Create a Joystick object whenever you think your game may be used on touch devices. The scene's getMouseX() and getMouseY() methods will respond to touch input.

joyName = new Joy() Creates a new Joystick object called joyName.
getMouseX(), getMouseY() Gets the X and Y position of the mouse if the user is touching the screen. Returns screen coordinates, not coordinates in the scene. Typically, you want to use the scene object's getMouseX() and getMouseY() methods instead.
getDiffX(), getDiffY() Returns how far the user's finger has moved from the initial touch position. Modify these values to get joystick-like behavior.
virtKeys = true; If you create a variable called virtKeys in your main code and set it to true before building a joystick object, the user can drag up to trigger the up arrow key, down to trigger the down arrow, and so on.