graphics - Implement Undo&Redo Java Blackboard Paint -

i've got blackboard, can draw dragging mouse. i'm trying implement undo & redo delete/restore last coordinates drawn. undo & redo works, blackboard doesn't refresh automatically, , have drag mouse again.

here code (too long):

public class clientblackboard extends jpanel implements keylistener { 

[...]

class mmotionlistener implements mousemotionlistener {      public void mousedragged(mouseevent e) {                     x = e.getx();         y = e.gety();         points.add(new pointerdraw(x, y));          _changed = true;         try {             drawcurrentshape(_bufimage.creategraphics());         } catch (exception e1) {             // todo auto-generated catch block             e1.printstacktrace();         }         repaint();     }      public void mousemoved(mouseevent e) { } }  // subscribe mouse events class mlistener implements mouselistener {      // required interface, not used     public void mouseclicked(mouseevent e) {}      public void mouseentered(mouseevent e) {}      public void mouseexited(mouseevent e) {}      public void mousepressed(mouseevent e) {}      public void mousereleased(mouseevent e) {         if (!_changed)             system.out.println("error");         else             system.out.println("changes saved");     }  }  // draw @override public void paintcomponent(graphics g){     super.paintcomponent(g);     graphics2d g2 = (graphics2d) g;     if (_bufimage == null){         int w = this.getwidth();         int h = this.getheight();         _bufimage = (bufferedimage) this.createimage(w, h);         graphics2d gc = _bufimage.creategraphics();         gc.setcolor(color_background);         gc.fillrect(0,  0,  w,  h);     }     g2.drawimage(_bufimage, null, 0, 0);     test = g2; }     private void drawcurrentshape(graphics2d g2) throws exception{     g2.setcolor(color_foreground);     if (!_flagundo)         g2.drawline(x,  y,  x,  y);     else{         (pointerdraw p : points){             g2.drawline(p.getstartxpoint(), p.getstartypoint(), p.getstartxpoint(), p.getstartypoint());         }         _flagundo = false;     } }    // shortcuts: ctrl-z , ctrl-y public void keyreleased(keyevent e) { } public void keypressed(keyevent e) {     if (e.iscontroldown() && e.getkeycode() == keyevent.vk_z){                   try {             pointerdraw point = connection.remote.readdrawfromdb(_counter++);             system.out.println("counter: " + _counter);             system.out.println("undo: " + "x: " + point.getstartxpoint() + " y: " + point.getstartypoint());             points.removelast();             getalllist();             _bufimage = null;             _flagundo = true;             repaint();             test.drawimage(_bufimage,  null,  0,  0);         } catch (nullpointerexception exc){             system.out.println("you can't undo again");             _counter--;         } catch (exception exc) {             // todo auto-generated catch block             exc.printstacktrace();         }     }      if (e.iscontroldown() && e.getkeycode() == keyevent.vk_y){         try {             if (_counter > 0){                 pointerdraw point = connection.remote.readdrawfromdb(_counter--);                 system.out.println("counter: " + _counter);                 system.out.println("redo: x: " + point.getstartxpoint() + " y: " + point.getstartypoint());                 points.add(new pointerdraw(point.getstartxpoint(), point.getstartypoint()));                 getalllist();                 _bufimage = null;                 _flagundo = true;                 (pointerdraw p : points){                     test.drawline(p.getstartxpoint(), p.getstartypoint(), p.getstartxpoint(), p.getstartypoint());                 }                 repaint();             }         } catch(nullpointerexception exc){             system.out.println("you can't redo again");             _counter++;         } catch (exception e1) {             // todo auto-generated catch block             e1.printstacktrace();         }     } }