Why oh why, does my scrollPane not scroll to anywhere?

Question

I can't figure out why this code doesn't make scrollbars appear. I'm a complete beginner at Swing so the scrollpanes are very confusing and I don't understand some of the solutions I have seen online. The annoying thing is this code briefly worked but I destroyed the successful part before backing it up when trying to add a component. Any help would be appreciated!

import javax.swing.*;
import java.awt.*;

public class Test extends JFrame
{
    private JPanel leftPanel;
    private JButton myButton;
    private JPanel scrollPanel;
    private JScrollPane scrollPane;
    
    public static void main(String[] args)
    {
        System.setProperty("swing.defaultlaf", "com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
        javax.swing.SwingUtilities.invokeLater(new Runnable()
        {
            public void run() {
                new Test();
            }
        });
    }
    
    /**
     * Constructor for objects of class Backup
     */
    public Test()
    {
        this.setTitle("testy");
        this.setSize(new Dimension(1280,622));
        
        leftPanel = new JPanel();
        leftPanel.setBackground(new Color(255,0,0));
        leftPanel.setBounds(0, 100, 640, 558);
        leftPanel.setEnabled(true);
        this.add(leftPanel);
        
        scrollPanel = new JPanel(null);
        scrollPanel.setBackground(new Color(100,100,100));
        scrollPanel.setPreferredSize(new Dimension(640, 550));
        scrollPanel.setEnabled(true);
        
        JScrollPane scrollPane = new JScrollPane(scrollPanel, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
        JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        scrollPane.setBounds(0, 0, 642, 550);
        scrollPane.setEnabled(true);
        leftPanel.add(scrollPane);
        
        for (int i = 1; i < 30; i++) 
        {
            scrollPanel.setLayout(new GridLayout(30, 1, 0, 1));
            myButton = new JButton("AAAAAAAAAAAAAAAAAAAAA " + i);
            myButton.setPreferredSize(new Dimension(630, 70));
            scrollPanel.add(myButton);
        }
        
        this.setBackground(new Color(0,0,0));
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.pack();
        this.setVisible(true);
}
}

I think I've massively improved it since then thanks to camickr Here is the current version:

   import javax.swing.*;
   import java.awt.*;

    public class Test extends JFrame
    {
    private JPanel leftPanel;
    private JButton myButton;
    private JPanel scrollPanel;
    private JScrollPane scrollPane;

    public static void main(String[] args)
    {
    System.setProperty("swing.defaultlaf", 
    "com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
    javax.swing.SwingUtilities.invokeLater(new Runnable()
    {
        public void run() {
            new Test();
        }
    });
    }


    public Test()
    {
    this.setTitle("testy");
    this.setSize(new Dimension(1280,622));
    
    leftPanel = new JPanel();
    leftPanel.setBackground(new Color(255,0,0));
    leftPanel.setBounds(0, 100, 640, 558);
    this.add(leftPanel);
    
    scrollPanel = new JPanel();
    scrollPanel.setLayout(new GridLayout(70, 1, 0, 1));
    //scrollPanel.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 
    20));
    scrollPanel.setBackground(new Color(100,100,100));
    //scrollPanel.setPreferredSize(new Dimension(640, 550));
    
    //JScrollPane scrollPane = new JScrollPane(scrollPanel);
    JScrollPane scrollPane = new JScrollPane(scrollPanel, 
    JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
    JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
    //scrollPane.setLayout(new GridLayout(30, 1, 0, 1));
    //scrollPane.setBounds(0, 0, 642, 550);
    scrollPane.setEnabled(true);
    //scrollPanel.add(scrollPane);
    
    leftPanel.add(scrollPanel);

    for (int i = 1; i < 71; i++) 
    {
        myButton = new JButton("AAAAAAAAAAAAAAAAAAAAA " + i);
        //myButton.setPreferredSize(new Dimension(640, 80));
        scrollPanel.add(myButton);
    }
    
    this.setBackground(new Color(0,0,0));
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.pack();
    this.setVisible(true);
    }}

Answer 1

1. Don't use a null layout.

2. Don't use setBounds().

3. Don't use setPreferredSize().

Each Swing component is responsible for determining its own preferred size. The layout manager will then set the size/location of each components added to the panel and it will then (dynamically) calculated the preferred size of the panel. The scroll bars will appear when the preferred size of the panel is greater than the size of the scroll pane.

    scrollPanel = new JPanel(null);
    scrollPanel.setBackground(new Color(100,100,100));
    // scrollPanel.setPreferredSize(new Dimension(640, 550)); // delete
    //scrollPanel.setEnabled(true); // not needed
    ...

    for (int i = 1; i < 30; i++) 
    {
        //scrollPanel.setLayout(new GridLayout(30, 1, 0, 1)); // set layout when panel created.
        //myButton.setPreferredSize(new Dimension(630, 70)); // not needed.

1. The layout manager should be set outside the loop, when you create the panel. It should NOT be null.

2. Don't hard code the preferred size. It will not dynamically adjust as components are added.

3. Swing components are enabled by default to setEnabled is not needed.

4. Don't use the "leftPanel". Just add the scroll pane directly to the frame. This will allow the scrollpane to resize dynamically as the frame is resized. Then the scroll bars will appear when requirement.

5. Don't set the preferreid size of the button. The size will be determined based on the text and the Font of the button.