Cant make my JButtons responsives. Everything resize except for that panel

I've been thinking for days trying to make my UI responsive, but I'm learning and cant find a way by myself.

This is what i want to do, as the img, but responsive (always showing the same number of buttons in that JScrollPanel). I set the starting size taking the actual screenSize, and it start maximized, so it starts with a 12 buttons column.

https://i.ibb.co/9hTwsh1/image.png

But when I resize it, my buttons don't, so my buttons panels stay the same size, when the rest of my program resize:

https://i.ibb.co/NFHvfLr/image.png

(cant expand imgs, maybe someone can edit this for me? thanks in advance)

The main problem I got is that I don't know how many buttons I'll need so I tried to do a dynamic and responsive layouts but don't know how to do it with my buttons.

EDIT-1:

I have made some changes since yesterday, and because of your comments (thanks a lot).

1. You told me to use GridLayout instead of GridBagLayout. I change to gridBag trying to achieve the responsive UI, but it didnt work. I roll back to GridLayout as you said... at least is cleaner.

2. I also made a reproducible example as you told me to do. About the coments, ill delete them. As you comment, is only noise (those are only for me, ideas). Sorry, should delete them before i post the first time.

3. About the change to a comboBox, im trying to simulate a POS program, so i need those big buttons

4. About the 4th comment.. I search about what you said of updateUI... The problem is that i use that because when i do mainPanel.removeAll(), it dont work unless i do "updateUI" after that(yo can see I use that like 4 times in my program). How should I update my panels after removeAll? - Just look for a solution. Maybe with revalidate is the correct way to do this?

You told me too not to use setPreferredSize.. but i need my buttons to be always the same(if maximized, resizable if not). No matter if i draw 10 or 100. How should i do that without preferredSize?
And about statics.. the truth is that i use statics when i dont know how to do it otherwise. But, as you say, ill try to change that.

New code (trying to do it shorter, but as you can see, im not good at this yet =) ):

