How to treat a swollen finger, jammed from playing basketball?

Jammed: Make sure it's not cracked first, if not, motion by flexing-extending the finger as tolerated to minimize more swelling.

See a doctor: Basketball injuries to the middle knuckle include simple ligament sprains, central slip (tendon-boutonneire) rupture and fracture dislocation. Unfortunately, even severe injuries could look innocuous. I personally feel it is important to have these injuries evaluated in order to rule out a more sinister injury. It could mean the difference between 3 weeks of buddy taping and surgery...