Is this Swing behavior a bug? Mysterious BoxLayout alignment

Question

I've been playing around with WindowBuilder Pro for Java Swing layouts, and encountered some strange behavior that I was eventually able to reduce down to a SSCCE. Consider the source code below:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FlowLayout;

import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class WBPTest2 {
    private JFrame frame;
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    WBPTest2 window = new WBPTest2();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
    public WBPTest2() {
        initialize();
    }
    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setBackground(Color.RED);
        frame.getContentPane().add(panel, BorderLayout.CENTER);
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));

        JPanel panel_1 = new JPanel();
        panel_1.setBackground(Color.GREEN);
        panel_1.setMaximumSize(new Dimension(32767, 100));
        panel.add(panel_1);
        panel_1.setLayout(new BoxLayout(panel_1, BoxLayout.Y_AXIS));

        JPanel panel_2 = new JPanel();
        panel_2.setBackground(Color.BLUE);
        panel_2.setMaximumSize(new Dimension(32767, 100));
        panel.add(panel_2);
        panel_2.setLayout(new FlowLayout(FlowLayout.CENTER, 5, 5));
    }
}

It gives the following layout:

For whatever reason, panel_1, the green one, floats to the right. Playing with some of the parameters in the code, I can only find two ways to completely kill the gap to the left of the green panel:

• change the hgap and vgap in the panel_2 constructor to 0.

OR

• change the minimumSize of panel_2 to (0,0).

Do either of those, and we get this:

Now, the question is, why on earth would either the minimumSize or the hgap for the FlowLayout in panel_2 have anything to do with preventing panel_1 from filling all the way to the left of its parent? What if I happened to want a minimumSize greater than zero, and an hgap greater than zero, for my FlowLayout in panel_2? How then would I eliminate the gap to the left of my BoxLayout in panel_1?

No matter what I do to panel_1, I cannot make it fill the whole width of its parent container (unless I edit panel_2 as described above). Ironically, if I set the alignmentX for panel_1 to RIGHT_ALIGNMENT, it actually floats to the left. But the gap still exists (just now on the right). As I fill panel_1 with content and increase its size, the gap to its right grows smaller and smaller, but never completely goes away, frustratingly.

This doesn't just happen when panel_2 is a FlowLayout... it also happens with JScrollPane and many other types... types which don't usually have the hgap parameter, which means the only way to fix the gap to the left of panel_1 is to change the new container's minimumSize to 0, which again seems silly and unrelated, and most importantly may not be my design intent.

Answer 1

Set alignmentX on panels 1 and 2 and it fills the screen.

It appears the two different layout managers cause the panel to set different alignment defaults.