scala - Class[_] 类型的模式匹配？

Question

我正在尝试在 Java Class[_] 上使用 Scala 模式匹配(在使用来自 Scala 的 Java 反射的上下文中)，但我遇到了一些意外错误。以下给出了 行上的“无法访问的代码”案例jLong

def foo[T](paramType: Class[_]): Unit = {
  val jInteger = classOf[java.lang.Integer]
  val jLong = classOf[java.lang.Long]
  paramType match {
    case jInteger => println("int")
    case jLong => println("long")
  }
}

任何想法为什么会发生这种情况？

Answer 1

如果您将变量名称更改为大写(或在模式中用反引号将它们括起来)，代码将按预期工作:

scala> def foo[T](paramType: Class[_]): Unit = {
     |   val jInteger = classOf[java.lang.Integer]
     |   val jLong = classOf[java.lang.Long]
     |   paramType match {
     |     case `jInteger` => println("int")
     |     case `jLong` => println("long")
     |   }
     | }
foo: [T](paramType: Class[_])Unit

scala> foo(classOf[java.lang.Integer])
int

在您的代码中 jInteger在第一个模式中是一个新变量——它不是 jInteger从周围的范围。来自 specification :

8.1.1 Variable Patterns

... A variable pattern x is a simple identifier which starts with a lower case letter. It matches any value, and binds the variable name to that value.

...

8.1.5 Stable Identifier Patterns

... To resolve the syntactic overlap with a variable pattern, a stable identifier pattern may not be a simple name starting with a lower-case letter. However, it is possible to enclose a such a variable name in backquotes; then it is treated as a stable identifier pattern.

见 this question了解更多信息。