Maps: ConcurrentHashMap compute is atomic per key

compute, merge and computeIfAbsent hold a lock on the bin for that key, so concurrent updates cannot lose a write the way get-then-put can.

Code

var hits = new ConcurrentHashMap<String, Integer>();
var pool = Executors.newFixedThreadPool(4);

for (int i = 0; i < 400; i++) {
    pool.submit(() -> hits.merge("page", 1, Integer::sum));
}
pool.shutdown();
pool.awaitTermination(10, TimeUnit.SECONDS);

System.out.println("expected 400, got " + hits.get("page"));
System.out.println("mappingCount = " + hits.mappingCount());

Output

expected 400, got 400
mappingCount = 1

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Published 2026-07-30