How can I convert Scala Map to Java Map with scala.Float to java.Float k/v conversion

Question

I would like to be able to perform the following, but it fails in the call to useMap. How can I perform this conversion?

scala> import scala.collection.JavaConversions._
import scala.collection.JavaConversions._

scala> import scala.collection.JavaConverters._
import scala.collection.JavaConverters._

scala> def useMap(m: java.util.Map[java.lang.Integer, java.lang.Float]) = m
useMap: (m: java.util.Map[Integer,Float])java.util.Map[Integer,Float]

scala> val v: Map[Int, Float] = Map()
v: Map[Int,Float] = Map()

scala> useMap(v)
<console>:10: error: type mismatch;
 found   : scala.collection.immutable.Map[Int,scala.Float]
 required: java.util.Map[Integer,java.lang.Float]
              useMap(v)
                     ^

It seems to work with Map[String, String], but not Map[Int, Float].

look at [http://stackoverflow.com/questions/6991340/scala-convert-listint-to-a-java-util-listjava-lang-integer](http://stackoverflow.com/questions/6991340/scala-convert-listint-to-a-java-util-listjava-lang-integer) — pagoda_5b, May 16 '13 at 12:46

score 45 · Answer 1 · edited May 23 '17 at 12:34

45

The solution linked to by @pagoda_5b works: Scala convert List[Int] to a java.util.List[java.lang.Integer]

scala> import collection.JavaConversions._
import collection.JavaConversions._

scala> val m = mapAsJavaMap(Map(1 -> 2.1f)).asInstanceOf[java.util.Map[java.lang.Integer, java.lang.Float]]
m: java.util.Map[Integer,Float] = {1=2.1}

scala> m.get(1).getClass
res2: Class[_ <: Float] = class java.lang.Float

edited May 23 '17 at 12:34

Community

1
1

answered May 17 '13 at 12:17

rvange

2,432
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6

For anyone stumbling upon this now, `scala.collection.JavaConversions` is deprecated since Scala 2.8. Please refer to @cyber4ron 's answer. – Pulasthi Bandara May 14 '18 at 11:37
An important thing to note here is `scala.Float` is not the same type as `java.lang.Float` – Litchy Dec 08 '21 at 06:46

score 32 · Accepted Answer · edited Aug 07 '19 at 08:57

32

Use scala predefined float2Float and use CollectionConverters to perform conversion explicitly.

import scala.jdk.CollectionConverters._
// Prior to Scala 2.13: import scala.collection.JavaConverters._
val scalaMap = Map(1 -> 1.0F)
val javaMap = scalaMap.map{ case (k, v) => k -> float2Float(v) }.asJava

edited Aug 07 '19 at 08:57

AdamBat

83
1
5

answered Dec 22 '15 at 18:06

cyber4ron

468
5
7

score 11 · Answer 3 · answered Oct 26 '18 at 00:59

11

Implicit conversions are sometimes hard to debug/understand and therefore I prefer explicit conversions as follows:

import scala.collection.JavaConversions.mapAsJavaMap

val scalaMap = Map(1 -> 1.0F)
val javaMap = mapAsJavaMap(scalaMap)

answered Oct 26 '18 at 00:59

Shrey

2,374
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score 3 · Answer 4 · answered May 16 '13 at 09:22

3

scala> v.asJava
<console>:16: error: type mismatch;
 found   : java.util.Map[Int,scala.Float]
 required: java.util.Map[Integer,java.lang.Float]

The type of v is Map[scala.Int, scala.Float], not Map[java.lang.Integer, java.lang.Float].

You could try this:

import java.lang.{Float => JFloat}
useMap(v.map{ case (k, v) => (k: Integer) -> (v: JFloat)})

answered May 16 '13 at 09:22

senia

37,745
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I did think of converting the data manually, but fortunately a more "automatic" approach is available, see the comment by pagoda_5b – rvange May 17 '13 at 12:20

How can I convert Scala Map to Java Map with scala.Float to java.Float k/v conversion

4 Answers4

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