If the line is to move at all, it is best to have an actor class describe and control the object. Just create a GreenfootImage object of whatever length and a narrow width and fill the image with any color. Set the image to the actor. As an actor, its rotation and location can be adjusted as necessary through time. If you unsure about GreenfootImage objects, refer to the API documentation (use the 'Documentation' link along the header of this page and go to the 'online' documentation of the classes greenfoot provides.