求您了,帮助看下,俺新来的.

[aolin]

在一个Frame 的某一菜单的监听器中也想显一个dialog对话框,但是想在对话框上加按纽的时候却提示
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
请问如何解决?
代码:


items[0].addActionListener(new NewItemListenerDialog());//菜单项所要实现的监听器
其监听器类如下

public class NewItemListenerDialog extends JFrame implements ActionListener {

public static final int DEFAULT_HEIGHT=280;
public static final int DEFAULT_WIDTH=400;
JPanel panel;
Toolkit kit = Toolkit.getDefaultToolkit();
Dimension screenSize = kit.getScreenSize();
int width = screenSize.width;
int height = screenSize.height;
Container contentPane = getContentPane();
public NewItemListenerDialog(){
super();
}
public void actionPerformed(ActionEvent e) {
// TODO 自动生成方法存根
contentPane.add(panel);
panel.add(new JButton("测试"));
JDialog dialog = new JDialog();
dialog.setTitle("新建的下载任务");
dialog.setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
dialog.setLocation((width-DEFAULT_WIDTH)/2,(height-DEFAULT_HEIGHT)/2);
dialog.show();
}

}
为什么不能显示这个dialog,却出现上面的错误呢?

[lining771]

监听注册有问题吧

[Ghosthuo]

mark up