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Resize a JLayeredPane based on user's input

I'm developping a sprite editor. I have a class that extends JPanel, in that class, I use a JLayeredPane as a container. On the bottom layer, there is a JLabel with an ImageIcon and on the top Layer there is a JPanel where I have drawn a grid. When the code is ran, I make some basic math to set the JLayeredPane's prefered size to be equal to about 85% of the height of the screen resolution.
My problem is that when the user wants a new canvas to draw on it, I ask with a JOptionPane the user the size of the canvas he wants. Then I call my class constructor to create a new canvas with the specified size. After that I apply the methods revalidate() and repaint() on the canvas. Unfortunately, it doesn't work. In addition, when I try to get the width and the height of my canvas, it gives 0 for both. However, when I set the size by myself directly into the code, it works well. Therefore, I would like to know how can I update the size of a JLayeredPane ?
On start with a size of 85% of the screen resolution
When I set the size to 640x640 directly inside my code
When I answer the JOptionPane with a size of 640x640, the grid has been adapted to the asking size. However, it's not the case for the JLayeredPane

Without more context, it's impossible to know what's going wrong with your code.
However, personally, I'd stop trying to reinvent the wheel and make use of the available layout management API to provide you with the support you need.
There's a number of ways you could approach the problem. Personally I'd wrap the rendering of the grid and the image up into a single component, but that's me. In that case, you wouldn't need the JLayeredPane.
Let's, for the moment, assume that the JLayeredPane is non-negotiable. I'd then apply a layout manager to the JLayeredPane, this makes interaction with the two other components (image and grid) simpler, as the layout manager takes over control. Because you want them overlaid onto of each other, I'd be tempted to use a GridBagLayout.
The problem here then becomes keeping the two component's sizes in sync with each other. To this end, I'd use setPreferredSize to change the size dynamically.
"insert internal screaming" - Any time I see setPreferredSize it sets of alarm bells. In this case, again, I'd fall back to using a single component and provide a "sizing" mechanism which worked with getPreferredSize, but at the end of day, this is moving us in the same direction.
For laying out the JLayeredPane at 85% height of the parent container, again, GridBagLayout would be my preferred choice.
The following example lays the JLayeredPane in such away as it will want to fill the available space of the parent container (up to 85% of the available height). It then allows the image and grid components to automatically position themselves within side this area.
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.LineBorder;
public class Test {
public static void main(String[] args) {
new Test();
}
public Test() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
} catch (IOException ex) {
ex.printStackTrace();
}
}
});
}
public class TestPane extends JPanel {
private JLayeredPane lp;
private ImagePane imagePane;
private GridPane gridPane;
public TestPane() throws IOException {
setLayout(new GridBagLayout());
lp = new JLayeredPane();
lp.setLayout(new GridBagLayout());
imagePane = new ImagePane();
gridPane = new GridPane();
gridPane.setForeground(new Color(255, 255, 255, 64));
imagePane.setImg(ImageIO.read(...)));
GridBagConstraints gbc = new GridBagConstraints();
gbc.gridx = 0;
gbc.gridy = 0;
lp.add(imagePane, gbc);
lp.add(gridPane, gbc);
lp.setLayer(imagePane, 0);
lp.setLayer(gridPane, 10);
lp.setBackground(Color.RED);
lp.setBorder(new LineBorder(Color.RED));
gbc = new GridBagConstraints();
gbc.gridx = 0;
gbc.gridy = 0;
gbc.weightx = 1;
gbc.weighty = 0.85;
gbc.fill = gbc.BOTH;
add(lp, gbc);
applyDesiredSize(200, 200);
JButton btn = new JButton("Change");
gbc = new GridBagConstraints();
gbc.gridx = 0;
gbc.gridy = 1;
gbc.weighty = 0.25;
add(btn, gbc);
btn.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
JTextField widthTF = new JTextField(4);
JTextField heightTF = new JTextField(4);
JPanel panel = new JPanel();
panel.add(new JLabel("Size: "));
panel.add(widthTF);
panel.add(new JLabel("x"));
panel.add(heightTF);
JOptionPane.showMessageDialog(TestPane.this, panel, "Change Size", JOptionPane.PLAIN_MESSAGE);
try {
int width = Integer.parseInt(widthTF.getText());
int height = Integer.parseInt(heightTF.getText());
applyDesiredSize(width, height);
} catch (NumberFormatException numberFormatException) {
JOptionPane.showMessageDialog(TestPane.this, "Invalid dimensions", "Error", JOptionPane.ERROR_MESSAGE);
}
}
});
}
protected void applyDesiredSize(int width, int height) {
Dimension size = new Dimension(width, height);
// lp.setPreferredSize(size);
imagePane.setPreferredSize(size);
gridPane.setPreferredSize(size);
// Stop GridBagLayout from shrinking the components
imagePane.setMinimumSize(size);
gridPane.setMinimumSize(size);
lp.revalidate();
lp.repaint();
}
}
public class ImagePane extends JPanel {
private BufferedImage img;
public ImagePane() {
}
public BufferedImage getImg() {
return img;
}
public void setImg(BufferedImage img) {
this.img = img;
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
g2d.drawImage(img, 0, 0, getWidth(), getHeight(), this);
g2d.dispose();
}
}
public class GridPane extends JPanel {
private int gridSize = 10;
public GridPane() {
setOpaque(false);
}
public int getGridSize() {
return gridSize;
}
public void setGridSize(int gridSize) {
this.gridSize = gridSize;
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
g2d.setColor(getForeground());
for (int x = 0; x < getWidth(); x += gridSize) {
g2d.drawLine(x, 0, x, getHeight());
}
for (int y = 0; y < getWidth(); y += gridSize) {
g2d.drawLine(0, y, getWidth(), y);
}
g2d.dispose();
}
}
}
So, before you shoot me down because it "doesn't fit with what you've done", understand that:
I've no context to go on as to what you've actually done, other then an overview of your problem description
I've provided a runnable example which demonstrates the basic concepts you "might" be able to use to solve your issue, which is more then you've done for us
This is not the only way you might achieve this, and by no means is it my preferred solution

