Implement a rate limiter that allows N operations per second.

The simplest correct version is a Semaphore with N permits and a scheduled refill:

Semaphore permits = new Semaphore(n);
scheduler.scheduleAtFixedRate(
        () -> permits.release(n - permits.availablePermits()), 1, 1, SECONDS);

void call() throws InterruptedException {
    permits.acquire();
    doWork();
}

Be ready for the critique: this is a fixed window, so 2N calls can land across a window boundary in a moment. A token bucket with fractional refill (tokens accumulate at n per second up to a burst capacity) smooths that out, and a leaky bucket removes bursts entirely.

Points for mentioning fairness (new Semaphore(n, true) so a caller is not starved), a timeout on tryAcquire so callers are not blocked forever, and that in real systems the limiter belongs where the resource is, not per JVM.