Pattern matching on Class[_] type?

On your pattern matching, each of these 2 cases try to create place holder names instead of matching the class type as expected.

If you use upper case in the starting character, you'll be fine

def foo[T](paramType: Class[_]): Unit = {
  val JInteger = classOf[Int]
  val JLong = classOf[Long]
  paramType match {
    case JInteger => println("int")
    case JLong => println("long")

scala> foo(1.getClass)

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])

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.