Unwanted border around JPanel

Answer 1

I guess you got the label on the screenshots wrong.

If you specify

g.fillRect(0, 0, 310, 210);

in PaintPanel and

paintPanel.setPreferredSize(new Dimension(320, 240));

in Window. You get that what the first screenshot shows. Which makes sense as the rectangle has 10px less width, and 30px less height.

If you set the same (width/height; 310, 210) for both, the blue rectangle obviously "fills "the PaintPanel out.

Answer 2

Here's a self-contained test case, based on your code:

import java.awt.*;
import javax.swing.*;

public class Test {

 public PaintPanel paintPanel;
 public JFrame frame;

 public Test() {
  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));

  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 static class PaintPanel extends JPanel {
    public Test window;

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

   @Override
   public void paintComponent(Graphics g) {
    g.setColor(Color.blue);
    g.fillRect(0, 0, 320, 240);
   }
  }

  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        new Test();
      }
    });
  }
}

When I run it on an IcedTea JVM on Linux, I see this.

Your problem is either due to the button container forcing the window to be wider, or possibly due to a Swing bug in the version of Java you're using.

My recommendation is not to bother with the built-in layout classes and to use MigLayout instead.

Answer 3

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.

Answer 4

believe it or not, but try inverting the order of pack() and setResizeable()

    ...
    frame.setResizable(false);
    frame.pack();  // should be called after any changes
    frame.setVisible(true);

EDIT: checked using this

    frame.pack();
    frame.setResizable(false);
    System.out.println(paintPanel.getSize());
    frame.setVisible(true);
    System.out.println(paintPanel.getSize());

and

    frame.setResizable(false);
    frame.pack();
    System.out.println(paintPanel.getSize());
    frame.setVisible(true);
    System.out.println(paintPanel.getSize());

but if size isn't important, you can fill the actual area with

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

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