compiler error when using Google guava from scala

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.

---

Answers for similar errors:

1. Scala Guava type mismatch issue