GUI Program - Process That Will Record All Ordered Items

I can't finish my project in my OOP subject,Ordering System Program,but i am finished with the layout and some processes.

I have complication in which i don't know how to code a process that will record all the ordered items.

i have items like [MS1]Taro Milk Tea.. and there's this textbox that will will get the inputted text and get the quantity of it by JOptionPane.

There's this another JOptionPane that will show the order(s) when "Show Order" button is clicked.

My main problem is i can't record additional items if i want to add more . and my variables are all stored in an array.

Any different way of getting the order,quantity,price of it and Sum all of it and Display the Ordered items...

My professor required that we used inheritance so i have 2 subclasses..

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class MilkteaGUI extends JFrame implements ActionListener,WindowListener {
MtGUIProcess mg = new MtGUIProcess();
Label cardNo = new Label("Card Number:");

[Code] ....