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 Gui extends JFrame {
    private JButton reg;
    private JButton custom;

    public Gui(){
        super("The Title");
        setLayout(new FlowLayout());

        reg = new JButton("Reg Button");
        add(reg);

        Icon b = new ImageIcon(getClass().getResource("b.png"));
        Icon x = new ImageIcon(getClass().getResource("x.png"));
        custom = new JButton("Custom Button", b);
        custom.setRolloverIcon(x);
        add(custom);

        HandlerClass handler = new HandlerClass();
        reg.addActionListener(handler);
        custom.addActionListener(handler);
    }

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