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Wrong size with JScrollPane containing updated image

I am coding a GUI containing a JScrollPane that displays an image that gets updated (with potential modification of its dimensions). The image is in an ImageIcon in a JLabel. The image size is retrieved using ImageIcon.getIconWith() and getIconHeight(). And the JLabel preferred size is updated with those dimensions.
When the application is started for the first time, the JScrollPane and its scrollbars have the right dimensions to view the whole image (potentially using scrolling). But when the image gets updated the JScrollPane and the scrollbars assume the image has the dimensions of the previous image. How do I get the JScrollPane to update correctly ?
Here is a curated version of my GUI. Visualizer.java uses the GUI VisualizerGUI.java. When the "Run" button is pushed, a new image is randomly generated using ImageDrawer.drawImage() (simulates the behavior of the real application) and the content of the JScrollPane is updated using the function VisualizerGUI.setTransitionsImage(String imgPath).
Visualizer.java:
import java.util.*;
import java.io.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class Visualizer implements ActionListener {
private VisualizerGUI gui = null;
public Visualizer() {
gui = VisualizerGUI.createAndStart(this);
}
public static void main(String[] args) {
Visualizer viz = new Visualizer();
}
public void actionPerformed(ActionEvent e) {
if (e.getActionCommand().equals("Run command")) {
run();
}
}
public void run() {
updateGUIwithSolution();
}
public void updateGUIwithSolution() {
gui.initGUIupdate();
try {
ImageDrawer.drawImage();
gui.setTransitionsImage("image.png");
} catch (Exception e) {
System.out.println("Error while generating image");
e.printStackTrace();
}
gui.finalizeGUIupdate();
}
}
VisualizerGUI.java:
import java.util.*;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.event.ActionListener;
import javax.swing.*;
import java.lang.reflect.InvocationTargetException;
public final class VisualizerGUI {
private JFrame frame;
private JButton runButton;
private JButton nextButton;
private JScrollPane transitionsDisplay;
private JTabbedPane executionsDisplay;
private JTabbedPane tracesDisplay;
private JTextArea textInfoArea;
public VisualizerGUI() {}
private void initGUI(ActionListener actionsHandler) {
//Create and set up the window.
frame = new JFrame("Visualizer");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel controlPanel = new JPanel(new FlowLayout());
runButton = new JButton("Run");
runButton.addActionListener(actionsHandler);
runButton.setActionCommand("Run command");
controlPanel.add(runButton);
nextButton = new JButton("Next");
nextButton.addActionListener(actionsHandler);
nextButton.setActionCommand("Find next solution");
controlPanel.add(nextButton);
transitionsDisplay = new JScrollPane();
executionsDisplay = new JTabbedPane();
tracesDisplay = new JTabbedPane();
JSplitPane ETspliter = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, executionsDisplay, tracesDisplay);
JSplitPane graphsSpliter = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, transitionsDisplay, ETspliter);
textInfoArea = new JTextArea();
textInfoArea.setLineWrap(true);
textInfoArea.setWrapStyleWord(true);
textInfoArea.setEditable(false);
JScrollPane textInfoAreaSP = new JScrollPane(textInfoArea);
JSplitPane topSpliter = new JSplitPane(JSplitPane.VERTICAL_SPLIT, graphsSpliter, textInfoAreaSP);
transitionsDisplay.setPreferredSize(new Dimension(200,200));
executionsDisplay.setPreferredSize(new Dimension(200,200));
tracesDisplay.setPreferredSize(new Dimension(200,200));
textInfoAreaSP.setPreferredSize(new Dimension(200,100));
frame.getContentPane().setLayout(new BorderLayout());
frame.getContentPane().add(controlPanel, BorderLayout.NORTH);
frame.getContentPane().add(topSpliter, BorderLayout.CENTER);
frame.pack();
frame.setVisible(true);
}
public static VisualizerGUI createAndStart(ActionListener actionsHandler) {
VisualizerGUI gui = new VisualizerGUI();
final Runnable guiRunner =
new Runnable() {
public void run() {
gui.initGUI(actionsHandler);
// gui.pack();
}
};
try {
javax.swing.SwingUtilities.invokeAndWait(guiRunner);
} catch (InterruptedException e) {
System.out.println(">>> WARNING <<< InterruptedException while creating the GUI");
} catch (InvocationTargetException e) {
System.out.println(">>> WARNING <<< InvocationTargetException while creating the GUI");
}
return gui;
}
public void clear() {
initGUIupdate();
finalizeGUIupdate();
}
public void initGUIupdate() {
// frame.setVisible(false);
transitionsDisplay.setViewportView(null);
executionsDisplay.removeAll();
tracesDisplay.removeAll();
textInfoArea.setText(null);
}
public void pack() {
frame.pack();
}
public void finalizeGUIupdate() {
// frame.validate();
// frame.repaint();
// frame.setVisible(true);
}
public void setTransitionsImage(String imgPath) {
ImageIcon icon = new ImageIcon(imgPath);
icon.getImage().flush();
int width = icon.getIconWidth();
int height = icon.getIconHeight();
JLabel label = new JLabel();
label.setVerticalAlignment(SwingConstants.CENTER);
label.setHorizontalAlignment(SwingConstants.CENTER);
label.setIcon(icon);
label.setPreferredSize(new Dimension(width,height));
//label.setPreferredSize(null);
transitionsDisplay.setViewportView(label);
label.revalidate();
label.repaint();
transitionsDisplay.getViewport().revalidate();
transitionsDisplay.getViewport().repaint();
transitionsDisplay.revalidate();
// transitionsDisplay.validate();
transitionsDisplay.repaint();
frame.revalidate();
// frame.validate();
frame.repaint();
}
public void setTransitionsImageInED(String imgPath) {
final Runnable guiRunner =
new Runnable() {
public void run() { setTransitionsImage(imgPath); }
};
// javax.swing.SwingUtilities.invokeLater(guiRunner);
try {
javax.swing.SwingUtilities.invokeAndWait(guiRunner);
} catch (InterruptedException e) {
System.out.println(">>> WARNING <<< InterruptedException while creating the GUI");
} catch (InvocationTargetException e) {
System.out.println(">>> WARNING <<< InvocationTargetException while creating the GUI");
}
}
}
ImageDrawer.java:
import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
public class ImageDrawer {
public static void drawImage() throws Exception {
try {
int width = 20 + (int)(Math.random() * 1000);
int height = 20 + (int)(Math.random() * 1000);
BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
Graphics2D ig2 = bi.createGraphics();
ig2.setPaint(Color.blue);
ig2.fillRect(0, 0, width, height);
ig2.setPaint(Color.red);
ig2.fillRect(5, 5, width - 10, height - 10);
ig2.setPaint(Color.blue);
ig2.drawLine(0, 0, width, height);
ig2.drawLine(0, height, width, 0);
ImageIO.write(bi, "PNG", new File("image.png"));
} catch (IOException ie) {
ie.printStackTrace();
}
}
}
Can someone explain why I have this problem? Thanks!

