I'm trying to do a Kafka tutorial in Kotlin and wo...

# getting-started

[waltermcq]

I'm trying to do a Kafka tutorial in Kotlin and wondered what the following code is in idiomatic Kotlin

// send data - asynchronous
            producer.send(record, new Callback() {
                public void onCompletion(RecordMetadata recordMetadata, Exception e) {
                    // executes every time a record is successfully sent or an exception is thrown
                    if (e == null) {
                        // the record was successfully sent
                        <http://logger.info|logger.info>("Received new metadata. \n" +
                                "Topic:" + recordMetadata.topic() + "\n" +
                                "Partition: " + recordMetadata.partition() + "\n" +
                                "Offset: " + recordMetadata.offset() + "\n" +
                                "Timestamp: " + recordMetadata.timestamp());
                    } else {
                        logger.error("Error while producing", e);
                    }
                }
            });
        }

[waltermcq]

Copy/pasting this code into IntelliJ and letting it do the Kotlin conversion yields something that is broken.

[waltermcq]

It has been long since I did any Java but it looks like Callback is being extended with an implementation of onCompletion() i.e. anonymous class?

[waltermcq]

Would it make sense to use an extension function on Callback here?
fun Callback.onCompletion(recordMetadata: RecordMetadata, e: Exception) { ... }

[akatkov]

// send data - asynchronous
    producer.send(record, object: Callback() {
        fun onCompletion(recordMetadata: RecordMetadata, e: Exception?) {
            // executes every time a record is successfully sent or an exception is thrown
            if (e == null) {
                // the record was successfully sent
                <http://logger.info|logger.info>("Received new metadata. \n" +
                        "Topic:" + recordMetadata.topic() + "\n" +
                        "Partition: " + recordMetadata.partition() + "\n" +
                        "Offset: " + recordMetadata.offset() + "\n" +
                        "Timestamp: " + recordMetadata.timestamp())
            } else {
                logger.error("Error while producing", e)
            }
        }
    })

[akatkov]

i believe that should be valid and does the same as the java version with creating an anonymous class (might need an override on the fun onCompletion )

[waltermcq]

I see you used the object keyword above

[waltermcq]

https://kotlinlang.org/docs/reference/nested-classes.html

[waltermcq]

ok, that creates an anonymous inner class

[waltermcq]

I'll try this out

[waltermcq]

Seems to work! Thanks @akatkov. My next step was to try the nested class / object keyword.

[waltermcq]

You beat me to it 🙂