Why is my JPanel empty after button is clicked?
<p>I'm learning how to create event listeners in Java for a button click. I want a panel to popup with form items. I've built the panel in the action listener's contructor, but it's empty when it opens. I thought it makes sense to only build this one, then just show it when the button is clicked (actionPerformed). Obviously not :)</p>

<p>Below is my ActionListener class:</p>

<pre><code>package biz.martyn.budget;

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class NewTransaction implements ActionListener {

    protected JPanel panel = new JPanel(new GridLayout(0, 1));

    public void NewTransaction() {      
        String [] category = {"Internet", "Clothes", "Rent", "Salary", "Groceries"};
        JComboBox combo = new JComboBox(category);

        panel.add(combo);

        panel.add(new JLabel("Description:"));
        JTextField desc = new JTextField();
        panel.add(desc);

        panel.add(new JLabel("Date:"));
        JTextField date = new JTextField();
        panel.add(date);

        panel.add(new JLabel("Amount:"));
        JTextField amount = new JTextField();
        panel.add(amount);
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        int result = JOptionPane.showConfirmDialog(null, panel, "New transaction",
            JOptionPane.OK_CANCEL_OPTION, JOptionPane.PLAIN_MESSAGE);
    }
}
</code></pre>

<p>Here is how I'm attaching the event:</p>

<pre><code>JButton newTransactionButton = new JButton("New transaction");
newTransactionButton.addActionListener(new NewTransaction());
toolbar.add(newTransactionButton);
</code></pre>

<p>I'd appreciate any additional advice on conventions when doing this sort of thing coz I'm quite the beginner, thanks.</p>