### Question:
Timer { public static Timer timer() { if (timeHolder.get() == null) { if (timeHolder.compareAndSet(timeHolder.get(), new Timer())) { timeHolder.get().start(); } } return timeHolder.get(); }  static Timer timer(); static long clockTime(); void stop(); long time(); long now();  }### Answer:
@Test public void testTimer() throws Exception { long now = Timer.timer().now(); long currentTime = Clock.systemUTC().millis(); Sys.sleep(1_000); puts(now, currentTime); for (int index = 0; index < 5; index++) { Sys.sleep(1000); now = Timer.timer().now(); currentTime = Clock.systemUTC().millis(); puts(now, currentTime, now - currentTime); assertTrue(Math.abs(now - currentTime) < 1000); } }