Swing JScrollPane gallery implementation

I am trying to make a JComponent function as a gallery. It must able to display saved images on it which are going to be showed in a component living in a JScrollPane. I am also going to add the functionality to remove an image. I managed to display the image on the component. How should I approach this?
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollBar;
import javax.swing.JScrollPane;
public class GalleryPanel extends JPanel
{
private static final long serialVersionUID = 1L;
private int currentImage;
private JLabel[] images;
private final int MAX_IMAGES = 12;
private JScrollPane scrollPane;
private JPanel imageGallery;
public void init()
{
setLayout(new BorderLayout());
images = new JLabel[MAX_IMAGES];
imageGallery = new JPanel();
imageGallery.setLayout(new GridLayout(12,1,10,10));
scrollPane = new JScrollPane();
scrollPane.setBackground(Color.RED);
scrollPane.add(imageGallery);
add(scrollPane, BorderLayout.CENTER);
setBackground(Color.GRAY);
}
public void addImageToGallery(File file)
{
if ( currentImage <= images.length - 1)
{
BufferedImage bufImage = null;
try
{
bufImage = ImageIO.read(file); //tries to load the image
}
catch (Exception e)
{
System.out.println("Unable to load file " + file.toString());
}
Image resizedImage = bufImage.getScaledInstance(bufImage.getWidth()/6, bufImage.getHeight()/6, Image.SCALE_SMOOTH);
ImageIcon icon = new ImageIcon(resizedImage);
images[currentImage] = new JLabel(icon, JLabel.CENTER);
scrollPane.add(images[currentImage]);
images[currentImage].setSize(120, 100);
scrollPane.add(images[currentImage]);
currentImage++;
//scrollPane.revalidate();
//scrollPane.repaint();
}
else
{
throw new ArrayIndexOutOfBoundsException("The gallery is full");
}
}
public final int getMaxImages()
{
return MAX_IMAGES;
}
public Dimension getPreferredSize()
{
return new Dimension(300, 700);
}
}

