Filling out available space in JScrollPane

Question

Hello all,
Is there a way to inform a JScrollPane that it's viewport's component should fill out the entire area of the viewport? I have a scroll pane whose viewport is a JPanel with a BoxLayout. The contents of the JPanel will change size depending on certain events. However, the JPanel at first is not very wide (it's components have a preferred size of 100), so it does not fill out the width of the viewport and is also displayed in the center of the scroll pane. Is there a way to have the JPanel fill out the entire viewport, or at least have it displayed on the left (I have set both the JViewport and JPanel's alignmentX values to 0.0 but it's still in the center)?
Thank you all for your time.

Answer 1

It looks like you have an intermediate panel between the viewport and the JPanel with the BoxLayout. In other words, the view is not the BoxLayout JPanel, but the intermediate one. A newly constructed JPanel will have FlowLayout and position the component top centered as displayed in your picture.
I wasn't lying when I said the JPanel should be the same size as the viewport. That's the default behavior. Example,
import javax.swing.*;
public class Demo {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
    private static void createAndShowGUI() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        String lines = "------------\n" +
                       "------------\n" +
                       "------------\n" +
                       "------------\n" +
                       "------------\n";
        JFrame frame = new JFrame();
        //Box musicPanel = Box.createVerticalBox();
        JPanel musicPanel = new JPanel();
        musicPanel.setBorder(BorderFactory.createEtchedBorder());
        musicPanel.setLayout(new BoxLayout(musicPanel,BoxLayout.PAGE_AXIS));
        Box tempo = Box.createHorizontalBox();
        tempo.add(new JLabel("Temp = 120"));
        tempo.add(new JButton("Change Tempo"));
        musicPanel.add(tempo);
        musicPanel.add(new JLabel("Tack 0: tuba"));
        musicPanel.add(new JTextArea(lines));
        musicPanel.add(new JLabel("Track 1: trombone"));
        musicPanel.add(new JTextArea(lines));
        musicPanel.add(new JLabel("Track 2: C_trumpet"));
        musicPanel.add(new JTextArea(lines));
        musicPanel.add(new JLabel("Track3: English_horn"));
        leftAlign(musicPanel);
        frame.setContentPane(new JScrollPane(musicPanel));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    private static void leftAlign(JComponent parent) {
        parent.setAlignmentX(JComponent.LEFT_ALIGNMENT);
        for(java.awt.Component c : parent.getComponents()) {
            leftAlign((JComponent) c);
} If you change
frame.setContentPane(new JScrollPane(musicPanel)); to
JPanel intermediate = new JPanel();
intermediate.add(musicPanel);
frame.setContentPane(new JScrollPane(intermediate)); in the above code, then you'll get something that looks remarkably like the picture you provided.