无法添加面板

Question

我的代码，创建一个设置按钮，但是垂直对齐

现在我要做的是在这个面板中添加一个JLabel，但是当我创建一个新的JPanel时，我在运行时遇到错误，但是代码没有给我任何冲突。如果可能的话，我想在flowlayout中重新排列按钮。

我的完整代码：

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public final class CharSearch extends Box {
    int i = 0;
    int error = 0;
    static JPanel panel;
    String original = "Dinosaur";
    String secret = new String(new char[original.length()]).replace('\0', '-');

    public CharSearch() {

        super(BoxLayout.Y_AXIS);
        for (char i = 'A'; i <= 'Z'; i++) {
            String buttonText = new Character(i).toString();
            JButton button = getButton(buttonText);
            add(button);
        }
        JLabel label = new JLabel(secret);
        JPanel panel = new JPanel();
        panel.add(label);
    }

    public JButton getButton(final String text) {
        final JButton button = new JButton(text);
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if (original.indexOf(text) != -1) {
                    JOptionPane.showConfirmDialog(null,
                            "Your word does contain" + text);
                } else {
                    JOptionPane.showConfirmDialog(null, "There is no" + text);
                }
                // If you want to do something with the button:
                button.setText("Clicked"); // (can access button because it's
                                            // marked as final)
            }
        });
        return button;
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(panel);
                frame.setContentPane(new CharSearch());
                frame.pack();
                frame.setVisible(true);
                new CharSearch();
            }
        });
    }
}

这是我运行时遇到的错误

Exception是：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at javax.swing.JFrame.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at org.informatica.com.CharSearch$2.run(CharSearch.java:50)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

您正在向JFrame添加未定义的JPanel对象，从而生成NullPointerException，如Container#add(Component)文档中所述。

frame.add(panel);

您必须先将panel实例化，然后再将其添加到JFrame的内容窗格容器中。

Answer 2

对于NullPointerException因为这一行：

frame.add(panel);

虽然您已经创建了一个全局变量panel，但在将其添加到frame

之前，您没有实例化它