Kotlin Kafka consumer - kotlin

Question

I am new to kotlin and kafka. I am trying to read message from a kafka consumer and then process it later in kotlin.
I am able to print the message consumed from kafka topic.
However, I am not able to add this message into a list and process it later.
How to add the message from kafka topic to a list to be processed later?
Below is my kafka consumer's consume method.
KafkaConsumer.kt
fun consume(handler: (value: String) -> Unit) = thread(start = true) {
keepGoing = true
consumer.use { consumer ->
while (keepGoing) {
consumer.poll(Duration.ofMillis(500))?.forEach {
println(it?.value() ?: "no value")
handler(it?.value() ?: "empty_message")
}
}
}
}
I am invoking the consume() as shown below
fun main(){
<read config>
val kafkaConsumer = KafkaConsumer(config.get(<topicname>))
kafkaConsumer.consume {
println(it)
}
}
PS: I tried to append the to a mutable list in the main() which did not work.

Answer 1

I'm not sure if it's a formatting issue or your actual code, but Kotlin lambdas are defined by { } and you call consume() with 2 pairs of braces:
kafkaConsumer.consume { it -> {
println(it)
} }
This is effectively passing a function that takes one argument and returns a function. That returned function takes itself an argument and prints it.
This should not even compile given the signature of consume().
What you probably want is this instead:
kafkaConsumer.consume { it ->
println(it)
}
Note that it is also the implicit argument of lambda expressions, so you don't even need to specify it at all:
kafkaConsumer.consume {
println(it)
}