painting from user defined class

[Ed Langley]

Hello everybody
I am trying to use painting methods from inside my own class in an application:

public class App extends Frame
{
picSegArray puzzle;

public static void main(String args[])
{
App mainFrame = new App();
mainFrame.setSize(800, 600 );
mainFrame.setTitle("App");
mainFrame.setVisible(true);
System.out.println("Hello World!");
}

public App()
{
addWindowListener(new WindowAdapter()
{
public void windowClosing(WindowEvent e)
{
dispose();
System.exit(0);
}
});
setLayout(new FlowLayout());

puzzle = new picSegArray();
add(puzzle);
}
}
class picSegArray extends Panel
{
public picSegArray()
{
super();
aFunction();
}
public void aFunction()
{
update();
}

public void update()
{
Graphics g = this.getGraphics();
g.drawString("picSegArray",200,100);

}
}

The drawstring call in update() generates a NullPointerException during execution. I have tried various other ways but can't get the text drawn in the frame.
What is wrong with the above code?
What class should I be extend to make picSegArray and is there a better way to paint from inside it?
Thanks in advance.

[Michael Hildner]

From the API on getGraphics():
Creates a graphics context for this component. This method will return null if this component is currently not displayable.
You are getting a NullPointerExcpetion because g is null, as the Panel is not currently disaplyed.

[Randall Twede]

your main problem is there is you are defining your own update() method. The update method of Component class is update(Graphics g)
Panel is ok as far as I know but Canvas is perhaps used more often.
I believe this is all you really need:

class picSegArray extends Canvas {  //or Panel
  public void paint(Graphics g) {
    g.drawString("picSegArray",200,100);
  }
}

The component should get painted automatically. If you do need a constructor this is probably all it needs to be:
public picSegArray() {
repaint();
}
super() is automatically called.
repaint() calls update(Graphics g), supplying the Graphics object. update(Graphics g) clears the component then calls paint(Graphics g)
I dont think you need a constructor but I wanted to explain about super() and repaint()
------------------
Dont blindly believe everything I say.

[Ed Langley]

Thanks a lot for that.
I discovered that when i did it other ways the panel wasn't showing because i had a button in the frame as well and was using a flowLayout. I was adding the button first though so I don't know why it showed and not picSegArray.
The class actually does a whole lot more than that but I simplified it for posting.
Anyway I wanted to use a lot of painting code in other functions and the double buffering technique seems very handy for this, but I can't get the image created properly:

private void resetOffScreen()
    {
        offScreenImage = createImage(this.getSize().width, this.getSize().height);
        offScreenGraphics = offScreenImage.getGraphics();
        repaint();
    }

the getGraphics line in this function inside picSegArray generates another nullPointerException, so my new question is how do I properly initialise offScreenImage?
resetOffScreen() isn't called until the component is displayed.
[This message has been edited by Ed Langley (edited February 12, 2001).]

[Randall Twede]

You might find your answer in this code somewhere.

class Text extends Canvas implements Runnable {
  String message = "Select an animal";
  int message_x, message_y = 16;
  int min_x, max_x;
  int scroll_frames = 100;
  int sleepTime = 100;
  Image OSC = null;
  Graphics OSC_graphics;
  int width = 400;
  int height = 20;
  public Text() {
    setSize(400,20);
  }
  synchronized public void paint(Graphics g) {
      if (OSC != null)  // if off-screen image exists, copy it to screen
          g.drawImage(OSC,0,0,this);
      else {  // otherwise, fill the canvas with the background color
          g.setColor(getBackground());
          g.fillRect(0,0,size().width,size().height);
      }
   }
       
   public void update(Graphics g) {
         // redefine update() so that it doesn't erase the applet;
         // this will avoid flickering when the applet is redrawn
      paint(g);
   }
   synchronized void drawNextFrame(int scroll_frame) {
    OSC = createImage(width,height);
    OSC_graphics = OSC.getGraphics();
    Font font = getFont();
    font = new Font("TimesRoman",Font.PLAIN,15
    
    
    
    );
    OSC_graphics.setFont(font);
    FontMetrics fm = getFontMetrics(font);
    min_x = -fm.stringWidth(message);
    max_x = width;
    message_x = max_x - ((scroll_frame * (max_x-min_x)) / scroll_frames);
    OSC_graphics.setColor(Color.white);
    OSC_graphics.fillRect(0,0,width,height);
    OSC_graphics.setColor(Color.black);
    OSC_graphics.drawString(message,message_x,message_y);
  }
  public void run() {
    int scroll_frame = 0;
    while(true) {
      switch(Farm.selection) {
        case 0:
          message = "Click the sheep";
          break;
        case 1:
          message = "Click the dog";
          break;
        case 2:
          message = "Click the bird";
          break;
        case 3:
          message = "Click the cow";
          break;
        case 4:
          message = "Click the rooster";
          break;
        case 5:
          message = "Click the donkey";
        }
      drawNextFrame(scroll_frame);
      repaint();
      scroll_frame++;
      if (scroll_frame > scroll_frames)
        scroll_frame = 0;
      try { Thread.sleep(sleepTime); }
      catch (InterruptedException e) { }
    }
  }
}