public class TestDrawingPaper {
  public static void main( String args[] ) {
    DrawingPaper aPaper = new DrawingPaper();
    Triangle a = new Triangle();
    Rectangle b = new Rectangle();
    
    // aとbに自分自身を描画させる
    aPaper.drawYourself( a );
    aPaper.drawYourself( b ); // エラー！
  }
}