    public class StackOverflow {
public static void main(String[] args) {
Producto.crearProductosInicio();
Categoria.crearCategorias();
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
PantallaGeneral fondo = new PantallaGeneral();
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
}
class ProductPane extends JScrollPane  {
static JPanel contenedorPanelProductos = new JPanel();
static JPanel panelProductos;
public ProductPane () {
super(
contenedorPanelProductos,
ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED
);
this.setLayout(new ScrollPaneLayout());
panelProductos=new JPanel();
}
public static void dibujarProductos ( int categoria) {
ArrayList<Categoria> categorias = Categoria.listaCategorias().stream().filter(x->x.getIDPadre()==categoria).collect(Collectors.toCollection(ArrayList::new));
ArrayList<Producto> productos = Producto.listaProductos().stream().filter(x->x.getID_Categoria()==categoria).collect(Collectors.toCollection(ArrayList::new));
panelProductos.removeAll();
panelProductos.updateUI();
JButton botones;
panelProductos.setLayout(new GridLayout(0,12));
panelProductos.setBackground(Color.yellow);
int x=0 ,y=0;
for (int i = 0; i<categorias.size();i++) {
botones=categorias.get(i).getBoton();
panelProductos.add(botones);
}
for (int i = 0; i<productos.size();i++) {
botones=productos.get(i).getBoton();
panelProductos.add(botones);
}
if (categoria>1) {
botones = crearCategoriaGeneral();
panelProductos.add(botones);
}
agregarPanel(panelProductos);
}
public static JButton crearCategoriaGeneral () {
JButton botonGeneral = new JButton("General");
botonGeneral.setMargin(new Insets(0, 0, 0, 0));
Font fuente = new Font("Arial",1,10);
botonGeneral.setFont(fuente);
botonGeneral.setBackground(Color.white);
botonGeneral.setPreferredSize(new Dimension((((java.awt.Toolkit.getDefaultToolkit().getScreenSize().width/2)-40)/12),(((Toolkit.getDefaultToolkit().getScreenSize().width/2)-40)/12)));
botonGeneral.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
dibujarProductos(1);
}
});
return botonGeneral;
}
public static void agregarPanel (JPanel panelProductos) {
contenedorPanelProductos.setBackground(Color.BLUE);
contenedorPanelProductos.setBorder(BorderFactory.createEmptyBorder(10, 20, 10, 20));
contenedorPanelProductos.setLayout(new GridBagLayout());
GridBagConstraints v = new GridBagConstraints();
v.weightx = 1;
v.weighty = 1;
v.anchor=GridBagConstraints.FIRST_LINE_START;
contenedorPanelProductos.add(panelProductos,v);
}
}
class Producto {
int id;
String nombre;
String rutaImagen;
double precio;
int ID_Categoria;
int ID_Impuesto;
JButton boton;
//Consulta a BD para creación de productos:
static ArrayList<Producto> productos = new ArrayList<>();
public Producto(int id, String nombre, String rutaImagen, double precio, int ID_Categoria, int ID_Impuesto) {
this.id = id;
this.nombre = nombre;
this.rutaImagen = rutaImagen;
this.precio = precio;
this.ID_Categoria = ID_Categoria;
this.ID_Impuesto = ID_Impuesto;
establecerBoton();
}
private void establecerBoton() {
boton = new JButton(this.nombre);
boton.setMargin(new Insets(0, 0, 0, 0));
boton.setBackground(Color.DARK_GRAY);
Font fuente = new Font("Arial", 1, 10);
boton.setFont(fuente);
Dimension tamBot = new Dimension((((java.awt.Toolkit.getDefaultToolkit().getScreenSize().width / 2) - 40) / 12), (((Toolkit.getDefaultToolkit().getScreenSize().width / 2) - 40) / 12));
boton.setPreferredSize(tamBot);
Producto temp = this;
boton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
TicketPane.añadirProductoTicket(temp);
}
});
}
public JButton getBoton() {
return boton;
}
public static ArrayList<Producto> listaProductos () {
return productos;
}
public static void crearProductosInicio() {
for (int i=0;i<15;i++) {
productos.add(new Producto(i, "Aquarius"," ", 0.30*i, 4, 1));
}
for (int i=15;i<75;i++) {
productos.add(new Producto(i, "7up"," ", 0.15*i, 2, 1));
}
}
public int getID_Categoria() {
return ID_Categoria;
}
public String getNombre() {
return nombre;
}
public double getPrecio() {
return precio;
}
}
class Categoria {
static ArrayList<Categoria> categorias = new ArrayList<>();
int ID;
String nombre;
int IDPadre;
JButton boton;
public Categoria(int ID, String nombre, int IDPadre) {
this.ID = ID;
this.nombre = nombre;
this.IDPadre = IDPadre;
establecerBoton();
}
public static void crearCategorias() {
categorias.add(new Categoria(1, "General", 0));
categorias.add(new Categoria(2, "Bebidas", 1));
categorias.add(new Categoria(3, "Comida", 1));
categorias.add(new Categoria(4, "Refrescos", 2));
}
public static ArrayList<Categoria> listaCategorias() {
return categorias;
}
private void establecerBoton() {
boton = new JButton(this.nombre);
boton.setMargin(new Insets(0, 0, 0, 0));
boton.setBackground(Color.white);
Font fuente = new Font("Arial", 1, 10);
boton.setFont(fuente);
//Con el primero, siempre miden lo mismo ya que referencio la resolución del ordenador. Con el segundo, referencio el tamaño del frame, con lo que si cambio el tamaño, se cambian (no responsive=) )
Dimension tamBot = new Dimension((((java.awt.Toolkit.getDefaultToolkit().getScreenSize().width / 2) - 40) / 12), (((Toolkit.getDefaultToolkit().getScreenSize().width / 2) - 40) / 12));
boton.setPreferredSize(tamBot);
