Using scala map in Java

Question

I have two files. One is scala and other is java.

Scala file has a function which returns scala immutable map.

Java file wants to use that map as dictionary.

Answer 1

Use the static forwarders from Java. In Scala 2.13 the API is simplified, with overloaded asJava for conversion to Java:

$ javap -cp ~/scala-2.13.0/lib/scala-library.jar scala.jdk.javaapi.CollectionConverters
Compiled from "CollectionConverters.scala"
public final class scala.jdk.javaapi.CollectionConverters {
  public static scala.collection.mutable.Map asScala(java.util.Properties);
  public static  scala.collection.mutable.Map asScala(java.util.Dictionary);
  public static  scala.collection.concurrent.Map asScala(java.util.concurrent.ConcurrentMap);
  public static  scala.collection.mutable.Map asScala(java.util.Map);
  public static  scala.collection.mutable.Set asScala(java.util.Set);
  public static  scala.collection.mutable.Buffer asScala(java.util.List);
  public static  scala.collection.Iterable asScala(java.util.Collection);
  public static  scala.collection.Iterable asScala(java.lang.Iterable);
  public static  scala.collection.Iterator asScala(java.util.Enumeration);
  public static  scala.collection.Iterator asScala(java.util.Iterator);
  public static  java.util.concurrent.ConcurrentMap asJava(scala.collection.concurrent.Map);
  public static  java.util.Map asJava(scala.collection.Map);
  public static  java.util.Dictionary asJavaDictionary(scala.collection.mutable.Map);
  public static  java.util.Map asJava(scala.collection.mutable.Map);
  public static  java.util.Set asJava(scala.collection.Set);
  public static  java.util.Set asJava(scala.collection.mutable.Set);
  public static  java.util.List asJava(scala.collection.Seq);
  public static  java.util.List asJava(scala.collection.mutable.Seq);
  public static  java.util.List asJava(scala.collection.mutable.Buffer);
  public static  java.util.Collection asJavaCollection(scala.collection.Iterable);
  public static  java.lang.Iterable asJava(scala.collection.Iterable);
  public static  java.util.Enumeration asJavaEnumeration(scala.collection.Iterator);
  public static  java.util.Iterator asJava(scala.collection.Iterator);
}

Given a Scala class that returns a Scala Map:

$ jshell --class-path .:~/scala-2.13.0/lib/scala-library.jar 
|  Welcome to JShell -- Version 11.0.3
|  For an introduction type: /help intro

jshell> Map vs = scala.jdk.javaapi.CollectionConverters.asJava(new mytools.MapMaker().m())
vs ==> {greeting=hi, reply=bye}