Convert java.util.HashMap to scala.collection.immutable.Map in java

Question

I'm using some Scala library from my Java code. And I have a problem with collections. I need to pass scala.collection.immutable.Map as a parameter of a method. I can convert or build immutable.Map from my Java code but I do not know how to do it. Suggestions?

Answer 1

It's entirely possible to use JavaConverters in Java code—there are just a couple of additional hoops to jump through:

import java.util.HashMap;
import scala.Predef;
import scala.Tuple2;
import scala.collection.JavaConverters;
import scala.collection.immutable.Map;

public class ToScalaExample {
  public static <A, B> Map<A, B> toScalaMap(HashMap<A, B> m) {
    return JavaConverters.mapAsScalaMapConverter(m).asScala().toMap(
      Predef.<Tuple2<A, B>>conforms()
    );
  }

  public static HashMap<String, String> example() {
    HashMap<String, String> m = new HashMap<String, String>();
    m.put("a", "A");
    m.put("b", "B");
    m.put("c", "C");
    return m;
  }
}

We can show that this works from the Scala REPL:

scala> val jm: java.util.HashMap[String, String] = ToScalaExample.example
jm: java.util.HashMap[String,String] = {b=B, c=C, a=A}

scala> val sm: Map[String, String] = ToScalaExample.toScalaMap(jm)
sm: Map[String,String] = Map(b -> B, c -> C, a -> A)

But of course you could just as easily call these methods from Java code.