Maps: ConcurrentHashMap.newKeySet is a concurrent Set

There is no ConcurrentHashSet class - ConcurrentHashMap.newKeySet() is the intended way to get a thread-safe set.

Code

Set<Integer> seen = ConcurrentHashMap.newKeySet();
var pool = Executors.newFixedThreadPool(4);

for (int i = 0; i < 100; i++) {
    int v = i % 20;
    pool.submit(() -> seen.add(v));
}
pool.shutdown();
pool.awaitTermination(10, TimeUnit.SECONDS);

System.out.println("distinct values = " + seen.size());
System.out.println("sorted = " + seen.stream().sorted().limit(5).toList() + " ...");

Output

distinct values = 20
sorted = [0, 1, 2, 3, 4] ...

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Published 2026-07-30