What the heck!? JPanel background problem - weird!

[Mike Lipay]

Ok, so I got the background image working on my first window, then I went to add it to a second window and the weirdest thing happened. Two different applications (one called by the other), two different images (different names), but both windows display the same image. Here's the code:

First application:

class BackgroundPanel extends JPanel
{
	Image image;
	public BackgroundPanel(int size[])
	{
		try
		{
			image = javax.imageio.ImageIO.read(new java.net.URL(getClass().getResource("jeep03.jpg"), "jeep03.jpg"));
			size[0] = image.getWidth(null);
			size[1] = image.getHeight(null);
		}
		catch (Exception e) { /*handled in paintComponent()*/ }
		return;
	}

	@Override
	protected void paintComponent(Graphics g)
	{
		super.paintComponent(g); 
		if (image != null)
			g.drawImage(image, 0,0,this.getWidth(),this.getHeight(),this);
	}
}

Second application, which is called by the first one:

class BackgroundPanel extends JPanel
{
	Image image;
	public BackgroundPanel(int size[])
	{
		try
		{
			image = javax.imageio.ImageIO.read(new java.net.URL(getClass().getResource("jeep02.jpg"), "jeep02.jpg"));
			size[0] = image.getWidth(null);
			size[1] = image.getHeight(null);
		}
		catch (Exception e) { /*handled in paintComponent()*/ }
		return;
	}

	@Override
	protected void paintComponent(Graphics g)
	{
		super.paintComponent(g); 
		if (image != null)
			g.drawImage(image, 0,0,this.getWidth(),this.getHeight(),this);
	}
}

jeep02.jpg and jeep03.jpg are different pictures, but both windows show jeep02.jpg. Here is how the first app calls the second (in case it matters):

	public void actionPerformed (ActionEvent event )
	{
		if( event.getSource() == addBtn)
		{
			AddVehicle.AddVehicle();
		}		
		
		if( event.getSource() == exitBtn)
		{
			System.exit(0);
		}	
	}

[Francois Nadeau]

Hi Mike,

I noticed that both of your classes are called BackgroundPanel (I assume that they are in different packages). I am curious to see what happen if you make their names unique (to make sure that you are not creating the same object twice). Or I suppose you could make sure that each BackgroundPanel is constructed at least once with a debugger.

Hope this helps.

[Mike Lipay]

That did it. I'm guessing that, although both panels were in different physical files java merges them at compile or execution time. Thanks!

[Rob Spoor]

It probably imported the same BackgroundPanel class in both cases. You thought that you were using package2.BackgroundPanel but in fact you were using package1.BackgroundPanel. Since both classes are actually similar you didn't notice.

And that brings me to another issue. The classes are too similar to warrant two classes. Why not use one class with all the major code:

class BackgroundPanel extends JPanel   
{   
    Image image;   

    public BackgroundPanel(String resource, int[] size)
    {
        this(getImage(resource), size);
    }

    public BackgroundPanel(Image image, int[] size)   
    {
        this.image = image;
        if (image != null)
        {
            size[0] = image.getWidth(this); // use this since JPanel implements ImageObserver
            size[1] = image.getHeight(this); // use this since JPanel implements ImageObserver
        }
    }   

    private static Image getImage(String resource)
    {
        URL url = getClass().getResource("jeep02.jpg"); // this will work just fine
        Image image = null;
        try
        {
            image = ImageIO.read(url);
        }
        catch (Exception e) { /*handled in paintComponent()*/ }   
        return image;
    }
  
    @Override   
    protected void paintComponent(Graphics g)   
    {   
        super.paintComponent(g);    
        if (image != null)   
            g.drawImage(image, 0,0,this.getWidth(),this.getHeight(),this);   
    }   
} 

And if you want to shield the resource name from the calling class, subclass the panel:

class Jeep02Panel extends BackgroundPanel
{
    Jeep02Panel(int[] size)
    {
        super("jeep02.jpg", size);
    }
}

As you see, you now only create such simple subclasses and leave the hard work to one single parent class.


Now, there is one more issue I have with this code. You use an int[] to store the size of the image. That leaves one potential danger (apart from it being null) - the size can be incorrect. I would use a java.awt.Dimension object instead:

    public BackgroundPanel(Image image, Dimension size)   
    {
        this.image = image;
        if (image != null && size != null)
        {
            size.width = image.getWidth(this); // use this since JPanel implements ImageObserver
            size.height = image.getHeight(this); // use this since JPanel implements ImageObserver
        }
    }   

Now you know that the object will always have space for the width and height, and it has easier ways to retrieve them as well - using the named fields instead of anonymous array indexes.