4))
        .expect(1L, 1L, 2L, 3L);
  }

  public void testFlatMapToDouble_nullStream() {
    SpliteratorTester.ofDouble(
            () ->
                CollectSpliterators.flatMapToDouble(
                    Arrays.spliterator(new Double[] {1.0, 0.0, 1.0, 2.0, 3.0}),
                    (Double i) -> i == 0.0 ? null : DoubleStream.of(i).spliterator(),