如何在 Scala 中从 Map[String, Any] 创建 Map[String,String]？

Question

Scala 新手 - 如何创建一个Map[String,String] fromMap[String, Any] 的值Map[String,Any]是字符串，但我不知道如何强制转换或以其他方式将 " Any" 类型强制转换为 " String" 类型。

Answer 1

正如您提到的，地图中的所有值都是字符串，您可以简单地使用asInstanceOf. 如果您的假设不正确，您将收到如下所示的运行时异常：

$ scala
Welcome to Scala version 2.9.2 (OpenJDK 64-Bit Server VM, Java 1.7.0_55).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val m:Map[String, Any] = Map("foo" -> 5, "bar" -> 7.6, "baz" -> "qux")
m: Map[String,Any] = Map(foo -> 5, bar -> 7.6, baz -> qux)

scala> val m2: Map[String, Any] = Map("foo" -> "5", "bar" -> "7.6", "baz" -> "qux")
m2: Map[String,Any] = Map(foo -> 5, bar -> 7.6, baz -> qux)

scala> m2.asInstanceOf[Map[String, String]]
res0: Map[String,String] = Map(foo -> 5, bar -> 7.6, baz -> qux)

当所有值实际上都是 type 时，这是完美的String。

scala> res0("foo")
res5: String = 5

注意你的错误假设：

scala> m.asInstanceOf[Map[String, String]]
res2: Map[String,String] = Map(foo -> 5, bar -> 7.6, baz -> qux)

scala> res2("foo")
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
    at .<init>(<console>:10)
    at .<clinit>(<console>)
    at .<init>(<console>:11)
    at .<clinit>(<console>)
    at $print(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:704)
    at scala.tools.nsc.interpreter.IMain$Request$$anonfun$14.apply(IMain.scala:920)
    at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43)
    at scala.tools.nsc.io.package$$anon$2.run(package.scala:25)
    at java.lang.Thread.run(Thread.java:744)