As you see, I finally finished the alienai.(Kind of) here's the changes
I made:
1. I found there's a bug which when you initialized gamescreen(when use
pressed start), it will also initialize alienaisystem. In alienaisystem,
I will get current gamescreen, and it will return an type changing error
beacuse it has not switching to the game screen yet. it will return a
startscreen type.
2.There's some problem in the Alien class in shooting method. I didn't
fix it because it will cost to much time.
3. there's some problem in the constracting method of alien class. don't
forget to put "break;" at the end of every case in switch.
4.I don't think every alien has to have a alienai because first, i think
it's useless. alienai doesn't need to store any information. so i will
just use one variable which is alienai type and when i'm using it i'll
just copy the alien i'm gotta control to the variable and do some stuff.