import org.eclipse.draw2d.RectangleFigure;

public class MyShape extends RectangleFigure {
    public MyShape() {
    }

    @Override
    protected void fillShape(Graphics graphics) {
        // Draw the rectangle. You can also just call graphics.fillRectangle(getBounds()), which is what RectangleFigure.fillShape() does.
        super.fillShape(graphics);

        // Code to draw triangle goes here...
    }
}