Java Bug? Centering a JLable with just an icon in a JScrollPane

Question

Hi,
I'm attempting to write a image viewing program. I would like the image centered on the window displaying them, but because some images are larger than the window, I would like the image to be in the upper left hand coner of the window with scroll bars to allow you to view the rest of the image.
I have attempted to implement this by placing a JScrollPane on a JFrame, and then placing a JLabel in the JScrollPane's Viewport. I set the Label's text to blank, and then set the image, wraped in an InageIcon, as the JLabel's Icon.
To center the JLabel, I have tried to set the JScrollPane's Viewport's LayoutManager to two different layout managers, but I've found buggy operation with each. The Layout I've tried are the following:
1. a GridLayout(1,1,0,0); - this works fine when the image is smaller that the available area of the JScrollPane. However, if the window is resized so that the image more than totally fills the JScrollPane, the image gets cut off. What happens is that the image is centered in the available scrollpane view, meaning the uper and left edges of the image get cut off. Scroll bars appear to let you view the lower and righer edges of the image, however, when you scroll, the newly uncovered areas of the image never get drawn, so you can't get to see the lower or right edges of the image either.
2. a GridbagLayout with one row, and one column each with a weight of 100, centered and told to fill both horizontally and vertically. Again, this correctly centers the JLabel when th eimage is smaler than the available space in the JScrollPane. Once the window is resized so that the image is bigger than the space to view the image in, strange problems occur. This time it seems that the JScrollPane's scrollbars show that there should be enough room to show the image if the image was in the upper left corner, however, the image isn't in the upper left corner, it is offset down and to the right if the upper left corner by an amount that appears equal to the amount of extra space needed to display the image. For example, if the image is 200x300 and the view area is 150x200, ite image is offest by 50 horizontally and 100 vertically. This results in the bottom and right edges of the image getting cut off because the scrollbars only scroll far enough to show the image if the image was placed at 0,0 and not at the offset location that it is placed at.
You may not understand what I'm trying to say from my description, but below is code that can reproduce the problem.
Questions. Am I doing anything wrong that is causing this problem? Is that another, better way to center the JLabel that doesn't have these problems? Is this a JAVA Bug in either the layoutmanagers, JScrollPane or JLabel when it only contains an icon and no text?
Code:
this is the class that creates and lays out the JFrame. It currently generates a 256x256 pixel image. By changing the imagesize variable to 512, you can get a larger image to work with which will make the bug more obvious.
When first run the application will show what happens when using the GridLayout and described in 1. There are two commented out lines just below the GridLayout code that use a GridbagLayout as descriped in method 2. Comment out the GridLayout code when you uncomment the GridBageLayout code.
package centertest;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.image.*;
public class Frame1 extends JFrame {
    JPanel contentPane;
    BorderLayout borderLayout1 = new BorderLayout();
    JScrollPane jScrollPane1 = new JScrollPane();
    JLabel jLabel1 = new JLabel();
    static final int imagesize=256;
    //Construct the frame
    public Frame1() {
        enableEvents(AWTEvent.WINDOW_EVENT_MASK);
        try {
            jbInit();
        catch(Exception e) {
            e.printStackTrace();
    //Component initialization
    private void jbInit() throws Exception  {
        contentPane = (JPanel) this.getContentPane();
        contentPane.setLayout(borderLayout1);
        this.setSize(new Dimension(400, 300));
        this.setTitle("Frame Title");
        //setup label
        jLabel1.setIconTextGap(0);
        jLabel1.setHorizontalAlignment(JLabel.CENTER);
        jLabel1.setVerticalAlignment(JLabel.CENTER);
        jLabel1.setVerticalTextPosition(JLabel.BOTTOM);
        //create image and add it to the label as an icon
        int[] pixels = new int[imagesize*imagesize];
        for(int i=0;i<pixels.length;i++){
            pixels=i|(0xff<<24);
MemoryImageSource mis = new MemoryImageSource(imagesize, imagesize,
pixels, 0, imagesize);
Image img = createImage(mis);
jLabel1.setIcon(new ImageIcon(img));
jLabel1.setText("");
contentPane.add(jScrollPane1, BorderLayout.CENTER);
//center image using a GridLayout
jScrollPane1.getViewport().setLayout(new GridLayout(1,1,0,0));
jScrollPane1.getViewport().add(jLabel1);
//Center the image using a GridBagLayout
/*jScrollPane1.getViewport().setLayout(new GridBagLayout());
jScrollPane1.getViewport().add(jLabel1,
new GridBagConstraints(0, 0, 1, 1, 100.0, 100.0,
GridBagConstraints.CENTER, GridBagConstraints.BOTH,
new Insets(0, 0, 0, 0), 0, 0));
//Overridden so we can exit when window is closed
protected void processWindowEvent(WindowEvent e) {
super.processWindowEvent(e);
if (e.getID() == WindowEvent.WINDOW_CLOSING) {
System.exit(0);
and here is an application with which to launch the frame
package centertest;
import javax.swing.UIManager;
import java.awt.*;
public class Application1 {
    boolean packFrame = false;
    //Construct the application
    public Application1() {
        Frame1 frame = new Frame1();
        //Validate frames that have preset sizes
        //Pack frames that have useful preferred size info, e.g. from their layout
        if (packFrame) {
            frame.pack();
        else {
            frame.validate();
        //Center the window
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        Dimension frameSize = frame.getSize();
        if (frameSize.height > screenSize.height) {
            frameSize.height = screenSize.height;
        if (frameSize.width > screenSize.width) {
            frameSize.width = screenSize.width;
        frame.setLocation( (screenSize.width - frameSize.width) / 2,
                          (screenSize.height - frameSize.height) / 2);
        frame.setVisible(true);
    //Main method
    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        catch (Exception e) {
            e.printStackTrace();
        new Application1();
}I'm running Java 1.4.1 build 21 on Windows 98. I used JBuilder8 Personal to write this code.

Answer 1

Hmmm,
You are correct. Not setting a layout for the scrollpane's viewport (using the default of javax.swing.ViewportLayout) results in the positioning of the image tat I want (center the image if image is smaller than the scrollpane, if the image is larger, place it in the upper-left corener and enable the scrollbars to scroll to see the rest of the image).
Anyone have any idea why the GridLayout and GridBagLayout act as they do? I gues it isn't that important tomy program, but I'm still not sure that they are operating correctly. (Specifically, GridLayout sha scrollbars, but when you scroll, no new parts of the image are revealed.)