6. Create buttons using Swing

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

import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;
import javax.swing.JFrame;
import javax.swing.JButton;

public class Gui extends JFrame{

    // Variables
    private JButton reg;
    private JButton custom;

    // Constructor
    public Gui(){

        // Basics
        super("JFrame's title");
        setLayout(new FlowLayout());
        Icon green = new ImageIcon(getClass().getResource("green.png"));
        Icon red = new ImageIcon(getClass().getResource("red.png"));

        // Buttons
        reg = new JButton("Regular button");
        reg.setSize(300, 300);
        add(reg);
        custom = new JButton("Custom button", red);
        reg.setSize(300, 300);
        custom.setRolloverIcon(green);
        add(custom);

        // Calling the handler - action listener
        HandlerClass handler = new HandlerClass();
        reg.addActionListener(handler);
        custom.addActionListener(handler);

    }


    // New class inside classes - ActionListener is abstract!
    // We promise that we will override the methods
    private class HandlerClass implements ActionListener{
        public void actionPerformed(ActionEvent event){
            JOptionPane.showMessageDialog(null, String.format("%s", event.getActionCommand()));
        }
    }

    // Main Function
    public static void main(String[] args){
        Gui gui = new Gui();
        gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        gui.setSize(600, 600);
        gui.setVisible(true);
    }

}