How to implement a Java abstract interface in Scala

Question

How do you implement a Java abstract interface in scala?

Abstract interface :

public abstract interface KeyIndex<K>
  extends Serializable
{
  public abstract long toIndex(K paramK);

  public abstract Seq<Tuple2<Object, Object>> indexRanges(Tuple2<K, K> paramTuple2);
}

It seems you are attempting to convert Java to Scala? The closest equivalent to an interface is a trait - give that a try. — StuartLC
– StuartLC, Commented Mar 9, 2016 at 6:05
You can leave out the abstract keyword from your interface and method declarations, an interface and its methods are always abstract. — Jesper
– Jesper, Commented Mar 9, 2016 at 10:40

Meiko Rachimow · Accepted Answer · 2016-03-09 08:20:28Z

1

It seems, that your KeyIndex class is written in Java. In Scala every field not labeled private or protected is public. There is no public keyword in Scala. But implementing a Java class in Scala is possible:

class KeyIndexImpl extends KeyIndex[geotrellis.spark.SpatialKey]{
  override def toIndex(paramK: geotrellis.spark.SpatialKey): Long = 
    1l

  override def indexRanges(paramTuple2: (geotrellis.spark.SpatialKey, geotrellis.spark.SpatialKey)): Seq[(AnyRef, AnyRef)] = 
    Seq((paramTuple2, paramTuple2))
}

edited Mar 9, 2016 at 8:20

answered Mar 9, 2016 at 6:09

Meiko Rachimow

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7 Comments

prasannads Over a year ago

thank you. When i implement toIndex , i get the output of type "long" . from the error, type mismatch; found : Long required: geotrellis.spark.io.index.KeyIndex[geotrellis.spark.SpatialKey] , i should get an output of type geotrellis.spark.io.index.KeyIndex

Meiko Rachimow Over a year ago

yes in my example i used Int as the Type Parameter, you have to use geotrellis.spark.SpatialKey. i edited my answer for you

prasannads Over a year ago

i get compilation errors by doing the above way. [ERROR] C:\Users\prasanna.s\workspace\RasterDataAnalysis\src\main\scala\RasterDataPOC\RasterDataAnalysis\ReadTiff.scala:24: error: overriding method toIndex in trait KeyIndex of type (key: geotrellis.spark.SpatialKey)Long; method toIndex has incompatible type [ERROR] override def toIndex(paramK: SpatialKey) : KeyIndex[SpatialKey] ^ [ERROR] C:\Users\prasanna.s\workspace\RasterDataAnalysis\src\main\scala\RasterDataPOC\RasterDataAnalysis\ReadTiff.scala:23: error: class KeyIndexImpl needs to be abstract, since:

Meiko Rachimow Over a year ago

override def toIndex(paramK: SpatialKey) : KeyIndex[SpatialKey] should be override def toIndex(paramK: SpatialKey): Long, the error is saying, that the return type of the implementation of toIndex not matches the definition in the base class (java)

prasannads Over a year ago

I tried that , but the return type becomes long instead of SpatialKey -- type mismatch; found : Long required: geotrellis.spark.io.index.KeyIndex[geotrellis.spark.SpatialKey]

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