To start with, the 'mouseClicked' method requires an object for the parameter (or 'null' to signify any click to return a true value). However, its implementation is specifically for World or Actor objects (objects that are visible -- a Class is not something that a click can be programmatically checked on. This is what you should do: after determining if a click (any click) is detected, called the 'getActor' method on the MouseInfo object and save the returned Actor object in a variable; then, before checking what type Actor it may be, you need to make sure that the returned value is not 'null' (no Actor object was clicked on); after that, you can use the 'instanceof' keyword to check what type Actor object it may be: Although this system of code might work, I do not see why you cannot check for clicks with the Enemy and Player classes themselves and then call methods in the world class when detected. For example, in the Enemy class, you could have in the act method: then, in the world: