Scala - Overriding a Java interface method in a Scala class

Question

I have a Java interface Writer defined as following:

public interface Writer<K, V> {

    Iterator<Product2<K, V>> iterator ();
}

And i am trying to implement this interface in a Scala class ExternalWriter which is as following:

private class ExternalWriter[K, V, C]
  extends Logging
  with Writer[K, V] {

    override def iterator(): Iterator[Product2[K, C]] = {
        partitionedIterator.flatMap(pair => pair._2)
  }
}

But when I try to compile this code, I get an error:

Error: Overriding method iterator in trait SortShuffleFileWriter of type ()Iterator[Product2[K,V]]; method iterator has incompatible type override def iterator(): Iterator[Product2[K, C]] = {

How do I fix this?

The java.util.Iterator interface is separate from the scala.collection.Iterator trait (in addition to the problem @Codebender mentioned). Did you account for that? — Silly Freak
– Silly Freak, Commented Aug 8, 2015 at 12:28

Codebender · Accepted Answer · 2015-08-08 11:02:06Z

2

Why did you change V to C?

Your override method should be,

override def iterator(): Iterator[Product2[K, V]] = {
    partitionedIterator.flatMap(pair => pair._2)

If you want to use C, then you should implement Writer with C as,

with Writer[K, C] {

answered Aug 8, 2015 at 11:02

Codebender

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3 Comments

jaywalker Over a year ago

When I change the override method to override def iterator(): Iterator[Product2[K, V]] I get the following error Error: type mismatch; found : Iterator[Product2[K,C]] required: scala.collection.GenTraversableOnce[Product2[K,V]] partitionedIterator.flatMap(pair => pair._2) ^

Codebender Over a year ago

@HaseebJaved, did you try to change your with instead as I have mentioned.

jaywalker Over a year ago

yes but that lead to all sorts of other errors like: Error:(90, 22) class ExternalWriter needs to be abstract, since method insertAll in trait Writer of type (x$1: Iterator[Product2[K,C]])Unit is not defined (Note that Iterator[Product2[K,C]] does not match Iterator[Product2[K,V]]: their type parameters differ) private[spark] class ExternalWriter[K, V, C]( I guess I have the structure of my program all wrong and may be I should start anew.

Pratyush Das · Accepted Answer · 2018-01-15 09:09:31Z

0

Try replacing Iterator in your scala class with java.util.Iterator as the scala Iterator and the java Iterator are different.

private class ExternalWriter[K, V, C]
  extends Logging
  with Writer[K, V] {

    override def iterator(): java.util.Iterator[Product2[K, C]] = {
        partitionedIterator.flatMap(pair => pair._2)
  }
}

The above would be the modified code.

answered Jan 15, 2018 at 9:09

Pratyush Das

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