• I'm creating an image viewer, and have extended JComponent for the viewing area.

    This component is placed in a JScrollPane. To make sure the scrollbars appear, I need to tell it that its contents are big...

    Image i = Toolkit.getDefaultToolkit().getImage(path);
    int height = i.getHeight(null);
    int width = i.getWidth(null);
    System.out.println(width + " " + height); //some debugging help...
    previewArea.setPreferredSize(new Dimension(width,height));

    My problem is that getWidth and getHeight nearly always return -1  (i.e. size not yet known). What should I give them as the "ImageObserver"  in order to make them wait?

  • just call setHorizontalScrollbarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS) on the scrollpane and the same for vertical scrollbar.

  • That makes it show scrollbars, but they don't work because the extended JComponent isn't making itself big enough to stretch the viewport...

  • What are you wanting?  Are you wanting the component to be larger or are you wanting the scroll bars to work?  If you are wanting the component to be larger I would suggest overriding the getWidth and getHeight methods of the component if they are not getting the correct width and height of the image. 



     I wrote an image manager once that loaded images into memory and this problem didn't occur for me.  I think I used ImageIcon and then if I needed the actual image I just called icon.getImage().   You might want to try that, but I don't remember if that's exactly how I did it and I don't have the code with me.

  • I have an extended Jcomponent in  a scrollpane.

     in the JComponent's paint method, I paint the current image onto the canvas, using AffineTransforms to allow rotation and zooming (rotation isn't supported by ImageIcon as far as I can see.

    I need to define the component's size for the scrollpane to think there's something that needs to be scrolled.


    I don't think it'll matter whether I override the standard JComponent's getWidth function to hardwire it to the Image.getWidth finction, or if i set the component's size normally, because I still don't know the image's size (it returns -1 becuase it doesn't know)



  • I've avoided the getWidth returning -1 problem by moving this section to inside the Paint method...

    <font face="Geneva,Arial,sans-serif" size="2">So my problem is now that
    neither using setPreferredSize(Dimension) nor setMinimumSize(Dimension)
    will make the JScrollPane realise that its contents are actually quite
    big... </font>

  • Try calling revalidate() on the image or its container after the image has been loaded or transformed.

  • My extended JComponent now implements scrollable, and works, except that if I stretch the window so it's bigger than the current image, it refuses to paint any future images; the dimensions and other data come up in the right places, but the image area remains grey.

  • In your paint method, call super(g);   (g being your Graphics object). Lemme know if that works.

  • edit: bah

  • D:\Java>javac *.java call to super must be first statement in constructor
    1 error

  • I think he meant super.paint(g)

  • The object you want to pass is previewArea:

    int height = i.getHeight(previewArea);
    int width = i.getWidth(previewArea);

    However, you will never be guaranteed that getWidth or getHeight will return a nonnegative value, because the image is being loaded in another thread.

    It's possible to use a MediaTracker to wait for the information to become available. However, if you're using Java 1.4 or later, there's a much easier solution:

    import javax.imageio.ImageIO;
    BufferedImage i = File(path));

    This call will not return until the image is fully loaded. And it returns a BufferedImage, so you can call the getWidth() and getHeight() methods which take no arguments.

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