Upon arriving at Hunterspoint Ave. there was another train also disembarking. The platform was crowded and passengers in the train had to wait on the train before being able to exit onto the platform. This has happened many times. As I was exiting the train [slowly] as the platform was crowded the doors closed and struck me hard on the head. Myself and the other passengers were stuck on the train. One passenger pushed the alert button. We moved to another car, the doors opened and we exited. I found a conducter to complain. How can you close the doors when passengers are still exitng? He said he didn't close the doors. I asked him ot malke sure whoever did understood what happend. This was not a light head contact.