Does JFrame not accurately display specified width and height values?

I'm building a simple 2D game in Java.

I'm using the JFrame class, but I don't think the width and height are what I specified, or perhaps the graphics are incorrect.

Here are some snippets of my code:

public final static int WIDTH = 600, HEIGHT = 900;

JFrame frame = new JFrame();
frame.setSize(WIDTH, HEIGHT);

g.setColor(Color.BLACK);
g.fillRect(0, 0, WIDTH, HEIGHT - 10);

The JFrame is displaying a black background. However, based on the arguments I gave to the fillRect function, there should still be a 10px tall sliver of white at the bottom of the frame. This is not the case. The white sliver only really starts to show after a 30px decrease from the height of the frame.

Thanks for your help.

The JFrame size includes the borders so you need to allow for them. To facilitate dealing with this don't specify the width and height of the JFrame. I recommend doing the following.

JFrame frame = new JFrame();
JPanel panel = new JPanel();
panel.setPreferredSize(new Dimension(width,height));
frame.add(panel);
// add other components in the panel
frame.pack();
// center on screen.
frame.setLocationRelativeTo(null);
frame.setVisible(true);

Now your panel will be the specified size.

Note, if your going to paint, make certain you override paintComponent(Graphics g) in JPanel and do your painting there.

@Override
public void paintComponent(Graphics g) {
   super.paintComponent(g);
   // your code here
}

</details>