compiler error when using Google guava from scala code

Question

I m using Google Guava from a scala code. And an issue occurs when I m trying to use Int as a key type like in the example:

CacheBuilder.newBuilder()
    .maxim         


        

Answer 1

On 1 not being an Int with AnyRef

The compile error in your question doesn't have anything to do with Guava. This snippet here produces the same error:

val h = new scala.collection.mutable.HashMap[Int with AnyRef, String]
h(3) = "hello"
println("Get 3: " + h.get(3))

gives

error: type mismatch;
found   : Int(3)
required: Int

This is caused by the Int with AnyRef: since Int is subtype of AnyVal, the intersection Int with AnyRef is empty, there simply cannot exist any instances with that type.

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Now to the root cause of the problem.

The problem is that when you call .build(), the scala compiler cannot find a version that would work as .build[Int, String], because there is no version for unboxed integers. So instead, the compiler infers .build[AnyRef with Int, String], and builds an unusable cache structure.

To avoid this, use java.lang.Integer instead of Int. This here compiles and runs with guava 15.0 scala 2.11:

import com.google.common.cache._
import java.util.concurrent._

val cache: LoadingCache[java.lang.Integer, String] = CacheBuilder.newBuilder()
  .maximumSize(2)
  .expireAfterWrite(24, TimeUnit.HOURS)
  .build[java.lang.Integer, String](
    new CacheLoader[java.lang.Integer, String] {
      def load(path: java.lang.Integer): String = {
        path + "hello"
      }
    }
  )

cache.put(42, "hello, world")
println(cache.get(42))

It should work seamlessly with scala's Int, because scala autoboxes Ints into java.lang.Integer anyway.

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Answers for similar errors:

1. Scala Guava type mismatch issue