Exception in thread "main" java.awt.AWTError: BoxLayout can't be shared

Question

I'm getting this error on this code:

    super("Trace Masker");
    setLayout(new BoxLayout(getContentPane(), BoxLayout.PAGE_AXIS));

    label1 = new JLabel("Source directory:");
    label2 = new JLabel("Target directory:");
    label3 = new JLabel("Defect number:");
    label4 = new JLabel("Slice tokens:");
    label4.setToolTipText("Seperate multiple tokens with comma");

    txtSourceDirectory = new JTextField(30);
    txtTargetDirectory = new JTextField(30);
    txtDefectNumber = new JTextField(30);
    txtSliceTokens = new JTextField(30);

    btnBrowseSourceDirectory = new JButton("...");
    btnBrowseTargetDirectory = new JButton("...");
    btnStart = new JButton("Start");
    btnCancel = new JButton("Cancel");

    pnlLabels = new JPanel(new BoxLayout(pnlLabels, BoxLayout.PAGE_AXIS));
    pnlText = new JPanel(new BoxLayout(pnlText, BoxLayout.PAGE_AXIS));
    pnlBrowseButtons = new JPanel(new BoxLayout(pnlBrowseButtons, BoxLayout.PAGE_AXIS));
    pnlTop = new JPanel(new BoxLayout(pnlTop, BoxLayout.LINE_AXIS));
    pnlActionButtons = new JPanel(new FlowLayout(FlowLayout.RIGHT));

    pnlLabels.add(label1);
    pnlLabels.add(label2);
    pnlLabels.add(label3);
    pnlLabels.add(label4);

    pnlText.add(txtSourceDirectory);
    pnlText.add(txtTargetDirectory);
    pnlText.add(txtDefectNumber);
    pnlText.add(txtSliceTokens);

    pnlBrowseButtons.add(btnBrowseSourceDirectory);
    pnlBrowseButtons.add(btnBrowseTargetDirectory);

    pnlTop.add(pnlLabels);
    pnlTop.add(pnlText);
    pnlTop.add(pnlBrowseButtons);

    pnlActionButtons.add(btnStart);
    pnlActionButtons.add(btnCancel);

    add(pnlTop);
    add(pnlActionButtons);

The error is on this line:

pnlLabels.add(label1);

Just to check if this related specifically to pnlLabels, I commented all of it's lines. The error then happens on:

pnlText.add(txtSourceDirectory);

I've already checked the other 2 questions here about this and fixed the setLayout declaration for the JFrame: Question1 Question2

Answer 1

Your problem comes from the following line (and all other lines looking the same):

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

When new BoxLayout(...) is called, pnlLabels is still null since it is not assigned yet. The correct way to do it is in two steps:

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

The problem should disappear (provided you do that for all other code lines similar to that one).