assertEquals(0, stopwatch.elapsed(NANOSECONDS));
  }

  public void testStart() {
    assertThat(stopwatch.start()).isSameInstanceAs(stopwatch);
    assertTrue(stopwatch.isRunning());
  }

  public void testStart_whileRunning() {
    stopwatch.start();
    assertThrows(IllegalStateException.class, stopwatch::start);
    assertTrue(stopwatch.isRunning());
  }

  public void testStop() {
    stopwatch.start();