How can I convert Scala Map to Java Map with scala

问题:

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].

回答1:

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

import scala.collection.JavaConverters._
val scalaMap = Map(1 -> 1.0F)
val javaMap = scalaMap.map{ case (k, v) => k -> float2Float(v) }.asJava

回答2:

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

回答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)})

回答4:

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)