I created JImagePanel for my CBIR java application using JPanel, JList, JLabel and ListCellRenderer
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package javax.swing;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.image.BufferedImage;
import java.util.Vector;
import javax.swing.border.LineBorder;
/**
*
* #author NiRRaNjAN RauT
*/
/**
* JImagePanel is used to display JList of BufferedImage
* This class is extended to JPanel
*/
public class JImagePanel extends JPanel {
/**
* JList is used to display display BufferedImage
*/
private JList imageList = null;
/**
* JScrollPane is used to add scroll bar to JList
*/
JScrollPane scroll_pane = null;
/**
* Vector<BufferedImage> is used to store the BufferedImages
*/
Vector<BufferedImage> listData = new Vector<BufferedImage>();
/**
* default constructor used to initialize JPanel
*/
public JImagePanel() {
this(null);
}
/**
* #param data
* This parameterized constructor is used to
* add the vector of BufferedImages to JList
*/
public JImagePanel(Vector<BufferedImage> data) {
setLayout(new BorderLayout());
if(data != null) {
listData = data;
}
imageList = new JList();
imageList.setFixedCellHeight(160);
imageList.setFixedCellWidth(160);
if(!listData.isEmpty()) {
imageList.setListData(listData);
}
imageList.setCellRenderer(new JCellRenderer());
imageList.setAutoscrolls(true);
imageList.setLayoutOrientation(JList.HORIZONTAL_WRAP);
imageList.setVisibleRowCount(-1);
scroll_pane = new JScrollPane(imageList);
this.add(scroll_pane, BorderLayout.CENTER);
this.setBorder(new LineBorder(Color.BLUE));
}
/**
* #param selection
* The parameter selection is used to set multi selection on or off
* When true, it allows to select multiple images in JList using
* Ctrl key.
*/
public void setMultiSelection(boolean selection) {
if(selection) {
imageList.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);
} else {
imageList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
}
}
public int[] getSelectedIndices() {
return imageList.getSelectedIndices();
}
public int getImageSize() {
return imageList.getModel().getSize();
}
/**
* #param image
* This is used to add BufferedImage to JList
* User can add new images to JList at runtime.
*/
public void addImageToList(BufferedImage image) {
listData.add(image);
imageList.setListData(listData);
}
/**
* The method is used to clear the old list data.
* It sets the list data to empty list.
*/
public void clear() {
listData.clear();
imageList.setListData(listData);
}
/**
* #param data
* This method is used to set list data to JList
* The data should be Vector of BufferedImage
*/
public void setListData(Vector<BufferedImage> data) {
if(data == null || data.isEmpty()) {
System.err.println("Empty Data");
return;
}
listData = data;
imageList.setListData(listData);
}
}
For this Panel ListCellRenderer component is required which is given below.
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package javax.swing;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import javax.swing.border.EmptyBorder;
/**
*
* #author NiRRaNjAN RauT
*/
/**
* JCellRenderer is extended to JLabel
* It is used to display the image on JList by implementing ListCellRenderer
*/
public class JCellRenderer extends JLabel implements ListCellRenderer {
/**
* The width and height is used to set size of a JLabel.
* It is default set to 200 * 200
*/
private static final int width = 140, height = 140;
/**
* default constructor used to set size of JLabel and set Border to JLabel.
*/
public JCellRenderer() {
setSize(width, height);
setOpaque(true);
setBorder(new EmptyBorder(10, 10, 10, 10));
}
/**
* #return Component
* it returns JLabel for a cell.
* The image from JList is set as icon on JLabel so that it displays the image as a JList item.
*/
#Override
public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
try {
if(value instanceof BufferedImage) {
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics2D g = image.createGraphics();
g.drawImage((BufferedImage) value, 0, 0, width, height, null);
g.dispose();
setIcon(new ImageIcon(image));
}
if(isSelected) {
setBackground(Color.DARK_GRAY);
} else {
setBackground(Color.LIGHT_GRAY);
}
} catch(Exception ex) {
System.err.println("" + ex.getMessage());
}
return this;
}
}
and This is how to use it.
JFrame frame = new JFrame("Image Panel Demo");
frame.setResizable(true);
frame.setSize(500, 500);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JImagePanel panel = new JImagePanel();
panel.setMultiSelection(true);
frame.setLayout(new BorderLayout());
frame.add(panel, BorderLayout.CENTER);
frame.setVisible(true);
try {
File files[] = new File("/SOFTWARES/NETBEANS_PROJECTS/BE/S_CBIR/dataset").listFiles();
for(File file : files) {
BufferedImage image = ImageIO.read(file);
panel.addImageToList(image);
}
} catch (Exception e) {
e.printStackTrace(System.err);
}
Hope it'll help.

scrollPane.add(images[currentImage]);
images[currentImage].setSize(120, 100);
scrollPane.add(images[currentImage]);
Don't add components directly to a scroll pane.
A JScrollPane uses a JViewport to hold the component to be displayed.
So if you want to use a JList you would use:
JList<Icon> list = new JList<Icon>();
JScrollPane scrollPane = new JScrollPane( list );
frame.add( scrollPane );
Then you add the Icon (not a JLabel) to the JList. A JList already has a custom renderer to display an Icon.
Read the section from the Swing tutorial on How to Use Lists for more information and working examples.
The tutorial also has a section on How to Use Scroll Panes you should read.

Positioning JButton on JFrame won't show up [duplicate]

This question already has answers here:
How to add JTable in JPanel with null layout?
(11 answers)
Closed 8 years ago.
I know this is horrible coding but I desperately need to fix this problem. I've tried multiple ways of trying to position the button but the button still stays in the top center with all the other buttons lined up after it.
import javax.imageio.ImageIO;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Graphics;
import java.io.IOException;
import java.net.URL;
public class Template extends JFrame {
/**
* #param args
*/
public static void main(String[] args) throws IOException{
// TODO Auto-generated method stub
JFrame frame = new JFrame("The Impossible Quiz");//Construct JFrame
frame.setLayout(null);//manual setting for button placement
frame.setContentPane(new JPanel() {//sets panel as background
BufferedImage image = ImageIO.read(new URL("https://pbs.twimg.com/media/BoyFVfXIUAA0Tik.png"));//uses url image
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawImage(image, 0, 0, 1360, 690, this);//sets image as jframe background
}
});
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//closes jframe when press exit
frame.setSize(1365, 730);//sets size of jframe
JPanel buttonpanel = new JPanel();//sets panel for all buttons
buttonpanel.setBounds(0, 0, 1460, 690);//sets placing and sizing of panel
buttonpanel.setOpaque(false);//makes panel transparent
JButton next = new JButton ("Start");//contructs correct start button
next.setBounds(10, 5, 40, 50);
buttonpanel.add(next);//adds button to panel
next.addActionListener(new ActionListener()//adds actionlistener to button
{
public void actionPerformed(ActionEvent e)
{
new Introduction();//continues to next question
}
});
JButton wrongstart = new JButton("Start!!");//constructs wrong start button
wrongstart.setSize(100, 400);//setes size of button
buttonpanel.add(wrongstart);//adds button to panel
wrongstart.addActionListener(new ActionListener()//adds actionlistener to button
{
public void actionPerformed(ActionEvent e)
{
new Wrong();//direct user to wrong panel
}
});
frame.add(buttonpanel);//adds panel to jframe
frame.setVisible(true);//sets jframe as visible
}
}

Your problem is that you're trying to use absolute positioning to position a component (your JButton) in a container (the containing JPanel) that uses FlowLayout as default, and FlowLayout completely ignores the components bounds. A quick solution is to set the JPanel's layout to null allowing for absolute positioning. A correct solution is to always avoid null layouts and absolute positioning and instead to nest JPanels, each using its own layouts, in order to create complex yet flexible and pleasing GUI's.
You're setting the JFrame contentPane's layout to null as well -- don't do that either.
And then adding a JPanel as the contentPane which uses a default FlowLayout -- don't do that. Let the contentPane's layout be BorderLayout.
Edit
For example, if we leave the contentPane alone with its BorderLayout, and add another image panel on top of it, one that uses GridBagLayout, we can easily position our JButton to the top left corner of the GUI if desired. ....
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.image.BufferedImage;
import javax.swing.*;
#SuppressWarnings("serial")
public class Template2 extends JPanel {
private static final int PREF_W = 1460;
private static final int PREF_H = 690;
private BufferedImage img;
private JButton startButton = new JButton("Start");
public Template2() {
setLayout(new GridBagLayout());
// TODO: .... read in your image here
GridBagConstraints gbc = new GridBagConstraints();
gbc.gridx = 1;
gbc.gridy = 1;
gbc.gridwidth = 1;
gbc.gridheight = 1;
gbc.insets = new Insets(5, 10, 0, 0);
gbc.anchor = GridBagConstraints.NORTHWEST;
gbc.fill = GridBagConstraints.NONE;
gbc.weightx = 1.0;
gbc.weighty = 1.0;
add(startButton, gbc);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
#Override
public void paintComponents(Graphics g) {
super.paintComponents(g);
if (img != null) {
g.drawImage(img, 0, 0, this);
}
}
private static void createAndShowGui() {
Template2 mainPanel = new Template2();
JFrame frame = new JFrame("Some Horrendous Program");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(mainPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}

