error upon assigning Layout: BoxLayout can't be shared

Question

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)
);

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.

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));

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!"));