I’m trying to use Scala pattern matching on Java Class[_] (in context of using Java reflection from Scala) but I’m getting some unexpected error. The following gives “unreachable code” on the line with case 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") } }
Any ideas why this is happening ?
Answer
The code works as expected if you change the variable names to upper case (or surround them with backticks in the pattern):
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
In your code the jInteger
in the first pattern is a new variable—it’s not the jInteger
from the surrounding scope. From the 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.
See this question for more information.
Attribution
Source : Link , Question Author : alphageek , Answer Author : Community