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Editorial Team
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Asked: 2026-05-12T16:59:31+00:00

I’m creating a form with a JPanel inside it for some graphics and some

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I’m creating a form with a JPanel inside it for some graphics and some buttons for controlling the thing. For some reason I have to specify the JPanel to be 10 px less wide and 30 px less high than the actual graphics I want to put inside it. What causes this problem?

This is the code:

public class Window {
 public Sheepness sheepness;

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

 public Window(Sheepness sheepness) {
  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(5, 5, 5, 5));

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

  paintPanel = new PaintPanel(this);
  paintPanel.setPreferredSize(new Dimension(320, 240));
  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, 320, 240);
 }
}

Screenshot with a preferredSize of 320 x 240:

Can't find source image http://www.cmbi.ru.nl/~estens/Sheepness/Sheepness_simulation_border.png.

You can see the 320 x 240 fillRect doesn’t fill the JPanel entirely, a border of 10 px width and 30 px height remains.

Screenshot with a preferredSize of 310 x 210:

Can't find source image http://www.cmbi.ru.nl/~estens/Sheepness/Sheepness_simulation_noBorder.png.

Now it fits the 320 x 240 fillRect exactly!

Any ideas?

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1 Answer

  1. Editorial Team

    Different layout managers use different rules for computing actual managed controls sizes, i.e. you can’t expect that the panel has particular size only if you call ‘setPreferredSize()’ on it.

    Feel free to check javadoc for all target layout managers for more details about used algorithm in every particular case.

    Also note that you can avoid using layout managers and define all sizes absolutely via ‘setBounds()’ method.

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