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COMPARE AND SWAP

What Is CAS?

• A CPU-level atomic instruction: "set this value to X only if it's currently Y"
• If another thread changed it in between, CAS fails and you retry
• Used by AtomicInteger, ConcurrentHashMap, and lock-free algorithms
• No blocking — threads don't wait, they just retry (called "spinning")

// Pseudocode for what AtomicInteger.incrementAndGet() does:
do {
    current = read(value);         // read current value
    next = current + 1;
} while (!cas(value, current, next)); // retry if someone else changed it

CAS vs Locking

Approach	How it works	Tradeoff
Locking (synchronized)	Block other threads	Safe but threads wait
CAS (lock-free)	Retry on conflict	No blocking but wastes CPU on retries under high contention

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References

• cpu_cache_and_memory_visibility
• java_concurrency_data_structures