roll over imeges on java

JAVA MAIN FILE
-----------------

package jbutton;

import javax.swing.JFrame;

public class Main
{
    public static void main(String[] args)
    {
    button obj = new button();
    obj.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    obj.setSize(400, 400);
    obj.setVisible(true);
    }

}

JAVA CLASS FILE
-------------------

package jbutton;

import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;

public class button extends JFrame
{
    private JButton reg;
    private JButton custom;

    public button()
    {
    super("the title");
    setLayout(new FlowLayout());

    reg = new JButton("reg button");
    add(reg);

    //Icon b = new ImageIcon(getClass.getResource("a.png"));
    //Icon x = new ImageIcon(getClass.getResource("b.png"));

    //custom = new JButton("Custom", b);
    custom = new JButton("Custom");
    //custom.setRolloverIcon(x);
    add(custom);
   
    HandlerClass handler = new HandlerClass();
    reg.addActionListener(handler);
    custom.addActionListener(handler);
    }

    private class HandlerClass implements ActionListener
    {
        public void actionPerformed(ActionEvent ae)
        {
        JOptionPane.showMessageDialog(null, String.format("%s", ae.getActionCommand()), "Is my title title", JOptionPane.PLAIN_MESSAGE);
        }
    }
}