Question

So I’d like to make a Canvas based Java application. I’ve extended my main class to Canvas and I size it in it’s constructor.

public CanvasApp() {
    Dimension size = new Dimension(640, 480);
    setSize(size);
    setPreferredSize(size);
    setMinimumSize(size);
    setMaximumSize(size);
}

and in the main function, I make a frame for it, like this:

CanvasApp cnv = new CanvasApp();
JFrame frame = new JFrame("");
JPanel panel = new JPanel(new BorderLayout());
panel.setBorder(null); //I've tried this
panel.setSize(640,480); //but still doesn't work =(
panel.add(cnv, BorderLayout.CENTER);

frame.setContentPane(panel);
frame.pack();
frame.setResizable(false);
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);

But the content pane appears 650×490 in size. Why is this?

I’ve attached a picture too.

I’ve drew a line from 0,0 to 640,480

Answer 1

Because a JFrame has a 5px border around it, look at any frame on your computer screen you’ll notice a kind of embossed border – that border is 5 px in width adding 10 pixels onto height and width. You’ve only assigned the panel to 640 x 480 and plonked it inside the frame – the frame then adds it’s own border onto that.