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 class Maus extends MouseAdapter {                 
     public void mousePressed(MouseEvent event) {     
       
       Graphics g = getGraphics();             
       int px = event.getX();
       int py = event.getY();              
       g.setColor(Color.RED);                                               
       g.fillRect(((int)px/bx)*bx, ((int)py/hy)*hy, bx, hy); // Quadrate werden gefärbt bei Mausklick               
     }  
     }					

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boolean clicked=false;
Point p=new Point();

public void paint(Graphics g) {
  
  
       if(clicked==true)
       g.setColor(Color.RED);                                                
      g.fillRect(((int)p.x/bx)*bx, ((int)p.y/hy)*hy, bx, hy);
   
   }

public void mousePressed(MouseEvent event) {     
      clicked=true;
      System.out.println(clicked); // Auf die Klicks wird reagiert                                 
       p.x=getX();
       p.y=getY();      
     repaint();         
       }