@Override
    public <VO, VR> KStream<K, VR> leftJoin(final KStream<K, VO> otherStream,
                                            final ValueJoiner<? super V, ? super VO, ? extends VR> joiner,
                                            final JoinWindows windows) {
        return leftJoin(otherStream, toValueJoinerWithKey(joiner), windows);
    }
@Test
    public void shouldNotAllowNullJoinWindowsOnLeftJoin() {
        final NullPointerException exception = assertThrows(
            NullPointerException.class,
            () -> testStream.leftJoin(testStream, MockValueJoiner.TOSTRING_JOINER, null));
        assertThat(exception.getMessage(), equalTo("windows can't be null"));
    }