从Scala代码使用Google番石榴时出现编译器错误 [英] compiler error when using Google guava from scala code
问题描述
我正在使用Scala代码中的Google Guava。当我尝试将Int用作示例中的键类型时,就会出现问题:
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"
}
}
)
这似乎很好,但是所创建对象的推断类型为 LoadingCache [Int with AnyRef,String] :
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"
}
}
)
当我尝试获取错误时会发生错误这样的元素:
And the error occurs when I m trying to get an element like in this example:
cache.get(1)
Scala编译器错误:
Scala compiler error:
[ERROR] error: type mismatch;
[INFO] found : Int(1)
[INFO] required: Int
[INFO] cache.get(1)
[INFO] ^
有人可以指出为什么出现这种错误以及我在做什么错吗?
Can someone point me out why such error appears and what I m doing wrong?
ENV:
- Google Guava 15.0
- Scala 2.11.5
推荐答案
在 1
上不是 Int with AnyRef
问题中的编译错误与番石榴无关。此代码段在此处产生相同的错误:
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))
给予
error: type mismatch;
found : Int(3)
required: Int
这是由<$ c引起的$ c> Int with AnyRef :因为 Int
是 AnyVal
的子类型,所以交集 Int with AnyRef
为空,根本就不存在该类型的实例。
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.
问题是,当您调用 .build()
时,scala编译器无法找到将用作 .build [Int,String]
,因为没有拆箱整数的版本。因此,相反,编译器推断 .build [带有Int,String的AnyRef]
,并构建了无法使用的缓存结构。
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.
为避免这种情况,请使用 java.lang.Integer
代替 Int
。这是在guava 15.0 scala 2.11上编译并运行的:
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))
它应该与Scala的 Int无缝运行
,因为scala会将 Int
s自动装箱到 java.lang.Integer
中。
It should work seamlessly with scala's Int
, because scala autoboxes Int
s into java.lang.Integer
anyway.
类似错误的答案:
- Scala Guava type mismatch issue
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