[Java] Boxlayout can't be shared error

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

Answer 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)
);

Answer 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.