boton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
ProductPane.dibujarProductos(ID);
}
});
}
public int getIDPadre() {
return IDPadre;
}
public JButton getBoton() {
return boton;
}
}
class PantallaGeneral {
JFrame pantalla;
//Container general:
private static JPanel contenedorContenido = new JPanel();
//Salon
private JButton boton1 = new JButton("Mesa 1");
private JButton boton2 = new JButton("Mesa 2");
private JButton boton3 = new JButton("Mesa 3");
private JButton botonB = new JButton("Barra");
//Contenedores ventas
JPanel ventasArriba = new JPanel();
JPanel ventasAbajo = new JPanel();
ProductPane panelProductos = new ProductPane();
TicketPane panelTicket = new TicketPane();
public PantallaGeneral ()  {
Producto.crearProductosInicio();
initPantalla();
dibujarPantallaVenta(0);
}
private void initPantalla () {
pantalla = new JFrame();
pantalla.setLayout(new BorderLayout());
//pantalla.setBounds(0,0,ancho,alto);
pantalla.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
pantalla.setResizable(true);
pantalla.setExtendedState(JFrame.MAXIMIZED_BOTH);
pantalla.setVisible(true);
pantalla.getContentPane().add(contenedorContenido,BorderLayout.CENTER);
}
private void dibujarPantallaVenta (int numeroMesa) {
limpiarPantalla();
panelTicket.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width / 2, Toolkit.getDefaultToolkit().getScreenSize().height / 2));
panelProductos.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width / 2, Toolkit.getDefaultToolkit().getScreenSize().height / 2));
ventasArriba.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width, Toolkit.getDefaultToolkit().getScreenSize().height / 2));
ventasAbajo.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width, Toolkit.getDefaultToolkit().getScreenSize().height / 2));
contenedorContenido.setLayout(new BoxLayout(contenedorContenido, BoxLayout.Y_AXIS));
contenedorContenido.add(ventasArriba);
contenedorContenido.add(ventasAbajo);
ventasArriba.setLayout(new BoxLayout(ventasArriba, BoxLayout.X_AXIS));
ventasArriba.add(panelTicket);
ventasArriba.add(panelProductos);
panelTicket.dibujarTicket(numeroMesa);
panelProductos.dibujarProductos(1);
}
public void limpiarPantalla () {
contenedorContenido.removeAll();
contenedorContenido.updateUI();
System.gc();
}
}
class TicketPane extends JPanel {
static TreeMap<Integer, DefaultTableModel> tickets=new TreeMap<>();
static int mesaActual=0;
JTable tablaTicket = new JTable();
public TicketPane () {
}
public void dibujarTicket (int numeroMesa) {
removeAll();
updateUI();
tablaTicket.removeAll();
tablaTicket.updateUI();
mesaActual=numeroMesa;
setLayout(null);
setBackground(Color.lightGray);
//setBounds(0,0,PantallaGeneral.ancho/2,PantallaGeneral.alto/2);
if (tickets.get(mesaActual)==null) {
tickets.put(mesaActual,new ModeloTabla());
tickets.get(mesaActual).setColumnIdentifiers(new Object[]  { "Producto","Precio"});
}
//tablaTicket.setBounds(0+(int)(PantallaGeneral.ancho/2*0.05),0+(int)(PantallaGeneral.alto/2*0.05),PantallaGeneral.ancho/2-(int)(PantallaGeneral.ancho/2*0.10),PantallaGeneral.alto/2-(int)(PantallaGeneral.alto/2*0.10));
tablaTicket.setVisible(true);
tablaTicket.setModel(tickets.get(mesaActual));
tablaTicket.addMouseListener(new MouseAdapter()
{
public void mouseClicked(MouseEvent e)
{
int fila = tablaTicket.rowAtPoint(e.getPoint());
System.out.println("Se selecciono la fila: " + fila);
}
});
tablaTicket.getColumn("Producto").setPreferredWidth(this.getWidth()-(int)(this.getWidth()*0.15));
javax.swing.JScrollPane jScrollPane1;
jScrollPane1 = new javax.swing.JScrollPane();
jScrollPane1.setViewportView(tablaTicket);
//add(tablaTicket);
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
this.setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addComponent(jScrollPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 388, Short.MAX_VALUE)
.addContainerGap())
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addComponent(jScrollPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 288, Short.MAX_VALUE)
.addContainerGap())
);
//pantalla.getContentPane().add(this);
updateUI();
}
public static void añadirProductoTicket(Producto producto) {
NumberFormat nw = NumberFormat.getInstance(new Locale("en", "EN"));
nw.setMaximumFractionDigits(2);
nw.setMinimumFractionDigits(2);
if (tickets.get(mesaActual).getRowCount()==0){
tickets.get(mesaActual).addRow(new Object[] {producto.getNombre(),nw.format(producto.getPrecio())});
tickets.get(mesaActual).addRow(new Object[] {"Total Ticket: ",nw.format(producto.getPrecio())});
}
else {
double total=Double.parseDouble( tickets.get(mesaActual).getValueAt(tickets.get(mesaActual).getRowCount()-1,tickets.get(mesaActual).getColumnCount()-1).toString());
tickets.get(mesaActual).removeRow(tickets.get(mesaActual).getRowCount()-1);
tickets.get(mesaActual).addRow(new Object[] {producto.getNombre(),nw.format(producto.getPrecio())});
total+=producto.getPrecio();
tickets.get(mesaActual).addRow(new Object[] {"Total Ticket: ",nw.format(total)});
}
}
public void calcularTotal () {
}
public static int getMesaActual() {
return mesaActual;
}
}
class ModeloTabla extends DefaultTableModel {
public boolean isCellEditable(int row, int column) {
return false;
}
}

