java swing paint on another class
public class MainInterface(){
   static JPanel drawBox = new JPanel;
   public MainInterface(){
      drawBox.setBorder(BorderFactory.createEtchedBorder(EtchedBorder.RAISED));
      drawBox.setLayout(null);
   }
}

public class PainterClass(){
   public void paint(Graphics g){
      MainInterface.drawBox.paint(g);
      g.drawRect(100,100,100,100);
   }
}