We defined the type of the surface variable as PSurfaceFX.
PSurfaceFX is a sub-type of PSurface.
super.initSurface() does indeed return a PSurfaceFX object but the compiler only knows that it is an object of type PSurface.
So, we need to cast the PSurface object that it returns to a PSurfaceFX type, so the type corresponds with the surface variable:
surface = (PSurfaceFX) super.initSurface();