transitionsDisplay.setPreferredSize(new Dimension(200,200));
executionsDisplay.setPreferredSize(new Dimension(200,200));
tracesDisplay.setPreferredSize(new Dimension(200,200));
textInfoAreaSP.setPreferredSize(new Dimension(200,100));
Don't use setPreferredSize(...). Each Swing component is responsible for determining its own size.
The image size is retrieved using ImageIcon.getIconWith() and getIconHeight(). And the JLabel preferred size is updated with those dimensions.
Not necessary. Again the JLabel will determine its own size based on the size of the Icon. This is done dynamically as the image/icon changes.
The scrollbars of the scrollpane will appear when the preferred size of the label is greater than the size of the scrollpane. Just let the layout managers do their job.

Unable to add other components to window after creating background using JLabel in Java

I want to create a java Application like a widget. Here is my code below
package newpackage;
public class MainFrame extends JFrame {
JLabel imageLabel = new JLabel();
public MainFrame() {
try {
this.setUndecorated(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setSize(new Dimension(360, 360));
ImageIcon ii = new ImageIcon(this.getClass().getResource("imageexcel.gif"));
imageLabel.setIcon(ii);
add(imageLabel, java.awt.BorderLayout.CENTER);
this.setVisible(true);
Shape shape=new Ellipse2D.Float(0,0,360,360);
AWTUtilities.setWindowShape(this, shape);
AWTUtilities.setWindowOpaque(this, false);
imageLabel.add(new JButton("START"));
} catch (Exception exception) {
exception.printStackTrace();
}
}
public static void main(String[] args) {
new MainFrame();
}
}
In the above code, I have done the following:
Created a Frame
Removed the Title Bar
Added the Background using JLabel
Changed the shape of window as circle according to the shape of image
Now I would like to add some components in to it and perform some action with them but no component is visible after adding.
I have tried adding to Frame as well as JLabel and no use from either.
This is the image i used for background
Please help me to proceed further....
Thanking you

