java - JDialog doesn't respect dimensions of child components

Question

I have a setup that usually works, but I'm finding it a bit of a pain at the moment.

I have a JDialog (formerly a JFrame, but I've changed the UI flow recently to remove redundant JFrames) set to BorderLayout that contains three main JPanels, header, content and footer.

The header JPanel has a background image that is loading fine. However, I'm calling all manner of setMinimumSize / setPreferredSize / etc (I even tried setSize and setBounds out of desperation) and the JPanel isn't being sized correctly. The layout for that component is BoxLayout, and the children sit comfortably within it, but that shouldn't affect what I'm trying to do with the header panel itself.

The only thing that works is setting a size on the parent JDialog. But I don't want to do that / from what I've read it seems like bad design. I'd also have to maths out the potential width / height of all the children, add the margins, etc.

Advice I am not looking for: "use a different LayoutManager."

I want to understand if there's a reason for the child components not being respected.

I can provide code, but isolating a small, runnable segment is difficult given the amount of interlocked systems I'm juggling. Here is the relevant snippet to set the size of the JPanel. The image dimensions are 480 * 96.

public JPanelThemed(BufferedImage image) {
    super();
    this.backgroundImage = image;
    setAllSimilarConstraints(new Dimension(image.getWidth(), image.getHeight()));
    setOpaque(false);
}

// for use with BoxLayout as it requires explicit bounds to be set
public void setAllSimilarConstraints(Dimension dim) {
    setPreferredSize(dim);
    setMinimumSize(dim);
    setMaximumSize(dim);
}

Answer 1

I would implement it a bit another way - to ensure that preferred/min/max/Size can not be changed from elsewhere:

public JPanelThemed(BufferedImage image) {
    this.backgroundImage = image;
    setOpaque(false);
}

public Dimension getPreferredSize() {
    return new Dimension(this.backgroundImage.getWidth(), this.backgroundImage.getHeight());
}