JScrollPane scroll bar won't show up?

package me.an.ugm;

import java.awt.EventQueue;

import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.ScrollPaneConstants;
import javax.swing.UIManager;

public class Application
{
    private JFrame frame;

    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                try
                {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                    Application window = new Application();
                    window.frame.setVisible(true);
                } catch (Exception e)
                {
                    e.printStackTrace();
                }
            }

        });
    }

    public Application()
    {
        initialize();
    }

    private void initialize()
    {
        frame = new JFrame();
        frame.setTitle("Application");
        frame.setBounds(100, 100, 401, 450);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);
        //frame.setResizable(false);
        frame.setLocationRelativeTo(null);

        JPanel panel = new JPanel();
        panel.setBounds(0, 0, 296, 710);
        panel.setLayout(null);

        JScrollPane scrollPane = new JScrollPane(panel, ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS, ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        scrollPane.setBounds(0, 0, 296, 399);
        frame.getContentPane().add(scrollPane);

        for (int i = 0, j = 10; i < 20; i++, j += 35)
        {
            JButton button1 = new JButton();
            button1.setBounds(10, j, 25, 25);
            panel.add(button1);

            JComboBox<String> selectorBox = new JComboBox<>();
            selectorBox.setBounds(40, j, 200, 25);
            panel.add(selectorBox);

            JButton button2 = new JButton();
            button2.setBounds(245, j, 25, 25);
            panel.add(button2);
        }
    }
}

I don't know why the scroll bar won't show up. The JPanel is bigger than the JScrollPane so I thought it should show up. Also when I try using setPreferredSize for the scroll pane instead of setBounds or setSize then just nothing shows up at all. My end goal for the program is to have a button off to the right to add another set of buttons (the ones in the loop), that would be used to select another item. I wanted the program to start with 10 rows of the buttons, but I set it to 20 to test the scroll bar. Is it a problem with there not being a layout or did I mess something up with the scroll pane?

The problem was not using the correct Swing layouts.

Here's the GUI I created.

I added text to the buttons and combo box, so the GUI would look more realistic. I commented out the look and feel to focus on the GUI. I changed the name of the class because I have one test package for all of the code I write for Stack Overflow.

I created the button JPanel in a separate method. I used a GridLayout. This allowed me to create 20 rows of 3 Swing components. This also allowed me to space out the components a bit.

I created the scroll JPanel in another separate method. The key was to use a BorderLayout for the scroll JPanel. I made the scroll JPanel half the size of the button JPanel so it would scroll. You can adjust this calculation however you wish.

Here's the complete runnable code.

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.GridLayout;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;

public class JButtonScrollGUI {
    private JFrame frame;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
//                  UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                    new JButtonScrollGUI();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

        });
    }
    
    private String[] greekAlphabet;

    public JButtonScrollGUI() {
        this.greekAlphabet = new String[] { "alpha", "beta", "gamma", "epsilon", "zeta" };
        initialize();
    }

    private void initialize() {
        frame = new JFrame();
        frame.setTitle("Application");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.add(createScrollPanel(), BorderLayout.CENTER);
        
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }
    
    private JPanel createScrollPanel() {
        JPanel panel = new JPanel(new BorderLayout());
        
        JPanel innerPanel = createButtonPanel();
        Dimension d = innerPanel.getPreferredSize();
        d.width += 50;
        d.height /= 2;
        panel.setPreferredSize(d);
        
        JScrollPane scrollPane = new JScrollPane(innerPanel);
        
        panel.add(scrollPane, BorderLayout.CENTER);
        return panel;
    }
    
    private JPanel createButtonPanel() {
        JPanel panel = new JPanel(new GridLayout(0, 3, 10, 10));
        panel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        
        for (int i = 0; i < 20; i++) {
            JButton button1 = new JButton("Previous " + i);
            panel.add(button1);

            JComboBox<String> selectorBox = new JComboBox<>(greekAlphabet);
            panel.add(selectorBox);

            JButton button2 = new JButton("Next " + i);
            button2.setPreferredSize(button1.getPreferredSize());
            panel.add(button2);
        }
        
        return panel;
    }
    
}