code before 1st edit:

    //Main sales drawing
            private void dibujarPantallaVenta (int numeroMesa) {
                limpiarPantalla();
                panelTicket.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width/2, Toolkit.getDefaultToolkit().getScreenSize().height/2));
                panelProductos.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width/2, Toolkit.getDefaultToolkit().getScreenSize().height/2));
                ventasArriba.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width, Toolkit.getDefaultToolkit().getScreenSize().height/2));
                ventasAbajo.setPreferredSize(new Dimension(java.awt.Toolkit.getDefaultToolkit().getScreenSize().width, Toolkit.getDefaultToolkit().getScreenSize().height/2));
        
                pantalla.getContentPane().add(contenedorContenido,BorderLayout.CENTER);
                contenedorContenido.setLayout(new BoxLayout(contenedorContenido,BoxLayout.Y_AXIS));
                contenedorContenido.add(ventasArriba);
                contenedorContenido.add(ventasAbajo);
                ventasArriba.setLayout(new BoxLayout(ventasArriba,BoxLayout.X_AXIS));
        
                ventasArriba.add(panelTicket);
                ventasArriba.add(panelProductos);
                panelTicket.dibujarTicket(numeroMesa); //Dibujo el ticket del numero de mesa indicado
                panelProductos.dibujarProductos(1,panelTicket); //Siempre que se abre una mesa, se pinta la categoria general por defecto
        
            }
        public class ProductPane extends JScrollPane  {
        static JPanel contenedorPanelProductos = new JPanel();
        JPanel panelProductos;
        public ProductPane () {
            super(
                    contenedorPanelProductos,
            ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
            ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED
    );
            this.setLayout(new ScrollPaneLayout());
            panelProductos=new JPanel();
        }
    
        //This is where i paint my JButtons:
          public void dibujarProductos ( int categoria,TicketPane ticketMesa) {
                //Pendiente: Establecer un tamaño inicial para los botones. Permitir agrandar y achicar este tamaño, asi como el numero de columnas para un ajuste personalizado
                ArrayList<Categoria> categorias = Categoria.listaCategorias().stream().filter(x->x.getIDPadre()==categoria).collect(Collectors.toCollection(ArrayList::new));
                ArrayList<Producto> productos = Producto.listaProductos().stream().filter(x->x.getID_Categoria()==categoria).collect(Collectors.toCollection(ArrayList::new));
        
                panelProductos.removeAll();
                panelProductos.updateUI();
        
                JButton botones;
        
                panelProductos.setLayout(new GridBagLayout());
                GridBagConstraints c = new GridBagConstraints();
                c.anchor = GridBagConstraints.FIRST_LINE_START;
                c.weightx = 1.0;
                c.weighty = 1.0;
                c.ipadx = 0;
                c.ipady = 0;
        
                panelProductos.setBackground(Color.yellow);
                int x=0 ,y=0;
                for (int i = 0; i<categorias.size();i++) {
                    botones=categorias.get(i).getBoton();
                    int finalI = i;
                    botones.addActionListener(new ActionListener() {
                        @Override
                        public void actionPerformed(ActionEvent e) {
                            dibujarProductos(categorias.get(finalI).getID(),ticketMesa);
                        }
                    });
                    c.gridx = x;
                    c.gridy = y;
                    x++;
                    if (x==12){
                        x-=12;
                        y++;
                    }
                    c.anchor = GridBagConstraints.FIRST_LINE_START;
                    panelProductos.add(botones ,c);
                }
                for (int i = 0; i<productos.size();i++) {
                    botones=productos.get(i).getBoton();
                    c.gridx = x;
                    c.gridy = y;
                    x++;
                    if (x==12){
                        x-=12;
                        y++;
                    }
                    panelProductos.add(botones ,c);
                }
                if (categoria>1) {
                    botones = new JButton("General");
                    botones.setMargin(new Insets(0, 0, 0, 0));
                    Font fuente = new Font("Arial",1,10);
                    botones.setFont(fuente);
                    botones.setBackground(Color.white);
                    botones.setPreferredSize(new Dimension((((java.awt.Toolkit.getDefaultToolkit().getScreenSize().width/2)-40)/12),(((Toolkit.getDefaultToolkit().getScreenSize().width/2)-40)/12)));
                    botones.addActionListener(new ActionListener() {
                        @Override
                        public void actionPerformed(ActionEvent e) {
                            dibujarProductos(1,ticketMesa);
                        }
                    });
                    c.gridx = x;
                    c.gridy = y;
                    x++;
                    if (x==12){
                        x-=12;
                        y++;
                    }
                    panelProductos.add(botones ,c);
                }
                agregarPanel(panelProductos);
        
