int choice;
private JButton add;
private JLabel l1;
public HomeController1() {
// Create panels and set layouts
JPanel p1 = new JPanel(new GridLayout(1,4));
JPanel p2 = new JPanel (new GridLayout(3,3));
JPanel p3 = new JPanel(new BorderLayout());
// Create buttons
add = new JButton("ADD APPLIANCE");
//Add ActionListeners
add.addActionListener(this);
// Add buttons to panel
p1.add(add);
// Default ImageIcon
ImageIcon blank = new ImageIcon("blank.jpg");
// Create Jlabels to hold blank image
JLabel l1 = new JLabel(blank);
// Add JLabels to Panel
p2.add(l1);
//Add contents to Frame
p3.add(p1,BorderLayout.SOUTH);
p3.add(p2,BorderLayout.CENTER);
add(p3);
}
public void actionPerformed(ActionEvent e)
{
if (e.getSource() == add)
{
choice = 0;
if (choice == 0)
{
Clock clock1 = new Clock();
ImageIcon icon = clock1.getPicture();
l1.setIcon(icon);
}
}
}
}
完整的错误信息:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at HomeController.actionPerformed(HomeController.java:107)
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:3312)
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: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)