JLabels use null layouts by default, and so your button will default to size 0,0. Try giving it a decent layout manager, even FlowLayout would likely work. Another solution is to keep the null layout and set the sizes and positions of added components, but this route is a dangerous route and one I don't recommend.
Actually a GridBagLayout works nice to center the components. Also add all components before calling setVisible(true):
imageLabel.setLayout(new GridBagLayout());
this.setUndecorated(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setSize(new Dimension(360, 360));
ImageIcon ii = new ImageIcon(this.getClass().getResource("imageexcel.gif"));
imageLabel.setIcon(ii);
add(imageLabel, java.awt.BorderLayout.CENTER);
imageLabel.add(new JButton("START"));
this.setVisible(true);
or better?
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.Image;
import java.awt.Shape;
import java.awt.Window;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.geom.Ellipse2D;
import java.net.URL;
import javax.swing.*;
import com.sun.awt.AWTUtilities;
#SuppressWarnings("serial")
public class MainPanelOvalFrame extends JPanel {
private static final String RESOURCE_PATH = "imageexcel.gif";
private Window window;
private Image img;
public MainPanelOvalFrame(Window window, Image image) {
this.window = window;
this.img = image;
setLayout(new GridBagLayout());
add(new JButton(new StartAction("Start", KeyEvent.VK_S)));
int w = image.getWidth(this);
int h = image.getHeight(this);
Shape shape = new Ellipse2D.Float(0, 0, w, h);
AWTUtilities.setWindowShape(window, shape);
AWTUtilities.setWindowOpaque(window, false);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
if (img != null) {
g.drawImage(img, 0, 0, this);
}
}
#Override
public Dimension getPreferredSize() {
if (isPreferredSizeSet() || img == null) {
return super.getPreferredSize();
}
int w = img.getWidth(this);
int h = img.getHeight(this);
return new Dimension(w, h);
}
private class StartAction extends AbstractAction {
public StartAction(String name, int mnemonic) {
super(name);
putValue(MNEMONIC_KEY, mnemonic);
}
#Override
public void actionPerformed(ActionEvent e) {
window.dispose();
}
}
private static void createAndShowGui() {
JFrame frame = new JFrame();
frame.setUndecorated(true);
URL imgUrl = MainPanelOvalFrame.class.getResource(RESOURCE_PATH);
Image image = new ImageIcon(imgUrl).getImage();
MainPanelOvalFrame mainPanel = new MainPanelOvalFrame(frame, image);
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.getContentPane().add(mainPanel);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}

You just need to set a Layout manager for imageLabel ,or use null as the Layout manager, then set the size and location of the JButton manually.
to use Layout Manager
imageLabel.setIcon(ii);
imageLabel.setLayout(new FlowLayout());
imageLabel.add(new JButton("START"));
//need to setLayout and add JButton before setVisible(true)
add(imageLabel, java.awt.BorderLayout.CENTER);
this.setVisible(true);
to use null layout
JButton j=new JButton("START");
j.setSize(100,50);
j.setLocation(imageLabel.getWidth()/2-j.getWidth()/2, imageLabel.getHeight()/2-j.getHeight()/2);
//then add Button into imageLabel
imageLabel.add(j);
Layout manager is usually recommended because it can fit different environment.

Placing button on top of image

I am back again. I was wondering how I would go about placing a button on top of an image in a GUI. Here is my current code:
private static JPanel titlePanel = new JPanel();
private static JLabel titleScreen = new JLabel();
private static JLabel titleScreenBackground = new JLabel();
private static JButton startGameButton = new JButton("START GAME");
private static ImageIcon titleScreenPic = new ImageIcon("http://icdn6.digitaltrends.com/image/battleship-650x0.jpg");
private static JFrame frame=new JFrame(); //creates frame
public static void main(String[] args) throws MalformedURLException{
titleScreen();
}
public static void titleScreen() throws IOException{
titleScreen.setLayout(new GridBagLayout());
titlePanel.setLayout(new GridBagLayout());
GridBagConstraints c1 = new GridBagConstraints();
c1.gridx = 0;
c1.gridy = 0;
c1.anchor = GridBagConstraints.PAGE_END;
titleScreenBackground.setIcon(titleScreenPic);
titlePanel.add(startGameButton);
titlePanel.setAlignmentY(SwingConstants.BOTTOM);
frame.add(titleScreenBackground);
frame.add(titlePanel);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(630, 300); //sets appropriate size for frame
frame.setVisible(true); //makes frame visible
}
I tried to make the panel a gridbaglayout so I could place the components in the same cell, but it still places the image first and then the button directly next to it.
EDIT: I have redone the code, making it do somewhat what I wanted. As you can see, the line where I try to set the location of the button does not do anything to the button.