            }
            public void agregarPanel (JPanel panelProductos) {
                contenedorPanelProductos.setBackground(Color.BLUE);
                contenedorPanelProductos.setBorder(BorderFactory.createEmptyBorder(10, 20, 10, 20));
                contenedorPanelProductos.setLayout(new GridBagLayout());
                GridBagConstraints v = new GridBagConstraints();
                v.weightx = 1;
                v.weighty = 1;
                v.anchor=GridBagConstraints.FIRST_LINE_START;
                contenedorPanelProductos.add(panelProductos,v);
            }

I use panelProductos because if i dont, when i add my buttons they seem to have same padding i coudlnt fix other way

Actual IU:

Without panelProductos:

Hopefully, I understand what you want to do. You don't want the horizontal scrollbar to appear at all. The fix is quite simple. Change the constructor of class ProductPane. Below is just the part that needs to be changed in the context of where it is in the code.

class ProductPane {
    public ProductPane() {
        super(contenedorPanelProductos,
              ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
              ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
    }
}

In your original code, you used HORIZONTAL_SCROLLBAR_AS_NEEDED. This is the default and so you don't have to explicitly set it. But you don't want the horizontal scrollbar to appear, so use HORIZONTAL_SCROLLBAR_NEVER instead. Try it and see for yourself. If it doesn't work, let me know and I will delete this answer.

I think I understand what you're trying to do. I have no idea what the bottom panel of your GUI does.

I tried running your code and had problems. The GUI would disappear from my monitor when I tried to reduce the size of the GUI. The buttons would disappear and reappear.

I created a POS Simulator GUI from scratch. I had to use a few Swing tricks, which I'll explain in more detail.

Here's the GUI.

When you expand the GUI to fill the screen, the buttons shift position to fill the space. The register tape panel stays the same size.

In general, when you create a Swing GUI, you try to make it as small as possible. It's a lot easier to see what happens when you expand the size of the GUI than trying to shrink the GUI to fit on a particular monitor.

I decided to use a JTextArea for the register display to save more GUI space.

I started simple. I created the JFrame and the Item and Inventory classes first. The Item class holds the name and price of an item, while the Inventory class holds multiple items. I saved the item price in pennies so that I could do integer arithmetic and not worry about rounding errors.

I removed all static Swing components, fields, and methods. It's a lot easier to keep track of instances than to deal with the side effects of static fields.

I used Swing components. I did not extend any Swing components. The only time you should extend a Swing component, or any Java class, is when you want to override one of the class methods.

I built this GUI one small piece at a time, testing each piece as I coded it. I rarely added more than 20 to 30 lines of code before running one or more tests. There's no way I could write 500 lines of code and get them all correct.

I wrote 6 classes. The POSSimulator class holds the JFrame. I made the remainder of the classes inner classes so I could post the code as a complete, executable example.

The ButtonPanel class creates the button panel. The RegisterPanel class creates the register tape panel. The ButtonListener class implements ActionListener and gives the buttons their function.

The Inventory class holds the inventory. The Item class holds an item.

Each class does what the name of the class says it does. The larger your GUI, the more important that each class does one thing and one thing well.

The ButtonPanel class creates an outer JPanel, which contains a JScrollPane, which contains an inner JPanel, which contains individual button JPanels. This sounds complicated, but it allows the outer JPanel to grow and for the individual button JPanels to maintain their size.

I created 40 buttons. I only created 4 items for the inventory, so I repeated the inventory 10 times. You would want to adjust the code in the createPanel method to create one button for each item in the inventory.

You're going to have to play with the JPanel preferred sizes depending on what preferred size you make the JButtons. The advice in the comments about not setting a preferred size is for ordinary GUIs. Your GUI is a special application that I didn't understand until you further explained what you were trying to do.

The RegisterPanel class is more straightforward. The appendItem method appends an item to the register tape panel. I use the format method of the String class to format the name and price of the item, as well as the total price.

The ButtonListener class gets the name of the item from the JButton, looks up the item in the inventory, and calls the appendItem method of the RegisterPanel class.

