How do you convert a java.util.Collections.unmodifiableMap to an immutable Scala Map?

Question

I'm trying to convert the parameter map from a ServletRequest to a Scala Map in Scala 2.9.0.1: ``` val params = request.getParameterMap.asInstanceOf[Map[String, Array[String]]] ``` I've imported collection.JavaConversions._, and at runtime this is thrown: ``` java.lang.ClassCastException: java.util.Collections$UnmodifiableMap cannot be cast to scala.collection.immutable.Map ```

Accepted Answer

How about just calling `.toMap` on it?

```
import collection.JavaConversions._
val x = java.util.Collections.unmodifiableMap[Int,Int](new java.util.HashMap[Int,Int]())
val y: Map[Int,Int] = x.toMap //y: Map[Int,Int] = Map()
```

Without calling `toMap`, `JavaConversions` only lets you implicitly convert to a mutable Scala map:

```
scala> val z: collection.mutable.Map[Int,Int] = x
z: scala.collection.mutable.Map[Int,Int] = Map()
```

Presumably this is because a Java `Map` is mutable, so it should only be represented in Scala as a `mutable.Map` until you explicitly convert it to an `immutable.Map`.

Note that when you just say `Map` in Scala, you are really talking about `collection.immutable.Map` since `Predef` aliases `Map` that way:

```
scala> Map()
res0: scala.collection.immutable.Map[Nothing,Nothing] = Map()
```

So when you say `request.getParameterMap.asInstanceOf[Map[String, Array[String]]]`, you are really asking Scala to implicitly convert a Java `Map` into Scala's `collection.immutable.Map`, which it doesn't want to do since Java's `Map` is mutable.

How do you convert a java.util.Collections.unmodifiableMap to an immutable Scala Map?

Solution

Problem