Word count demo of Apache Kafka.

WordCountDemo.java

// Serializers/deserializers (serde) for String and Long types
final Serde<String> stringSerde = Serdes.String();
final Serde<Long> longSerde = Serdes.Long();
 
// Construct a `KStream` from the input topic "streams-plaintext-input", where message values
// represent lines of text (for the sake of this example, we ignore whatever may be stored
// in the message keys).
KStream<String, String> textLines = builder.stream("streams-plaintext-input",
    Consumed.with(stringSerde, stringSerde);
 
KTable<String, Long> wordCounts = textLines
    // Split each text line, by whitespace, into words.
    .flatMapValues(value -> Arrays.asList(value.toLowerCase().split("\\W+")))
 
    // Group the text words as message keys
    .groupBy((key, value) -> value)
 
    // Count the occurrences of each word (message key).
    .count()
 
// Store the running counts as a changelog stream to the output topic.
wordCounts.toStream().to("streams-wordcount-output", Produced.with(Serdes.String(), Serdes.Long()));