The Inventory and Item classes are plain Java classes to hold the inventory and item, respectively.

Here's the code.

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.List;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;
public class POSSimulator implements Runnable {
public static void main(String[] args) {
SwingUtilities.invokeLater(new POSSimulator());
}
private ButtonPanel buttonPanel;
private Inventory inventory;
private JFrame frame;
private RegisterPanel registerPanel;
public POSSimulator() {
this.inventory = new Inventory();
}
@Override
public void run() {
frame = new JFrame("POS Si8mulator");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
registerPanel = new RegisterPanel();
frame.add(registerPanel.getPanel(), 
BorderLayout.BEFORE_LINE_BEGINS);
buttonPanel = new ButtonPanel();
frame.add(buttonPanel.getPanel(), BorderLayout.CENTER);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public class ButtonPanel {
private JPanel panel;
public ButtonPanel() {
createPanel();
}
private void createPanel() {
panel = new JPanel(new BorderLayout());
panel.setPreferredSize(new Dimension(360, 600));
JPanel innerPanel = new JPanel(new FlowLayout(
FlowLayout.LEADING));
innerPanel.setPreferredSize(new Dimension(340, 2320));
ButtonListener listener = new ButtonListener();
List<Item> items = inventory.getInventory();
int size = items.size();
int j = 0;
// We're creating more buttons than inventory
for (int i = 0; i < 40; i++) {
JPanel buttonPanel = createButtonPanel(
items.get(j), listener);
innerPanel.add(buttonPanel);
j++;
j = (j >= size) ? 0 : j;
}
JScrollPane scrollPane = new JScrollPane(innerPanel);
panel.add(scrollPane, BorderLayout.CENTER);
}
private JPanel createButtonPanel(Item item, 
ButtonListener listener) {
JPanel panel = new JPanel(new BorderLayout());
panel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
JButton button = new JButton(item.getName());
button.addActionListener(listener);
button.setPreferredSize(new Dimension(150, 100));
panel.add(button, BorderLayout.CENTER);
return panel;
}
public JPanel getPanel() {
return panel;
}
}
public class RegisterPanel {
private int total;
private JPanel panel;
private JTextArea textArea;
private String totalString;
public RegisterPanel() {
this.totalString = "Register total";
setTotal(0);
createPanel();
}
private void setTotal(int total) {
this.total = total;
}
private void createPanel() {
panel = new JPanel(new BorderLayout());
textArea =  new JTextArea(10, 40);
textArea.setText("");
textArea.setFont(new Font(
"Courier New", Font.BOLD, 12));
JScrollPane scrollPane = new JScrollPane(textArea);
panel.add(scrollPane, BorderLayout.CENTER);
}
public void appendItem(Item item) {
int price = item.getPrice();
total += price;
String tape = textArea.getText();
if (!tape.isEmpty()) {
int pos = tape.indexOf(totalString);
tape = tape.substring(0, pos);
}
String line = String.format("%-30s", item.getName());
line += String.format("%7.2f", 0.01d * price);
tape += line + System.lineSeparator();
line = String.format("%-30s", totalString);
line += String.format("%7.2f", 0.01d * total);
tape += line + System.lineSeparator();
textArea.setText(tape);
}
public JPanel getPanel() {
return panel;
}
}
public class ButtonListener implements ActionListener {
@Override
public void actionPerformed(ActionEvent event) {
JButton button = (JButton) event.getSource();
String name = button.getText();
Item item = inventory.getItem(name);
registerPanel.appendItem(item);
}
}
public class Inventory {
private List<Item> inventory;
public Inventory() {
this.inventory = new ArrayList<>();
addInventory();
}
private void addInventory() {
inventory.add(new Item("Coke Zero", "3.33"));
inventory.add(new Item("Sprite Zero", "3.33"));
inventory.add(new Item("Dark Kidney Beans", ".99"));
inventory.add(new Item("Tomato Sauce", "1.19"));
}
public void addItem(Item item) {
inventory.add(item);
}
public Item getItem(String name) {
for (Item item : inventory) {
if (item.getName().equals(name)) {
return item;
}
}
return null;
}
public List<Item> getInventory() {
return inventory;
}
}
public class Item {
private int price;
private String name;
public Item(String name, String price) {
this.name = name;
setPrice(price);
}
public void setPrice(String price) {
Double value = Double.valueOf(price);
this.price = (int) Math.round(100d * value);
}
public int getPrice() {
return price;
}
public String getName() {
return name;
}
}
}