error upon assigning Layout: BoxLayout can't be shared

Question

I have this Java JFrame class, in which I want to use a boxlayout, but I get an error saying java.awt.AWTError: BoxLayout can't be shared. I've seen others with this problem, but they solved it by creating the boxlayout on the contentpane, but that is what I'm doing here. Here's my code:

class EditDialog extends JFrame {
    JTextField title = new JTextField();
    public editDialog() {
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        setTitle("New entity");
        getContentPane().setLayout(
            new BoxLayout(this, BoxLayout.PAGE_AXIS));
        add(title);
        pack();
        setVisible(true);
    }
}

Solution 1

Your problem is that you're creating a BoxLayout for a JFrame (this), but setting it as the layout for a JPanel (getContentPane()). Try:

getContentPane().setLayout(
    new BoxLayout(getContentPane(), BoxLayout.PAGE_AXIS)
);

Solution 2

I've also found this error making this:

JPanel panel = new JPanel(new BoxLayout(panel, BoxLayout.PAGE_AXIS));

The JPanel isn't initialized yet when passing it to the BoxLayout. So split this line like this:

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

This will work.

Solution 3

I think that one important thing to highlight from the previous answers is that the BoxLayout's target (the first parameter) should be the same Container that the setLayout method is being called upon as in the following example:

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

Solution 4

If you're using the layout on a JFrame like:

JFrame frame = new JFrame();
frame.setLayout(new BoxLayout(frame, BoxLayout.Y_AXIS));
frame.add(new JLabel("Hello World!"));

The control is actually being added to the ContentPane so it will look like it's 'shared' between the JFrame and the ContentPane

Do this instead:

JFrame frame = new JFrame();
frame.setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));
frame.add(new JLabel("Hello World!"));

View more solutions

Comments

[palantus]

Yes, but removing it would confuse the issue, now wouldn't it?

[Alexander McNulty]

Dang you saved me, - why is this the only answer that mentions getContentPane() ?

[alife]

@AlexanderMcNulty, probably because JFrames don't normally require it (unlike the AWT Frame). From the JFrame documentation: As a convenience, the add, remove, and setLayout methods of this class are overridden, so that they delegate calls to the corresponding methods of the ContentPane. For example, you can add a child component to a frame as follows: frame.add(child); And the child will be added to the contentPane. The content pane will always be non-null. By frame they're referring to a JFrame instance.

[alife]

@AlexanderMcNulty, further, there is only one content pane in a JFrame, and it is always guaranteed to be there.