Weird JPanel Resizing Problem

Question

This program works as I want it to with one exception: sometimes when resizing the frame, the contained JPanels do not fully cover the frame's BorderLayout center region. If you run the program you'll discover that a light gray area often appears on the right and bottom edges of the frame when it's resized. Any ideas? Thanks. I'm using version 1.3.1._01 on Windows 98.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Grid extends JPanel implements ComponentListener
private int rows, columns;
private Block[][] block;
public Grid()
rows = 20;
columns = 10;
block = new Block[rows][columns];
setLayout(new GridLayout(rows,columns,0,0));
for (int i = 0; i < rows; i++)
for (int j = 0; j < columns; j++)
block[j] = new Block();
if ((i % 4) >= 1)
block[j].turnOn();
else
block[j].turnOff();
add(block[j]);
}//end inner for loop
}//end outer for loop
setBorder(BorderFactory.createLineBorder(Color.black,4));
addComponentListener(this);
}//end default constructor
public Dimension getPreferredSize()
int horizontalInsets = getInsets().left + getInsets().right;
int verticalInsets = getInsets().top + getInsets().bottom;
return new Dimension((block[0][0].getBlockWidth() * columns) + horizontalInsets,
(block[0][0].getBlockHeight() * rows) + verticalInsets);
}//end getPreferredSize
public void componentHidden(ComponentEvent ce)
//Invoked when the component has been made invisible.
public void componentMoved(ComponentEvent ce)
//Invoked when the component's position changes.
public void componentResized(ComponentEvent ce)
//Invoked when the component's size changes.
int horizontalInsets = getInsets().left + getInsets().right;
int verticalInsets = getInsets().top + getInsets().bottom;
setPreferredSize(new Dimension(getWidth() + horizontalInsets,getHeight() + verticalInsets));
setMinimumSize(new Dimension(getWidth() + horizontalInsets,getHeight() + verticalInsets));
public void componentShown(ComponentEvent ce)
//Invoked when the component has been made visible.
public static void main(String[] args)
JFrame frame = new JFrame();
frame.getContentPane().add(new Grid(),BorderLayout.CENTER);
frame.pack();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Block extends JPanel implements ComponentListener
private int blockWidth, blockHeight;
private Color blockBackgroundColor, blockBorderColor, onColor;
private String blockOwner;
private boolean isOn;
public Block()
blockWidth = 20;
blockHeight = 20;
//setSize(20,20);
//setPreferredSize(new Dimension(20,20));
blockBackgroundColor = Color.gray;
blockBorderColor = Color.black;
setLayout(new BorderLayout(0,0));
setBorder(BorderFactory.createLineBorder(blockBorderColor,1));
setBackground(blockBackgroundColor);
isOn = false;
blockOwner = "1";
onColor = Color.orange;
addComponentListener(this);
setVisible(true);
}//end default constructor
public Block(int blockWidth, int blockHeight)
this();
this.blockWidth = blockWidth;
this.blockHeight = blockHeight;
}//end two-arg constructor
public void setBlockWidth(int blockWidth)
this.blockWidth = blockWidth;
public int getBlockWidth()
return blockWidth;
public void setBlockHeight(int blockHeight)
this.blockHeight = blockHeight;
public int getBlockHeight()
return blockHeight;
public void setBlockBorderColor(Color blockBorderColor)
this.blockBorderColor = blockBorderColor;
public Color getBlockBorderColor()
return blockBorderColor;
public void setBlockBackgroundColor(Color blockBackgroundColor)
this.blockBackgroundColor = blockBackgroundColor;
public Color getBlockBackgroundColor()
return blockBackgroundColor;
public void setOnColor(Color onColor)
this.onColor = onColor;
public Color getOnColor()
return onColor;
public void turnOn()
setBackground(onColor);
isOn = true;
public void turnOff()
setBackground(blockBackgroundColor);
isOn = false;
public void setBlockOwner(String blockOwner)
this.blockOwner = blockOwner;
public String getBlockOwner()
return blockOwner;
public Dimension getPreferredSize()
int horizontalInsets = getInsets().left + getInsets().right;
int verticalInsets = getInsets().top + getInsets().bottom;
return new Dimension(blockWidth + horizontalInsets,
blockHeight + verticalInsets);
}//end getPreferredSize
public void paintComponent(Graphics g)
super.paintComponent(g);
if (isOn)
FontMetrics fm = g.getFontMetrics();
int blockOwnerWidth = fm.stringWidth(blockOwner);
int blockOwnerAscent = fm.getAscent();
int x = (blockWidth / 2) - (blockOwnerWidth / 2);
int y = (blockHeight / 2) + (blockOwnerAscent / 2);
g.drawString(blockOwner,x,y);
}//end paintComponent
public void componentHidden(ComponentEvent ce)
//Invoked when the component has been made invisible.
public void componentMoved(ComponentEvent ce)
//Invoked when the component's position changes.
public void componentResized(ComponentEvent ce)
//Invoked when the component's size changes.
int horizontalInsets = getInsets().left + getInsets().right;
int verticalInsets = getInsets().top + getInsets().bottom;
blockWidth = this.getWidth();
blockHeight = this.getHeight();
setPreferredSize(new Dimension(blockWidth + horizontalInsets,blockHeight + verticalInsets));
setMinimumSize(new Dimension(blockWidth + horizontalInsets,blockHeight + verticalInsets));
public void componentShown(ComponentEvent ce)
//Invoked when the component has been made visible.

Answer 1

Your problem is due to the use of a GridLayout.
With a GridLayout, all components must have the same dimension. And the extra space has to be divisible by the number of components to be distributed to each component :
For example : In a component with a GridLayout, you have a line with 10 JPanels. The width of the component is initially 100. In this case, each JPanel will have a width of 10 (10*10=100).
If you resize the main component to a width of 110, each JPanel will have a size of 11 (11*10=110).
But in the case where the width is not divisible by the number of JPanels, there will be an extra-space :
if the width of the component is 109, the width of each JPanel will be 10 and there will be an extra-space of 9 (10*10+9 = 109).
I hope this helps,
Denis