JScrollPane resize containing JPanel when scrollbars appear

I have a small problem when using JScrollPane in my Java application.
I have a JScrollPane containing a JPanel.
This JPanel is dynamically updated with buttons (vertically ordered) that can be of any width.
The JPanel automatically adjusts its width to the largest JButton component inside.
Now when the vertical scrollbar appears, it takes away some space on the right side of my JPanel, which causes the largest buttons not to appear completely. I don't want to use a horizontal scrollbar in addition to display the whole button.
Is there a way to resize my JPanel when a scrollbar appears, so it appears nicely next to my buttons? Or is there any other way to have the scrollbar appear next to my JPanel?
Thanks in advance!
EDIT: Here is a demo of my problem. When you resize the window to a smaller height, a little part of the buttons on the right side gets covered.
import java.awt.BorderLayout;
import java.awt.EventQueue;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import java.awt.GridLayout;
/**
* #author Dylan Kiss
*/
public class Demo {
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
JFrame myFrame = new JFrame("Demo");
JPanel sideBar = new JPanel();
JPanel centerPanel = new JPanel();
centerPanel.add(new JLabel("This is the center panel."));
JPanel buttonContainer = new JPanel();
JButton myButton = null;
for (int i = 0; i < 20; i++) {
buttonContainer.setLayout(new GridLayout(20, 1, 0, 0));
myButton = new JButton("This is my button nr. " + i);
buttonContainer.add(myButton);
}
sideBar.setLayout(new BorderLayout(0, 0));
JScrollPane scrollPane = new JScrollPane(buttonContainer);
sideBar.add(scrollPane);
myFrame.getContentPane().add(sideBar, BorderLayout.WEST);
myFrame.getContentPane().add(centerPanel, BorderLayout.CENTER);
myFrame.setLocationByPlatform(true);
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myFrame.pack();
myFrame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
}

Here is a simple, not pretty, solution:
scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
EDIT:
I thought that might not do the job in your case. Here is a better solution although it has quite a lot of boilerplate:
private class ButtonContainerHost extends JPanel implements Scrollable {
private static final long serialVersionUID = 1L;
private final JPanel buttonContainer;
public ButtonContainerHost(JPanel buttonContainer) {
super(new BorderLayout());
this.buttonContainer = buttonContainer;
add(buttonContainer);
}
#Override
public Dimension getPreferredScrollableViewportSize() {
Dimension preferredSize = buttonContainer.getPreferredSize();
if (getParent() instanceof JViewport) {
preferredSize.width += ((JScrollPane) getParent().getParent()).getVerticalScrollBar()
.getPreferredSize().width;
}
return preferredSize;
}
#Override
public int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction) {
return orientation == SwingConstants.HORIZONTAL ? Math.max(visibleRect.width * 9 / 10, 1)
: Math.max(visibleRect.height * 9 / 10, 1);
}
#Override
public boolean getScrollableTracksViewportHeight() {
if (getParent() instanceof JViewport) {
JViewport viewport = (JViewport) getParent();
return getPreferredSize().height < viewport.getHeight();
}
return false;
}
#Override
public boolean getScrollableTracksViewportWidth() {
return true;
}
#Override
public int getScrollableUnitIncrement(Rectangle visibleRect, int orientation, int direction) {
return orientation == SwingConstants.HORIZONTAL ? Math.max(visibleRect.width / 10, 1)
: Math.max(visibleRect.height / 10, 1);
}
}
It implements Scrollable to get full control of scrolling, does a fancy trick with tracking the viewport height to ensure the buttons expand when the space is available and adds on the width of the vertical scroll bar to the preferred width at all times. It could expand when the vertical scroll bar is visible but that looks bad anyway. Use it like this:
scrollPane = new JScrollPane(new ButtonContainerHost(buttonContainer));
It looks to me like this workaround is required because of a possible bug in javax.swing.ScrollPaneLayout:
if (canScroll && (viewSize.height > extentSize.height)) {
prefWidth += vsb.getPreferredSize().width;
}
Here extentSize is set to the preferred size of the viewport and viewSize is set to viewport.getViewSize(). This does not seem correct, AFAIK the size of the view inside the viewport should always equal the preferred size. It seems to me that the view size should be compared to the actual size of the viewport rather than its preferred size.

