Why does getBounds return 0?

[SammyB]

Our homework called for us to move a gif around in a canvas in an applet. Since it is an intro course, they used constants but I'm thinking, what if the user resizes the applet? So, I wrote the following code:

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
 
public class MoveIt extends Applet implements ActionListener
{
   private Image cupImage;
   private int iTop = 15;
   private int iLeft = 15;
   private Canvas drawingCanvas = new Canvas();
   public void init()
   {
      cupImage = getImage(getDocumentBase(), "cup.gif");
      this.setLayout(new BorderLayout());
      this.setSize(300, 300);
      this.setBackground(Color.BLUE);
      drawingCanvas.setBackground(Color.BLUE);
      this.add(drawingCanvas, BorderLayout.NORTH);
      Panel keyPanel = new Panel();
      keyPanel.setLayout(new BorderLayout());
      Button upButton = new Button("Up");
      keyPanel.add(upButton,BorderLayout.NORTH);
      upButton.addActionListener(this);
      Button centerButton = new Button("Center");
      keyPanel.add(centerButton,BorderLayout.CENTER);
      centerButton.addActionListener(this);
      Button downButton = new Button("Down");
      keyPanel.add(downButton,BorderLayout.SOUTH);
      downButton.addActionListener(this);
      Button leftButton = new Button("Left");
      keyPanel.add(leftButton,BorderLayout.WEST);
      leftButton.addActionListener(this);
      Button rightButton = new Button("Right");
      keyPanel.add(rightButton,BorderLayout.EAST);
      rightButton.addActionListener(this);
      this.add(keyPanel, BorderLayout.SOUTH);
   }
   public void paint(Graphics g)
   {
      g.drawImage(cupImage, iLeft, iTop, this);
   }
   public void actionPerformed(ActionEvent e)
   {
      String sItem = e.getActionCommand();
      char cFirst = sItem.charAt(0);
      switch (cFirst)
      {
      case 'C':
         Rectangle rCanvas = drawingCanvas.getBounds();
         int iImageWidth = cupImage.getWidth(this);
         int iImageHeight = cupImage.getHeight(this);
         iLeft = (rCanvas.width - iImageWidth)/2;
         iTop = (rCanvas.height - iImageHeight)/2;
         break;
      case 'U':
         iTop -= 15;
         break;
      case 'D':
         iTop += 15;
         break;
      case 'L':
         iLeft -= 15;
         break;
      case 'R':
         iLeft += 15;
         break;
      }
      repaint();
   }
}
 

My problem is in the Case 'C': rCanvas.height is zero! Why is that? How should I get the height & width of the Canvas. Also, I would like for the gif to be centered initially, but I don't know how to get its size without displaying it. Note, just use any small gif for cup.gif; my gif is 69x85. TIA. --Sam

[Ganon11]

I haven't done very much work at all with Java graphics...but it looks like drawingCanvas never actually gets a size. Your class gets a size (when you call this.setSize(blah blah blah)), but I'm not sure if that implicitly sets drawingCanvas' size as well. To check that, throw a System.out.println() line there, if you can, with drawingCanvas' height and/or width, just to check that it is being initialized properly.

As far as centering the image, your code for getting the center point looks good. So now you have the center point, and you have the dimensions of the image (by using it's getWidth/getHeight/getBounds method) - it should be a simple task to get the top left corner point of the image.

I'm not sure why you can't get the image's dimensions before displaying it - once you get the image in init(), you should be able to access the dimensions and place it as expected.

[JosAH]

As far as I can see you put a keyPanel in the SOUTH part of the Applet and a
Canvas in the NORTH part, but there's nothing in the Canvas itself, so the height
can be zero while the width will be equal to the width of the keyPanel. If you
increase the height of the entire Applet the keyPanel will 'eat up' that height.

Check the setMinimumSize and the setPreferedSize methods for the Canvas.

kind regards,

Jos

[SammyB]

Or, just delete the Canvas! :oD>>>
The buttons don't seem to resize their height, so everthing works fine without the Canvas. The only unanswerable question is why the book said to put it in! We have never used the Canvas, so it's a mystery.

The good news is that next chapter we switch to Swing.

Thanks for your help! --Sam

[JosAH]

Or, just delete the Canvas! :oD>>>
The buttons don't seem to resize their height, so everthing works fine without the Canvas. The only unanswerable question is why the book said to put it in! We have never used the Canvas, so it's a mystery.

The good news is that next chapter we switch to Swing.

Thanks for your help! --Sam

Also note that your 'paint()' method is your Applet's paint() method so you're
not drawing on te canvas, you're drawing somewhere on the Applet itself.

kind regards,

Jos