how I would go about placing a button on top of an image in a GUI.
If you want to place a Swing button on top of an image then you need to follow 2 steps.
set a layout manager for the label containing the image.
add the button to the label (not the panel).
See Background Panel for more information and examples.
Edit:
To center a component the easiest approach is:
label.setLayout( new GridBagLayout() );
label.add(button, new GridBagConstraints());

If you want button on the image you can just use image in paint method of JPanel.
Example (with resource im
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class TestFrame extends JFrame {
BackgroundPane bgPane;
private JButton startButton;
public TestFrame() {
super();
initComponents();
}
private void initComponents() {
try {
URL url = getClass().getResource("battleship-650x0.jpg");
BufferedImage image = ImageIO.read(url);
bgPane = new BackgroundPane(image);
bgPane.setLayout(new GridBagLayout());
startButton = new JButton("Start");
bgPane.add(startButton);
setContentPane(bgPane);
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* #param args
*/
public static void main(String[] args) {
TestFrame frame = new TestFrame();
frame.setVisible(true);
}
class BackgroundPane extends JPanel {
Image image;
public BackgroundPane(Image backGroundImage) {
super();
image = backGroundImage;
setPreferredSize(new Dimension(image.getWidth(this), image.getHeight(this)));
}
#Override
public void paint(Graphics g) {
super.paint(g);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawImage(image, 0, 0, null);
}
}
}

How to force component in JScrollPane to show

I have a problem with showing specific component placed in JScrollPane. I have horizontal JScrollPane with GridLayout(1,0) and it contains variable number of JPanels - each containing image. It's like a preview of frames in GIF image. I use button to move among these JPanels (by changing borders and keeping index of chosen one), but I don't know how to force JScrollPane to show me JPanel if it's chosen (and center it if possible).
So I want this
force to do this:
Thanks in advance!
EDIT: almost working code with scrollRectToVisible() method
public class MiniatursPanel extends JPanel{
private int indexOfChosenFrame = 0;
private ArrayList<JPanel> frames;
private JScrollPane scrollPane;
private JPanel innerPanel;
public MiniatursPanel(){
setBorder(BorderFactory.createCompoundBorder(BorderFactory.createRaisedBevelBorder(),BorderFactory.createLoweredBevelBorder()));
setPreferredSize(new Dimension(1200,170));
setLayout(null);
}
public void initialize(){
int width = GifImageStats.getInstance().getWidth();
int height = GifImageStats.getInstance().getHeight();
int numberOfFrames = GifImageStats.getInstance().getNumberOfFrames();
frames = new ArrayList(numberOfFrames);
for (int i = 0; i < numberOfFrames; i++) {
JPanel frameBox = new JPanel();
frameBox.setLayout(new FlowLayout(FlowLayout.CENTER));
JButton button = new JButton(String.valueOf(i+1));
button.setPreferredSize(new Dimension(2*width,2*height));
button.setBackground(Color.white);
button.setFocusable(false);
frameBox.add(button);
frames.add(frameBox);
}
innerPanel = new JPanel();
innerPanel.setLayout(new GridLayout(1,0,10,10));
for (JPanel button : frames) {
innerPanel.add(button);
}
scrollPane = new JScrollPane(innerPanel);
scrollPane.setBounds(10, 10, 1180, 145);
scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
highlightFrame(frames.get(0));
add(scrollPane);
}
public void nextFrame(){
if (indexOfChosenFrame == frames.size() - 1) {
unhighlightFrame(frames.get(indexOfChosenFrame));
indexOfChosenFrame = 0;
highlightFrame(frames.get(0));
}else{
unhighlightFrame(frames.get(indexOfChosenFrame));
indexOfChosenFrame++;
highlightFrame(frames.get(indexOfChosenFrame));
}
}
public void previousFrame(){
if (indexOfChosenFrame == 0) {
unhighlightFrame(frames.get(0));
indexOfChosenFrame = frames.size()-1;
highlightFrame(frames.get(indexOfChosenFrame));
}else{
unhighlightFrame(frames.get(indexOfChosenFrame));
indexOfChosenFrame--;
highlightFrame(frames.get(indexOfChosenFrame));
}
}
private void highlightFrame(JPanel frame){
Rectangle rect = frame.getBounds();
rect.setBounds(frame.getX()-550, frame.getY(), frame.getWidth()+1050, frame.getHeight());
innerPanel.scrollRectToVisible(rect);
frame.setBorder(BorderFactory.createLineBorder(Color.red,2));
}
private void unhighlightFrame(JPanel frame){
frame.setBorder(null);
}

