Iterate over java.util.Map with custom objects in Scala

Question

I'm new to writing code in Scala. I'm trying to iterate over a java Map with custom objects as key value pairs. Specifically I'm trying to go over the Map of TopicPartitions and OffSetMetadata Map when committing offsets to Kafka.

Here is the code that i have written

override def onComplete(map: util.Map[TopicPartition, OffsetAndMetadata], e: Exception): Unit = {
  val sb = new StringBuffer()
  map.forEach((partition:TopicPartition ,offsets : OffsetAndMetadata) => {
    sb.append(partition.topic()+","+partition.partition()+","+offsets.offset()+"\n")
  })

However I get a compilation error saying

error: type mismatch; found : (org.apache.kafka.common.TopicPartition, org.apache.kafka.clients.consumer.OffsetAndMetadata) => StringBuffer [ERROR] required: java.util.function.BiConsumer[_ >: org.apache.kafka.common.TopicPartition, _ >: org.apache.kafka.clients.consumer.OffsetAndMetadata] [ERROR] map.forEach((partition:TopicPartition ,offsets : OffsetAndMetadata) => { [ERROR] ^

It indicates the => operator while pointing the error, any help is appreciated.

Seems to work here (with maps of other classes). Could you make a complete example which reproduces the error? — Paŭlo Ebermann
– Paŭlo Ebermann, Commented Jan 23, 2019 at 19:35
Why don't you try converting the map from Java to Scala first? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Jan 23, 2019 at 19:36

SergGr · Accepted Answer · 2019-01-23 22:53:17Z

3

The best bet I have is that you use Scala 2.11 that is still not really aware of Java 8 and functional interfaces. That's why the compiler can't automatically convert your Scala lambda to a matching BiConsumer. According to my understanding the simplest workaround is to create a BiConsumer explicitly as in

override def onComplete(map: util.Map[TopicPartition, OffsetAndMetadata], e: Exception): Unit = {
  val sb = new StringBuffer()
  map.forEach(new java.util.function.BiConsumer[TopicPartition, OffSetMetadata]() {
    def accept(partition: TopicPartition, offsets: OffSetMetadata ) = {
      sb.append(partition.topic() + "," + partition.partition() + "," + offsets.offset() + "\n")
    }
  })
}

Well the other work-around is to wrap Java-Map into a Scala-Map first and use Scala interface. And of course you can upgrade to Scala 2.12 where this conversion of lambda to BiConsumer is done by the compiler.

answered Jan 23, 2019 at 22:53

SergGr

23.8k2 gold badges34 silver badges52 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user3679686 Over a year ago

You are right, I am indeed using Scala 2.11 , this worked! Thanks for the help!

Collectives™ on Stack Overflow

Iterate over java.util.Map with custom objects in Scala

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related