ActionEvent -改变按钮按下的JLabel

Question

我还是Java新手，当单击按钮时，我在JLabel中更改图像时遇到了麻烦。我没有得到任何编译错误，但我得到一个运行时错误。

Line 107: l1.setIcon(icon); 

"线程"AWT-EventQueue-0"异常。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)

Answer 1

你的问题来自于你在home controller中声明了一个全局变量:

private JLabel l1; 

和另一个本地的，在你的构造函数中:

// Create Jlabels to hold blank image
JLabel l1 = new JLabel(blank);

在构造函数中，定义了一个与类同名的变量，但它是不同的变量。构造函数中的那个隐藏了类中的那个。因此，最后，只有构造函数变量被初始化。如果在构造函数中像这样修改代码，

// Create Jlabels to hold blank image
l1 = new JLabel(blank);

那么你将只有一个全局变量在你的类，你不会得到一个NullPointerException