compiler error when using Google guava from scala code

问题

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()
    .maximumSize(2)
    .expireAfterWrite(24, TimeUnit.HOURS)
    .build(
      new CacheLoader[Int, String] {
        def load(path: Int): String = {
          path + "hello"
        }
      }
    )

It seems to be fine, but the inferred type of created object is LoadingCache[Int with AnyRef, String]:

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

And the error occurs when I m trying to get an element like in this example:

cache.get(1)

Scala compiler error:

[ERROR] error: type mismatch;
[INFO]  found   : Int(1)
[INFO]  required: Int
[INFO]   cache.get(1)
[INFO]             ^

Can someone point me out why such error appears and what I m doing wrong?

ENV:

Google Guava 15.0
Scala 2.11.5

回答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.

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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