@Test
public void testTransform() {

    KStreamBuilder builder = new KStreamBuilder();
    ValueTransformerSupplier<Integer, Integer> valueTransformerSupplier =
        new ValueTransformerSupplier<Integer, Integer>() {
            public ValueTransformer<Integer, Integer> get() {
                return new ValueTransformer<Integer, Integer>() {
                    private int total = 0;
                    @Override
                    public void init(ProcessorContext context) {
                    }
                    @Override
                    public Integer transform(Integer value) {
                        total += value;
                        return total;
                    }
                    @Override
                    public Integer punctuate(long timestamp) {
                        return (int) timestamp;
                    }
                    @Override
                    public void close() {
                    }
                };
            }
        };
    final int[] expectedKeys = {1, 10, 100, 1000};
    KStream<Integer, Integer> stream;
    MockProcessorSupplier<Integer, Integer> processor = new MockProcessorSupplier<>();
    stream = builder.stream(intSerde, intSerde, topicName);
    stream.transformValues(valueTransformerSupplier).process(processor);
    driver = new KStreamTestDriver(builder);
    for (int i = 0; i < expectedKeys.length; i++) {
        driver.process(topicName, expectedKeys[i], expectedKeys[i] * 10);
    }
    assertEquals(4, processor.processed.size());
    driver.punctuate(2);
    driver.punctuate(3);
    String[] expected = {"1:10", "10:110", "100:1110", "1000:11110", "null:2", "null:3"};
    for (int i = 0; i < expected.length; i++) {
        assertEquals(expected[i], processor.processed.get(i));
    }
}