Many LLD problems hide a concurrency requirement in plain sight: two cars reach the gate at once, two users book the same seat, many threads hit the cache. Senior candidates are expected to notice the shared mutable state and reach for the right tool without being told. This topic is the practical concurrency toolkit for the LLD round — just enough to write correct, thread-safe class designs, not a full JMM treatise.
The one mental model
Every concurrency bug in LLD reduces to unguarded shared mutable state. The fix is always one of three moves:
1. DON'T SHARE thread-confinement, copies 2. DON'T MUTATE immutability, final fields, value objects 3. GUARD MUTATION synchronized / locks / atomics / concurrent collections
Reach for immutability first — an immutable object is automatically thread-safe and needs no locks. Only guard what must genuinely change.
The classic LLD race
The recurring trap is check-then-act: a thread reads state, decides, then acts — but another thread slips in between. Two cars both see a spot free and both take it; two users both see a seat available and both book it.
if (spot.isFree()) { // T1 and T2 both read true
spot.occupy(); // both occupy the same spot — corrupt
}Making the check and the act atomic (a synchronized region, a compare-and-set, or a concurrent collection's atomic method) is the fix interviewers look for — and naming the race before being prompted is the senior signal.
The toolkit you'll use
synchronized/ReentrantLock— coarse mutual exclusion; the default.ReadWriteLock/StampedLock— many readers, few writers.ConcurrentHashMap,CopyOnWriteArrayList,BlockingQueue— concurrent collections that do the locking for you.AtomicInteger/AtomicReference— lock-free counters and CAS.- Immutability — the lock you never have to take.
How to use this topic
The questions below cover when the interviewer expects thread-safety, the coarse-versus-fine locking trade-off, the concurrent-collections toolkit with when-to-use guidance, and the canonical thread-safe Singleton (the five idioms with code). Cross-reference the Collections module for the concurrent-collection internals; here the focus is choosing the right tool inside a class design under interview time pressure.