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javaswingboxlayout

How can get components to stretch across an entire row using BoxLayout?

I'm looking at the How To Use BoxLayout documentation, which clearly says that

What if none of the components has a maximum width? In this case, if all the components have identical X alignment, then all components are made as wide as their container.

Let's assume we're adding a lot of JButton instances to a JPanel. If the maximum width of these buttons are none AND we invoke setAlignmentX(Component.LEFT_ALIGNMENT) on all of these buttons - then each of these buttons should stretch across its entire row. The documentation even illustrates this using the below picture.

enter image description here

I can't get this to work!

I've tried doing setMaximumSize(null) and setMaximumSize(new Dimension(-1,-1)) and setMaximumSize(new Dimension(0,0)) on the buttons but nothing gives me the described behaviour.

What excactly does the documentation mean when it says :

What if none of the components has a maximum width?

What is a maximum width of none?

The best I've been able to produce is the below. Reading the documentation I would expect that the buttons should be able to stretch across their entire rows. I know I can use other layout managers as well for this, but I would like to achieve this with BoxLayout (granted the documentation is right / I've understood the documentation right).

enter image description here

public class CustomList extends JPanel {

    private final Box box = Box.createVerticalBox();

    public CustomList() {
        for (int i = 0; i < 10; i++) {
            JButton b = new JButton("Button item" + i);
            //b.setMaximumSize(new Dimension(0,0));
            b.setAlignmentX(Component.LEFT_ALIGNMENT);
            box.add(b);
        }
        setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
        add(box, BorderLayout.CENTER);
    }

    public static void main(String[] args) {
        CustomList l = new CustomList();
        l.setSize(200, 200);
        l.setBackground(Color.red);

        JFrame frame = new JFrame("Vertical Box");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(l, BorderLayout.CENTER);
        frame.setSize(300, 200);
        frame.setVisible(true);
    }

}
Solution

  • Your buttons actually have a maximum width.

    What you can do is create JPanel objects with BorderLayout in your loop, add each button to each panel (to BorderLayout.CENTER, which is the default anyway).

    BorderLayout.CENTER doesn't care about the maximum size of its child Component, so you end up with a JPanel whose whole content is filled by a JButton.

    Since the JPanel itself has a huge default maximum size of new Dimension(Short.MAX_VALUE, Short.MAX_VALUE) (this is width=32767,height=32767 !!) which is the default maximum size of Component, you will get the expected result :

    public CustomList() {
    for (int i = 0; i < 10; i++) {

        JPanel panel = new JPanel(new BorderLayout());
        JButton b = new JButton("Button item" + i);
        //b.setMaximumSize(new Dimension(0,0));
        b.setAlignmentX(Component.LEFT_ALIGNMENT);
        panel.add(b);
        box.add(panel);
    }
    setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
    add(box, BorderLayout.CENTER);
}