Explain deadlock in Java and how to prevent it.

Deadlock occurs when two or more threads are blocked forever, waiting for each other. Prevention methods: 1) Avoid nested locks, 2) Avoid unnecessary locks, 3) Use timeout with tryLock(), 4) Lock ordering (always acquire locks in same order), 5) Use concurrent collections. Example: Thread1 locks A then B, Thread2 locks B then A - this can cause deadlock.