Item inside BoxLayout panel not occupying width of JPanel

Below panel has BoxLayout and the button's at the bottom are not occupying width of the panel and instead there is gap on the left and right side.

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

    JPanel framesPanel[] = new JPanel[8];
    
    for(int i=0;i<8;i++) {
        framesPanel[i] = new JPanel();
        framesPanel[i].setLayout(new BoxLayout(framesPanel[i], BoxLayout.X_AXIS));

        JButton jb1 = new JButton("Button 1");
        jb1.setAlignmentX(Component.LEFT_ALIGNMENT);
        JButton jb2 = new JButton("Button 2");
        jb2.setAlignmentX(Component.LEFT_ALIGNMENT);

        JButton jb3 = null;

        framesPanel[i].add(jb1);
        framesPanel[i].add(jb2);

        if (i < 6) {
            jb3 = new JButton("Button 3");
            jb3.setAlignmentX(Component.LEFT_ALIGNMENT);
            framesPanel[i].add(jb3);
        }
        panel.add(framesPanel[i]);
    }


    // Set size for the frame
    panel.setSize(300, 300);
    frame.setSize(300, 300);

    // Set the window to be visible as the default to be false
    frame.add(panel);

I have tried with setAlignmentX(Component.LEFT_ALIGNMENT) but this doesn't seem to have any effect.

A BoxLayout respects the maximum size of a component.

The buttons maximum size is the same as its preferred size.

When you create the buttons you could do something like:

JButton button = new JButton(...);
Dimension maximum = button.getMaximumSize();
maximum.width = Integer.MAX_VALUE;
button.setMaximumSize( maximum );

Note:

This is only a quick fix that will work if you never change the Font size of the button. For a proper solution you should really creat a custom button that extends JButton and override the getMaximumSize() method to return the above size. Then this will work if the properties of the button are changed dynamically.