### Question:
Concurrents { public static LongAdder longAdder() { return new LongAdder(); }  static LongAdder longAdder(); static CountDownLatch countDownLatch(int count); static CyclicBarrier cyclicBarrier(int count); static Semaphore nonFairSemaphore(int permits); static Semaphore fairSemaphore(int permits); static RateLimiter rateLimiter(int permitsPerSecond); static RateLimiter rateLimiter(int permitsPerSecond, int maxBurstSeconds); static Sampler sampler(double selectPercent); static TimeIntervalLimiter timeIntervalLimiter(long interval, TimeUnit timeUnit);  }### Answer:
@Test public void longAdder() { LongAdder counter = Concurrents.longAdder(); counter.increment(); counter.add(2); assertThat(counter.longValue()).isEqualTo(3L); }