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(),

  /**
   * Returns a {@link DoubleStream} containing the elements of the first stream, followed by the
   * elements of the second stream, and so on.
   *
   * <p>This is equivalent to {@code Stream.of(streams).flatMapToDouble(stream -> stream)}, but the
   * returned stream may perform better.
   *
   * @see DoubleStream#concat(DoubleStream, DoubleStream)
   */
  public static DoubleStream concat(DoubleStream... streams) {