按钮的返回方法不起作用

Question

Someone on stackoverflow helped me with this code so I can use it in a method to return the button I clicked... Now netbeans doesn't show any conflicting code, but when i run it, it gives errors

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";
JLabel label = new JLabel();
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);
    }
}

public JButton getButton(final String text){
    final JButton button = new JButton(text);
    button.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e) {
            JOptionPane.showMessageDialog(null, "You have clicked: "+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.add(panel);
            frame.pack();
            frame.setVisible(true);
        }
    });
}
}

When I run the program I get this error

Answer 1

Farily obvious, you never initialize the static atribute panel:

static JPanel panel;

So the NPE is generated in this part of the code:

public static void main(String[] args){
    EventQueue.invokeLater(new Runnable(){
        public void run(){
           JFrame frame=new JFrame();
            frame.add(panel); // <--------------- NullPointer to panel
            frame.pack();
            frame.setVisible(true);
        }
    });
}

Answer 2

You have panel declared but it is never instantiated:

static JPanel panel;

so when you go to add it:

JFrame frame = new JFrame();
frame.add(panel);

it throws a NPE.