public void onPeriodicEmit() {
        updateCombinedWatermark();
    }
@Test
    void deferredOutputDoesNotImmediatelyAdvanceWatermark() {
        TestingWatermarkOutput underlyingWatermarkOutput = createTestingWatermarkOutput();
        WatermarkOutputMultiplexer multiplexer =
                new WatermarkOutputMultiplexer(underlyingWatermarkOutput);

        WatermarkOutput watermarkOutput1 = createDeferredOutput(multiplexer);
        WatermarkOutput watermarkOutput2 = createDeferredOutput(multiplexer);

        watermarkOutput1.emitWatermark(new Watermark(0));
        watermarkOutput2.emitWatermark(new Watermark(1));

        assertThat(underlyingWatermarkOutput.lastWatermark()).isNull();

        multiplexer.onPeriodicEmit();

        assertThat(underlyingWatermarkOutput.lastWatermark()).isEqualTo(new Watermark(0));
    }