The relevant method here is JComponent#scrollRectToVisible(Rectangle). It has to be called on the component that is in the viewport of the scroll pane. (In your case, this is the panel with the grid layout, which contains the other sub-panels).
The rectangle that is passed to this method can be the bounds of one sub-panel. In this case, the scoll pane will do the "minimum" scrolling that is necessary to make the given rectangle visible. If you want to make sure that the respective sub-panel is in the center, then you can increase the size of this rectangle - that is, you define a rectangle in a way that the desired sub-panel will be in the center.
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.Rectangle;
import java.util.ArrayList;
import java.util.List;
import javax.swing.BorderFactory;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSpinner;
import javax.swing.SpinnerNumberModel;
import javax.swing.SwingConstants;
import javax.swing.SwingUtilities;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
public class ScrollToVisible
{
public static void main(String[] args)
{
SwingUtilities.invokeLater(new Runnable()
{
#Override
public void run()
{
createAndShowGUI();
}
});
}
private static void createAndShowGUI()
{
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
int n = 20;
final JPanel panel = new JPanel(new GridLayout(1,0));
final List<JComponent> components = new ArrayList<JComponent>();
for (int i=0; i<n; i++)
{
JComponent component = new JLabel(String.valueOf(i), SwingConstants.CENTER);
component.setPreferredSize(new Dimension(100,100));
component.setBorder(BorderFactory.createLineBorder(Color.BLACK));
components.add(component);
panel.add(component);
}
final JScrollPane scrollPane = new JScrollPane(panel);
final JSpinner spinner = new JSpinner(new SpinnerNumberModel(0, 0, n-1, 1));
spinner.addChangeListener(new ChangeListener()
{
JComponent selectedComponent = components.get(0);
#Override
public void stateChanged(ChangeEvent e)
{
selectedComponent.setBorder(BorderFactory.createLineBorder(Color.BLACK));
int index = (Integer)spinner.getValue();
JComponent component = components.get(index);
Rectangle bounds = component.getBounds();
// This would make the component "just" visible:
//panel.scrollRectToVisible(bounds);
// This will center the component:
int cx = bounds.x + bounds.width / 2;
int w = scrollPane.getViewport().getWidth();
Rectangle r = new Rectangle(cx-w/2, bounds.y, w, bounds.height);
panel.scrollRectToVisible(r);
selectedComponent = component;
selectedComponent.setBorder(BorderFactory.createLineBorder(Color.RED));
}
});
f.getContentPane().setLayout(new BorderLayout());
f.getContentPane().add(scrollPane, BorderLayout.CENTER);
f.getContentPane().add(spinner, BorderLayout.NORTH);
f.setSize(800, 300);
f.setLocationRelativeTo(null);
f.setVisible(true);
}
}
EDIT: You should NOT use setLayout(null), and you should not do manual calls to setBounds, and you should rarely use setPreferredSize. And... when you post code that already is so close to a https://stackoverflow.com/help/mcve (or even was created from a runnable example of another post) then you should make it really runnable. It's annoying to re-insert the boilerplate code and waste some time with debugging until you realize that initialize() is not called at all...
However, change the code according to this:
private void highlightFrame(JPanel frame){
Rectangle rect = frame.getBounds();
int c = rect.x + rect.width / 2;
int w = scrollPane.getViewport().getWidth();
int x = c-w/2;
rect.setBounds(x, rect.y, w, rect.height);
innerPanel.scrollRectToVisible(rect);
frame.setBorder(BorderFactory.createLineBorder(Color.red,2));
}
private void unhighlightFrame(JPanel frame){
frame.setBorder(BorderFactory.createEmptyBorder(2, 2, 2, 2));
}
The most important thing is to make sure that the size of the components is correct, by setting an empty border with the same size as the "highlighting" border.

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);