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 — pagoda_5b
– pagoda_5b, Commented May 16, 2013 at 12:46

Community · Accepted Answer · 2017-05-23 12:34:17Z

46

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, 2017 at 12:34

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answered May 17, 2013 at 12:17

rvange

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2 Comments

Pulasthi Bandara Over a year ago

For anyone stumbling upon this now, scala.collection.JavaConversions is deprecated since Scala 2.8. Please refer to @cyber4ron 's answer.

Litchy Over a year ago

An important thing to note here is scala.Float is not the same type as java.lang.Float

AdamBat · Accepted Answer · 2019-08-07 08:57:33Z

35

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 7, 2019 at 8:57

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answered Dec 22, 2015 at 18:06

cyber4ron

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Comments

Shrey · Accepted Answer · 2018-10-26 00:59:39Z

14

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, 2018 at 0:59

Shrey

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Comments

senia · Accepted Answer · 2013-05-16 09:22:43Z

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, 2013 at 9:22

senia

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1 Comment

rvange Over a year ago

I did think of converting the data manually, but fortunately a more "automatic" approach is available, see the comment by pagoda_5b

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