Why is object indended using Swing BoxLayout

Question

Can someone explain why the text "Options" is indented here? This looks like a bug to me in BoxLayout. TIA

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

public class BoxLayoutIssue {

    public static void main(String[] args) {

        JFrame f = new JFrame();
        f.setSize(240, 250);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));

        JLabel label = new JLabel("Option");
        panel.add(label);

        JLabel a = new JLabel("A");
        JLabel b = new JLabel("B");
        JLabel c = new JLabel("C");
        JLabel d = new JLabel("D");
        JLabel e = new JLabel("E");

        Box box = Box.createVerticalBox();
        box.add(a);
        box.add(b);
        box.add(c);
        box.add(d);
        box.add(e);

        JScrollPane jscrlpBox = new JScrollPane(box);
        jscrlpBox.setPreferredSize(new Dimension(140, 90));
        panel.add(jscrlpBox);

        f.add(panel);

        f.setVisible(true);
    }
}

Answer 1

Read the section from the Swing tutorial on Using a Box Layout. There are some examples there that try to explain how the "X alignment" of each component can affect the layout.

If you describe what you are trying to do, or what you expect to happen, maybe we can suggest a different combination of layout managers to use.

Answer 2

The issue here is that the JLabel is lining up its right edge with the center of the panel, while the scrollpane is lining up its center with the center of the panel.

I was able to fix this by adding two lines, setting the horizontal alignment of both label and jscrlpBox to Component.LEFT_ALIGNMENT:

JLabel label = new JLabel("Option");
label.setAlignmentX(Component.LEFT_ALIGNMENT); // Added

panel.add(label);

JLabel a = new JLabel("A");
JLabel b = new JLabel("B");
JLabel c = new JLabel("C");
JLabel d = new JLabel("D");
JLabel e = new JLabel("E");

Box box = Box.createVerticalBox();
box.add(a);
box.add(b);
box.add(c);
box.add(d);
box.add(e);

JScrollPane jscrlpBox = new JScrollPane(box);
jscrlpBox.setPreferredSize(new Dimension(140, 90));
panel.add(jscrlpBox);
jscrlpBox.setAlignmentX(Component.LEFT_ALIGNMENT); // Added

In general, when troubleshooting something like this, I try adding a brightly colored line border to the components (in this case, label) that aren't where I want them to be. That's when I realized that it was lining up its right edge with the same line that the other component was using for lining up its center.