assertTrue(stopwatch.isRunning());
  }

  public void testStop() {
    stopwatch.start();
    assertSame(stopwatch, stopwatch.stop());
    assertFalse(stopwatch.isRunning());
  }

  public void testStop_new() {
    assertThrows(IllegalStateException.class, stopwatch::stop);
    assertFalse(stopwatch.isRunning());
  }

  public void testStop_alreadyStopped() {
    stopwatch.start();
    stopwatch.stop();