JFrame中的背景图片

Question

This question has been asked a lot but everywhere the answers fall short. I can get a JFrame to display a background image just fine by extending JPanel and overriding paintComponent, like so:

class BackgroundPanel extends JPanel {
    private ImageIcon imageIcon;
    public BackgroundPanel() {
        this.imageIcon = Icons.getIcon("foo");
  }

    @Override
    protected void paintComponent(Graphics g) {
       super.paintComponent(g);
        g.drawImage(imageIcon.getImage(), 0,0,imageIcon.getIconWidth(),imageIcon.getIconHeight(),this);
    }
}

But now, how do you add a component on top of that background? When I go

JFrame w = new JFrame() ;
Container cp = w.getContentPane();
cp.setLayout(null);

BackgroundPanel bg = new BackgroundPanel();
cp.add(bg);

JPanel b = new JPanel();
b.setSize(new Dimension(30, 40));
b.setBackground(Color.red);
cp.add(b);
w.pack()
w.setVisible(true)

It shows the little red square (or any other component) and not the background, but when I remove cp.setLayout(null); , the background shows up but not my other component. I'm guessing this has something to do with the paintComponent not being called by the null LayoutManager, but I'm not at all familiar with how LayoutManagers work (this is a project for college and the assignment specifically says not to use a LayoutManager).

When i make the image the background has to display null (and so, transparant (??)) the red square shows up so it might be that the background is actually above my other components.

Does anyone anyone have any ideas?

Thanks

Answer 1

When using null layout (and you almost never should) you have to supply a bounds for every component, otherwise it defaults to (0 x,0 y,0 width,0 height) and the component won't display.

BackgroundPanel bg = new BackgroundPanel();
cp.add(bg);

isn't supplying a bounds. You'll need to do something like:

BackgroundPanel bg = new BackgroundPanel();
bg.setBounds(100, 100, 100, 100);
cp.add(bg);

Which would make bg size 100 x 100 and place it at 100 x, 100 y on the frame.

Answer 2

Look in the documentation on the Root Pane for all the information you need. Note the availability of the layered pane and the glass pane as well as the content pane.

Answer 3

By default all components have a 0 size. Just because you do some painting on a component doesn't give the component a size. You are still responsible for setting the size. That is why you should always use a layout manager. It looks after all this size stuff for you so you don't have to worry.

I don't know why newbies always think they can't use a layout manager. Yes it takes a couple of more minutes to learn, but it saves you a lot of grief in the long run.

Background Panel shows a couple of approaches. Again they both assume you use a layout manager, so you may need to set the size manually.