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Class derived from Graphics

mhebert
mhebert asked
on
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Last Modified: 2013-11-23
It is possible to derive a class from the Graphics one ?  

I made this simple applet that draws cross in the screen at mouse click point with double buffering.  To make the cross, I wanted to derive my own class from the Graphics one, and then, create the drawCross function.

In compilation, all fits.  But, when I start the applet under J++, Vstudio returns me this error "Unhandled exception in Object: 0x00000000 : Java Exception" and breaks at the line "tmp2 = (GraphicsEnhanced)tmp;".

What's wrong ?

How can I implement my own Graphics class ?

==========================================
import GraphicsEnhanced;
import java.applet.*;
import java.awt.*;

public class Object extends Applet
{
      //Double buffering
      private Graphics tmp;
      private GraphicsEnhanced tmp2;
      private Image img;
    private Dimension dim = new Dimension();
      //---------------------

      public Object(){
      }

      public void init(){
            dim.height = getSize().height;
        dim.width = getSize().width;
        img = createImage(dim.width, dim.height);
       
            tmp = img.getGraphics();
            tmp2 = (GraphicsEnhanced)tmp;
      }

      public void destroy(){
      }

      public void paint(Graphics g){
            update(g);
      }

      public void update(Graphics g){
            //Double buffering
        g.clearRect(0,0, dim.width, dim.height);
        g.drawImage(img, 0, 0, this);
            //------------------------
      }

      public boolean mouseDown(Event evt, int x, int y){
            tmp.setColor(new Color(0,0,255));
            tmp.drawLine(x-10, y-10, x+10,  y+10);    
            tmp.drawLine(x-10, y+10, x+10,  y-10);    
            repaint();            
            return true;
      }

      public boolean mouseUp(Event evt, int x, int y){
            tmp.setColor(new Color(255,0,0));
            tmp.drawLine(x-10, y-10, x+10,  y+10);    
            tmp.drawLine(x-10, y+10, x+10,  y-10);    
            repaint();            
            return true;
      }

      public boolean mouseDrag(Event  evt, int  x, int  y){
            tmp.setColor(new Color(0,0,0));
            tmp.drawLine(x-10, y-10, x+10,  y+10);    
            tmp.drawLine(x-10, y+10, x+10,  y-10);    
            repaint();            
            return true;
      }

      public void Clear(){
            tmp.clearRect(0,0, dim.width, dim.height);
            repaint();
      }

      public void Trait(int x, int y){
            tmp.setColor(new Color(0,0,0));
            tmp.drawLine(x-10, y-10, x+10,  y+10);    
            tmp.drawLine(x-10, y+10, x+10,  y-10);    
            repaint();
      }
}
=========================================
import java.awt.Graphics;

public abstract class GraphicsEnhanced extends Graphics
{
      protected GraphicsEnhanced(){
            //super();
      }
      
      public void drawCross(int x, int y, int width, int thickness){
            int xPoints[] = new int[2];
            int yPoints[] = new int[2];

            xPoints[0] = x - (width / 2);
            xPoints[1] = x + (width / 2);
            yPoints[0] = y - (width / 2);
            yPoints[1] = y + (width / 2);

            fillPolygon(xPoints, yPoints, thickness);
      }

}
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Commented:
For example you could write a class:

class Cross
{  int width,thickness;

  public Cross(int w,int t)
  {  width=w;
     thickness=t;
     return;
  }
   public void draw(Graphics g,int x, int y){
   int xPoints[] = new int[2];
   int yPoints[] = new int[2];

   xPoints[0] = x - (width / 2);
   xPoints[1] = x + (width / 2);
   yPoints[0] = y - (width / 2);
   yPoints[1] = y + (width / 2);

   g.fillPolygon(xPoints, yPoints, thickness);
   }
}


So for this example you would create a Cross object for every "type" of Cross you wanted to use. It could also, of course, simply be done as a static method.

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