code: @Override
      public void drawLine(int x1, int y1, int x2, int y2) {
          if (this.line == null) {
              this.line = new Line2D.Double(x1, y1, x2, y2);
          } else {
              this.line.setLine(x1, y1, x2, y2);
          }
          draw(this.line);
      }
docs: Draws a line from {@code (x1, y1)} to {@code (x2, y2)} using
  the current {@code paint} and {@code stroke}.
  
  @param x1  the x-coordinate of the start point.
  @param y1  the y-coordinate of the start point.
  @param x2  the x-coordinate of the end point.
  @param y2  the x-coordinate of the end point.