Question

我有一个脚本。它在没有警告的情况下运行。

$ cat ~/tmp/so1.scala 
import org.yaml.snakeyaml.Yaml

class JavaMapIteratorWrapper[K,V] (map: java.util.Map[K,V]) {
  def foreach (f: Tuple2 [K, V] => Unit): Unit = {
    val iter = map.entrySet.iterator
    while (iter.hasNext) {
      val entry = iter.next
      f (entry.getKey, entry.getValue)
    }
  }
}

implicit def foreachJavaMap[K,V] (map: java.util.Map[K,V]): JavaMapIteratorWrapper[K,V] = new JavaMapIteratorWrapper[K,V](map)

val yaml = new Yaml;
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map [_, _] => {
    for (entry <- map) {
      entry match {
        case ("id", id: String) => System.out.println ("ID is " + id)
        case (n: String, v: String) => System.out.println (n + " = " + v)
      }
    }
  }
}

$ scala -unchecked -classpath jar/snakeyaml-1.7.jar ~/tmp/so1.scala eg/default.yaml
(program output as expected)

我想将循环提取到自己的函数中。所以我试试。

$ cat ~/tmp/so2.scala 
import org.yaml.snakeyaml.Yaml

class JavaMapIteratorWrapper[K,V] (map: java.util.Map[K,V]) {
  def foreach (f: Tuple2 [K, V] => Unit): Unit = {
    val iter = map.entrySet.iterator
    while (iter.hasNext) {
      val entry = iter.next
      f (entry.getKey, entry.getValue)
    }
  }
}

implicit def foreachJavaMap[K,V] (map: java.util.Map[K,V]): JavaMapIteratorWrapper[K,V] = new JavaMapIteratorWrapper[K,V](map)

val processMap = (map: java.util.Map [_, _]) => {
  for (entry <- map) {      // line 16
    entry match {
      case ("id", id: String) => System.out.println ("ID is " + id)
      case (n: String, v: String) => System.out.println (n + " = " + v)
    }
  }
}

val yaml = new Yaml;
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map [_, _] => processMap (map)
}

$ scala -unchecked -classpath jar/snakeyaml-1.7.jar ~/tmp/so2.scala eg/default.yaml
(fragment of so2.scala):16: error: type mismatch;
 found   : map.type (with underlying type java.util.Map[_, _])
 required: java.util.Map[_$1,_$2] where type _$2, type _$1
  for (entry <- map) {
                 ^
one error found
!!!
discarding <script preamble>

循环在其自身的功能中意味着它需要更具体的类型。好。

我会尝试使用java.util.Map [AnyRef, AnyRef]代替java.util.Map [_, _]。

$ cat ~/tmp/so3.scala 
import org.yaml.snakeyaml.Yaml

class JavaMapIteratorWrapper[K,V] (map: java.util.Map[K,V]) {
  def foreach (f: Tuple2 [K, V] => Unit): Unit = {
    val iter = map.entrySet.iterator
    while (iter.hasNext) {
      val entry = iter.next
      f (entry.getKey, entry.getValue)
    }
  }
}

implicit def foreachJavaMap[K,V] (map: java.util.Map[K,V]): JavaMapIteratorWrapper[K,V] = new JavaMapIteratorWrapper[K,V](map)

val processMap = (map: java.util.Map [AnyRef, AnyRef]) => {
  for (entry <- map) {
    entry match {
      case ("id", id: String) => System.out.println ("ID is " + id)
      case (n: String, v: String) => System.out.println (n + " = " + v)
    }
  }
}

val yaml = new Yaml;
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map [AnyRef, AnyRef] => processMap (map)      // line 26
}

$ scala -unchecked -classpath jar/snakeyaml-1.7.jar ~/tmp/so3.scala eg/default.yaml
(fragment of so3.scala):26: warning: non variable type-argument AnyRef in type pattern is unchecked since it is eliminated by erasure
  case map: java.util.Map [AnyRef, AnyRef] => processMap (map)
                       ^
one warning found
!!!
discarding <script preamble>
(program output as expected)

现在它运行了，但它给了我一个警告。如何消除该警告？

注意：

org.yaml.snakeyaml.Yaml是用Java编写的，所以我不能使用类型清单。（我可以吗？）
我真正的程序使用了几个Java库，所以当我对哪些类型做出错误的假设时，我想要收到警告我被给了。但是我如何告诉编译器“是的，我已经检查了这个，这是正确的，不要再次警告我了吗？”
我正在使用scala 2.7.7（因为这是与Ubuntu一起打包的版本）。

Answer 1

您可以尝试删除自定义包装器。（2.8.1）Scala标准库已经包含一个包装器，可以在scala.collection.JavaConverters中更具惯用地使用Java集合类型。 （注意：导入时不需要scala.前缀

我还使processMap成为方法而不是函数，并添加类型参数：

import collection.JavaConverters._

def processMap[K,V](map: Map[K, V]): Unit = {
  for (entry <- map) {
    entry match {
      case ("id", id: String) => System.out.println ("ID is " + id)
      case (n: String, v: String) => System.out.println (n + " = " + v)
    }
  }
}

val yaml = new Yaml
(yaml load (io.Source.fromFile(argv(0)).mkString)) match {
  case map: java.util.Map[_, _] => processMap(map.asScala)
}

请注意倒数第二行的asScala方法......

在处理Java / Scala互操作时，通常最好的做法是尽早从Java转换为Scala集合，并尽可能晚地转换回来。

Answer 2

您必须使用Scala 2.7.X.如果您使用2.8.1，您的Map [_，_]示例工作正常。

如果需要使用2.7.X，请尝试将processMap值转换为方法： def processMap [K，V] =（map：java.util.Map [K，V]）=＆gt; {...} 这似乎为我编译，但请注意我使用YAML库“删除”部分。我用过：

val m1 = new java.util.HashMap[String,String]
m1.put("one", "1")
m1.put("id", "123")
m1.put("two", "2")

m1 match {
  case map: java.util.Map [_, _] => processMap (map)
}

如何摆脱此类警告/错误？

2 个答案: