Java

//global variables :
//params for the circle i draw :
 public int positionY=10;
 public int positionX=20;
 public int width=30;
 public int height=30;

 //used to set dimension of the viewport :
 private Dimension dim=new Dimension();

 //the viewport in which i draw, it extends JViewport
 private MyJViewport drawHere=new MyJViewport();

 private JScrollPane displayResult;

 private JButton jButton1;

class MyJViewport extends JViewport {

       @Override
       public void paint(Graphics g) {
           g.setColor(Color.BLUE);
           g.fillOval(positionX, positionY, width, height);

       }//paint

   }//MyJViewport

displayResult = new javax.swing.JScrollPane(drawHere);
displayResult.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
displayResult.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
       //the new circle will be positioned under the previous
       positionY+=height;

       // if the point where the circle gets drawn exceeds the bounds of the viewport
       // resize the viewport
       if (positionY > drawHere.getHeight()) {
               dim.height = drawHere.getHeight() + height;
               dim.width = drawHere.getWidth();
               drawHere.setPreferredSize(dim);
               drawHere.revalidate();

           }//if

       drawHere.repaint();
   }