For an actor to move horizontally back and forth across the world, you only need track the current direction of movement and have a check determining if the actor is at either edge of the world. Using an integer for the direction is helpful in that the movement can be based on the value (just multiply the speed by its value). Simply put (in pseudo-code):

You need to keep track of the direction the actor is moving horizontally. The easiest type of field to use will be an 'int' because it can be used in the movement code. The field can simply be called 'direction' and will be declared in the class with: The allowable values for the direction are 1 and -1, for moving right and moving left. The movement code would simply be: For changing direction, just change the sign of 'direction' (or negate it) For changing the image, the 'mirrorHorizontally' method of the GreenfootImage class will come in handy:

You could check your dog's x or y position and move according to that. The code would be simple. Assuming your world's size is 600 pixels wide, you can tell your dog the following. If your X position is greater than or equal to 590 (10 pixels less than your world's total width), move left (a negative number). If your X position is less than or equal to 10 (10 pixels more than your world's minimum width), move right (a positive number). The following code is exactly what is written here, except I created a variable named speed to hold our positive/negative number. With just this code, your image will not change, but it is important to get this first. This is the code to the entire "dog.class". import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) public class dog extends Actor { int speed = 3; public void act() { move(); checkEdges(); } public void move() { move(speed); } public void checkEdges() { if(getX() >= 590) { speed = -3; } if(getX() <= 10) { speed = 3; } } }