Wrong packing of JFrame using JPanel in borderlayout

Question

Hello there. I am writing a simple application in Java that does some particle simulation on a bunch of sheep (don't ask). For this I want a window with a JPanel for graphics (which will be resizable with a simple combobox that contains some standard resolutions) and some other elements like buttons to start and pause the simulation etc.

My question: I'm using the JFrame.pack method to pack everything nicely together using a borderLayout. But for some reason the JPanel is packed wrong, it seems like the packing ignores it, so the window is resized to fit the size of only the two buttons that I have now. What am I doing wrong?

This is the code so far (bit of a newbie, so no comments on my dumbness if there is any ;)):

public class Window {
 public Sheepness sheepness;

 public ButtonPanel buttonPanel;
 public PaintPanel paintPanel;
 public JFrame frame;

 public Window(Sheepness sheepness, int width, int height) {
  this.sheepness = sheepness;

  frame = new JFrame("Sheepness simulation");
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  //frame.setSize(width, height);

  BorderLayout frameLayout = new BorderLayout();
  JPanel background = new JPanel(frameLayout);
  background.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));

  buttonPanel = new ButtonPanel(this);
  background.add(BorderLayout.SOUTH, buttonPanel.buttonBox);

  paintPanel = new PaintPanel(this);
  paintPanel.setSize(600, 600);
  background.add(BorderLayout.CENTER, paintPanel);

  frame.getContentPane().add(background);
  frame.pack();
  frame.setResizable(false);
  frame.setVisible(true);
 }
}

public class PaintPanel extends JPanel {
 public Window window;

 public PaintPanel(Window window) {
  this.window = window;
 }

 @Override
 public void paintComponent(Graphics g) {
  g.setColor(Color.blue);
  g.fillRect(0, 0, 300, 200);
 }
}

public class ButtonPanel {
 public Window window;
 public Box buttonBox;

 public JButton startButton;
 public JButton resetButton;

 public ButtonPanel(Window window) {
  this.window = window;

  buttonBox = new Box(BoxLayout.X_AXIS);

  startButton = new JButton("Start");
  startButton.addActionListener(new startButtonListener());
  buttonBox.add(startButton);

  resetButton = new JButton("Reset");
  resetButton.addActionListener(new resetButtonListener());
  buttonBox.add(resetButton);
 }
}

Answer 1

Try:

paintPanel.setPreferredSize(600, 600);

As Window.pack() sizes to the preferred sizes of its subcomponents and JPanel gets its preferred size from its child components (in your case there are none).