Scala Map to Java HashMap

Question

I have a Scala function f(s1: String, s2: String): Map[String,String]

I want to allow a colleague coding in Java to call a Java method I'm writing:

HashMap<String, String> f(String s1, String s2)

This calls my Scala function. What I've found, on the Java side, is that Scala has returned a scala.collection.immutable.Map.

How do I make a Java HashMap out of it? Or should I be doing something else?

possible duplicate of Convert Scala Set into Java (java.util.Set)? — om-nom-nom
– om-nom-nom, Commented Apr 29, 2014 at 14:19
Although Set was mentioned in that question, in fact the same goes for most Scala -> Java collections. — om-nom-nom
– om-nom-nom, Commented Apr 29, 2014 at 14:19

Community · Accepted Answer · 2017-05-23 12:26:10Z

10

I think what you are looking for is here. Specifically the mapAsJavaMap method.

Where the preferred usage is explained in this SO question Using imported implicits and asJava

edited May 23, 2017 at 12:26

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answered Apr 28, 2014 at 20:45

Justin Pihony

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5 Comments

gknauth Over a year ago

Thanks, Justin. That got me on the right path. Over on the Scala side I did this: val im = f(s1,s2); val mm = collections.mutable.Map(im:toSeq: _*); JavaConversions.mapAsJavaMap(mm) and that resulted in a mutable map on the Java side (which I think is what Java was looking for, and my map was immutable).

som-snytt Over a year ago

Actually, you want import JavaConverters._ and m.asJava. This is common wisdom and must have a duplicate question if someone could find it. The style guide is to use converters and explicitly ask for the conversion.

Travis Brown Over a year ago

Yeah, sorry, but I'm with som-snytt—this is essentially a link-only answer that suggests an unidiomatic use of a package that most Scala developers agree should be avoided.

Justin Pihony Over a year ago

@som-snytt Sorry, I was relying on the built-in documentation too much. Edited to be more specific.

gknauth Over a year ago

Thanks, som-snytt, I'll follow your suggestion going forward.

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