A 27-year-old man who threw a rock at his former roommate, hitting the 53-year-old man in the head, was arrested on suspicion of second-degree assault in Kapolei, police said.

During an unprovoked attack, the suspect threw the rock about 4:30 p.m. yesterday, causing substantial injuries to the older man, police said. While the victim was incapacitated by the blow, the suspect choked him before bystanders intervened, police said. The victim was treated and released from the hospital.