🗺️ConcurrentHashMap & Concurrent Collections

Use purpose-built concurrent collections instead of manual locking around HashMap/List.

📌 Key IdeaHashMap is unsafe for concurrent mutation. Prefer ConcurrentHashMap, ConcurrentLinkedQueue, CopyOnWriteArrayList, or blocking queues based on read/write profile.

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Why It Matters

Concurrent writes on plain HashMap can cause races and corrupted state.
ConcurrentHashMap uses fine-grained/CAS-based concurrency, so throughput is far better than one global lock in many workloads.
Concurrent maps/queues give thread-safe access patterns without forcing a single global lock around every operation.

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When to Use

Shared mutable maps/queues accessed by many threads.
High-throughput caches/counters/registries.
Need weakly-consistent iteration under concurrent updates.

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When NOT to Use

Single-thread confined state.
CopyOnWrite structures under heavy writes (expensive copies).

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Code Variations

1?? Atomic map update without external lockjava

ConcurrentHashMap<String, LongAdder> counts = new ConcurrentHashMap<>();
counts.computeIfAbsent(key, k -> new LongAdder()).increment();

Avoids race from get+put pattern.

2?? Unsafe pattern to avoidjava

Map<String, Integer> m = new HashMap<>();
// two threads mutate/read m concurrently -> undefined behavior

Wrap with synchronization or replace with concurrent collection.

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Collection choice quick guide

Aspect	Type	Best for
ConcurrentHashMap	Read/write maps, high concurrency	No null keys/values
CopyOnWriteArrayList	Many reads, rare writes	Writes are O(n) copy
ConcurrentLinkedQueue	Non-blocking producer/consumer	No built-in backpressure
BlockingQueue	Backpressure + producer/consumer	Threads block on put/take

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Interview Q&A

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Quick Cheat Sheet

Default mapConcurrentHashMap for shared mutable key-value state.

Counter patternConcurrentHashMap + LongAdder.

AvoidRaw HashMap under concurrent writes.

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