Implementing a java interface in Scala

Question

Dear All,

I have the following code for building a cache using google collections:

val cache = new MapMaker().softValues().expiration(30,
TimeUnit.DAYS).makeComputingMap(
   new com.google.common.base.Function[String,Int] {
      def apply(key:String):Int ={
        1
     }
   })

And I am getting the following error message:

error: type mismatch;
 found   : java.lang.Object with
com.google.common.base.Function[java.lang.String,Int]{ ... }
 required: com.google.common.base.Function[?, ?]
   new com.google.common.base.Function[String,Int] {
   ^

I am wondering why the types don't match ?

The actual code is:

import com.google.common.collect.MapMaker
trait DataCache[V] {
  private val cache = new MapMaker().softValues().makeComputingMap(
    new com.google.common.base.Function[String,V] {
      def apply(key:String):V = null.asInstanceOf[V]
    })
  def get(key:String):V = cache.get(key)
}

Kind regards, Ali

PS - I am using google-collections v1

Answer 1

Does the following works?

new com.google.common.base.Function[_,_] {

If that doesn't work, you may wish to keep the declaration as it is right now, and then add a : com.google.common.base.Function[_, _] after it, like this:

val cache = new MapMaker().softValues().expiration(30,
TimeUnit.DAYS).makeComputingMap(
   new com.google.common.base.Function[String,Int] {
      def apply(key:String):Int ={
        1
     }
   }: com.google.common.base.Function[_, _])

I have heard that some Google stuff use raw types, which are rather hard to integrate well with Scala. And, in fact, should be banished back to hell, where they came from, but that's just imho.

Also, if you could compile that with -explaintypes, we may get a better notion of what is failing.

Answer 2

You need to supply type parameters to the final method call. You are going through the raw type interface and scala cannot reconstruct the type information.

val cache = new MapMaker().softValues().expiration(30,
TimeUnit.DAYS).makeComputingMap[String, Int](
   new com.google.common.base.Function[String,Int] {
      def apply(key:String):Int ={
        1
     }
   })