How to convert a Java Map to a Scala Map using JavaConverters

Here’s a quick look at how to convert a Java Map (such as HashMap) to a Scala Map:

// import what you need
import java.util._
import scala.collection.JavaConverters._

// create and populate a java map
val jMap = new HashMap[String, String]()
jMap.put("first_name", "Alvin")
jMap.put("last_name",  "Alexander")

// convert the java map to a scala map
val sMap = jMap.asScala

The key there is knowing that importing the JavaConverters object creates a method named asScala on the Java Map, and that method creates a Scala Map from the Java Map.

A second key is knowing that the JavaConversions object is deprecated, and you should use JavaConverters instead of JavaConversions.

Example in the REPL

Here’s what that sequence looks like in the Scala REPL:

scala> import java.util._
import java.util._

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

scala> val jMap = new HashMap[String, String]()
jMap: java.util.HashMap[String,String] = {}

scala> jMap.put("first_name", "Alvin")
res3: String = null

scala> jMap.put("last_name",  "Alexander")
res4: String = null

scala> val sMap = jMap.asScala
sMap: scala.collection.mutable.Map[String,String] = 
    Map(last_name -> Alexander, first_name -> Alvin)

toMap

As shown, that creates a Scala mutable Map. If you need an immutable Map, just call toMap (which is a normal Scala method on a mutable Map, and has nothing to do with JavaConverters):

scala> val iMap = sMap.toMap
iMap: scala.collection.immutable.Map[String,String] = 
    Map(last_name -> Alexander, first_name -> Alvin)

asJava

The JavaConverters object also lets you convert a Scala Map to a Java Map using asJava:

scala> val jMap2 = iMap.asJava
jMap2: java.util.Map[String,String] = {last_name=Alexander, first_name=Alvin}

The Scala JavaConverters object and Maps

The JavaConverters scaladoc shows that you can make the following conversions to/from Java and Scala map-related types using asScala, asJava, and asJavaDictionary:

// asScala and asJava
scala.collection.mutable.Map    <=> java.util.Map
scala.collection.concurrent.Map <=> java.util.concurrent.ConcurrentMap

// asScala, asJavaDictionary
scala.collection.mutable.Map <=> java.util.Dictionary

// asJava
scala.collection.Map => java.util.Map

I’ll write more about the Scala JavaConverters object over time, but until then, I hope this example is helpful.

The Scala Cookbook

This tutorial is sponsored by the Scala Cookbook, which I wrote for O’Reilly. You can find the Scala Cookbook here on Amazon.com.

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