STUDYECRAT Java : Concurrent Collections
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Java : Concurrent Collections
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★ Java Concurrent Collections: Key Interview Points
1. ConcurrentHashMap Thread Safety
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("A", 1);
map.compute("A", (k, v) -> v + 1); // Thread-safe atomic update
// Uses lock striping for high concurrency
map.put("A", 1);
map.compute("A", (k, v) -> v + 1); // Thread-safe atomic update
// Uses lock striping for high concurrency
- Tip: Prefer
compute()overget()+put()for atomic updates - Real Use: Shared cache in web applications with frequent reads/writes
2. CopyOnWriteArrayList Iterator Behavior
CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("A");
Iterator<String> it = list.iterator();
list.add("B"); // Modification after iterator creation
while(it.hasNext()) System.out.print(it.next()); // Prints "A"
// Iterators work on snapshot at creation time
list.add("A");
Iterator<String> it = list.iterator();
list.add("B"); // Modification after iterator creation
while(it.hasNext()) System.out.print(it.next()); // Prints "A"
// Iterators work on snapshot at creation time
- Tip: Ideal for read-heavy scenarios with rare writes
- Real Use: Maintaining listener lists in event-driven systems
3. BlockingQueue Producer-Consumer Pattern
BlockingQueue<Task> queue = new ArrayBlockingQueue<>(10);
// Producer:
queue.put(new Task()); // Blocks if queue is full
// Consumer:
Task t = queue.take(); // Blocks if queue is empty
// Built-in thread coordination
// Producer:
queue.put(new Task()); // Blocks if queue is full
// Consumer:
Task t = queue.take(); // Blocks if queue is empty
// Built-in thread coordination
- Tip: Use
poll(timeout)for graceful shutdowns - Real Use: Task scheduling in thread pools (like ExecutorService)
4. ConcurrentSkipListMap Sorting
ConcurrentSkipListMap<Integer, String> map = new ConcurrentSkipListMap<>();
map.put(3, "C"); map.put(1, "A");
System.out.println(map.firstEntry()); // 1=A
System.out.println(map.ceilingKey(2)); // 3
// Maintains natural ordering concurrently
map.put(3, "C"); map.put(1, "A");
System.out.println(map.firstEntry()); // 1=A
System.out.println(map.ceilingKey(2)); // 3
// Maintains natural ordering concurrently
- Tip: More scalable than
Collections.synchronizedSortedMap() - Real Use: Real-time leaderboards in multiplayer games
5. Atomic Updates in ConcurrentHashMap
ConcurrentHashMap<String, Long> counters = new ConcurrentHashMap<>();
counters.put("visits", 0L);
// Atomic increment:
counters.compute("visits", (k, v) -> v + 1);
// Merge alternative:
counters.merge("visits", 1L, Long::sum);
// Both methods are thread-safe
counters.put("visits", 0L);
// Atomic increment:
counters.compute("visits", (k, v) -> v + 1);
// Merge alternative:
counters.merge("visits", 1L, Long::sum);
// Both methods are thread-safe
- Tip:
merge()is cleaner for simple increments - Real Use: Hit counters in web analytics
💡 Pro Interview Tip
When asked about concurrent collections, always mention the trade-offs: CopyOnWriteArrayList has expensive writes but safe reads, while ConcurrentHashMap scales better than synchronized maps but has no lock-free reads. Tailor your answer to the use case.