A simple workaround to meet your demands regarding
Is there a way to resize my JPanel when a scrollbar appears, so it
appears nicely next to my buttons?
is the use of EmptyBorder, this will let you achieve what you feel like, should happen, as shown in the image below :
I just added this line written below after this line JPanel buttonContainer = new JPanel();
ADDED LINE
buttonContainer.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
Here is your code with that added line :
import java.awt.BorderLayout;
import java.awt.EventQueue;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import java.awt.GridLayout;
/**
* #author Dylan Kiss
*/
public class Demo
{
public static void main(String[] args)
{
EventQueue.invokeLater(new Runnable()
{
#Override
public void run()
{
try
{
JFrame myFrame = new JFrame("Demo");
JPanel sideBar = new JPanel();
JPanel centerPanel = new JPanel();
centerPanel.add(new JLabel("This is the center panel."));
JPanel buttonContainer = new JPanel();
buttonContainer.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
JButton myButton = null;
for (int i = 0; i < 20; i++)
{
buttonContainer.setLayout(new GridLayout(20, 1, 0, 0));
myButton = new JButton("This is my button nr. " + i);
buttonContainer.add(myButton);
}
sideBar.setLayout(new BorderLayout(0, 0));
JScrollPane scrollPane = new JScrollPane(buttonContainer);
sideBar.add(scrollPane);
myFrame.getContentPane().add(sideBar, BorderLayout.WEST);
myFrame.getContentPane().add(centerPanel, BorderLayout.CENTER);
myFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
myFrame.pack();
myFrame.setLocationByPlatform(true);
myFrame.setVisible(true);
}
catch (Exception e)
{
e.printStackTrace();
}
}
});
}
}

You could resize the JPanel by calling setPreferredSize when the JPanel needs to be resized.
buttonContainer.setPreferredSize(Dimension d);

Drawing a rectangle with a button

I am a beginner, starting a simple project on GUI. The RectangleComponent should draw a Rectangle on the form with a button click. A rectangle won't draw with the following code, but if I put the same 2 lines of code outside the listener, it certainly works. I would appreciate any help.
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.BorderLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class EllipseRectViewer {
/**
* #param args
*/
public static void main(String[] args)
{
final JFrame frame = new JFrame();
final int FRAME_WIDTH = 400;
final int FRAME_HEIGHT = 400;
frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("Rectangle and Ellipse Draw");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
JPanel panel = new JPanel();
frame.add(panel, BorderLayout.NORTH);
class RectangleDrawListener implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
RectangleComponent r2 = new RectangleComponent();
frame.add(r2);
}
}
JButton rectButton = new JButton("Rectangle");
ActionListener rectDrawListener = new RectangleDrawListener();
rectButton.addActionListener(rectDrawListener);
panel.add(rectButton);
frame.setVisible(true);
}
}
import java.awt.Rectangle;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JComponent;
public class RectangleComponent extends JComponent
{
Rectangle rect;
public RectangleComponent()
{
rect = new Rectangle(20, 20, 30, 30);
}
public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D) g;
g2.draw(rect);
}
}
Thank you.

After adding the RectangleComponent to the frame, either revalidate the newly added component or the frame's root pane:
public void actionPerformed(ActionEvent event) {
RectangleComponent r2 = new RectangleComponent();
frame.add(r2);
// Option 1
r2.revalidate();
// Option 2
frame.getRootPane().revalidate();
}
Note1: the frame itself can't be revalidated (upto JDK 1.6)
Note2: the frame itself can be revalidated (JDK 1.7+)

i think you need to revalidate() the frame.
frame.revalidate();
put it like this:
public void actionPerformed(ActionEvent event)
{
RectangleComponent r2 = new RectangleComponent();
frame.add(r2);
frame.revalidate();
}

Try to use LineBorder. Create a JPanel with LineBorder and add the JButton to the JPanel.

rect = new Rectangle(20, 20, 30, 30);
A second problem is that your component doesn't have a preferred size. Your component displays in a simple frame because you add the comonent to the center of a BorderLayout so the preferred size of the component is ignored. However, this won't work if you try to use the component when using other layout managers.
You should also override the getPreferredSize() method to return the preferred size of your component at a minimum you need to use:
return new Dimension(50, 50);
to accomodate the size and location of the painted rectangle.