Deadlock Four Conditions in Java

Deadlock is when two or more threads are permanently blocked, each waiting for a resource the other holds. For deadlock to happen, you need all four Coffman conditions present at the same time. Let me name them. First, Mutual Exclusion — the resource can't be shared, like a synchronized lock that only one thread can hold. Second, Hold and Wait — a thread holds one lock while trying to acquire another. Third, No Preemption — you can't forcibly take a lock away, it has to be released voluntarily. Fourth, Circular Wait — there's a circular chain where thread A waits for thread B's lock, and B waits for A's lock. Here's the key insight: you only need to break one condition to prevent deadlock. In Java code, the easiest one to break is Circular Wait by using lock ordering. That means if you need to lock two accounts, always lock them in ascending ID order. Thread one locks account ten then account twenty. Thread two also locks ten then twenty. Now there's no cycle, no deadlock. Some freshers think synchronized alone prevents deadlock, but that's wrong — deadlock happens when you have multiple synchronized blocks acquired in inconsistent order. The interviewer is testing whether you know the formal conditions, not just the intuitive explanation.