scala match Class [X]仅适用于在大小写之外定义的类

Question

I'm working with scala to transform different java types to Spark DataType, so I wrote the following method:

def convertDataType(x : Class[_]): DataType = {
x match {
  case i: Class[Integer] => IntegerType
  case s: Class[String] => StringType
  case d: Class[Double] => DoubleType
  case l: Class[Long] => LongType
  case s: Class[Short] => ShortType
  case b: Class[Byte] => ByteType
  case f: Class[Float] => FloatType
  case ab: Class[Array[Byte]] => BinaryType
  case bl: Class[Boolean] => BooleanType
  case ts: Class[Timestamp] => TimestampType
  case dt: Class[Date] => DateType
  case _ => StringType
}}

However, with an input x (java.lang.String), it breaks out with a IntegerType. I tried another way:

def convertDataType(x : Class[_]): DataType = {
val S = classOf[String]
val I = classOf[Integer]
val D = classOf[Double]
val L = classOf[Long]
val SH = classOf[Short]
val B = classOf[Byte]
val F = classOf[Float]
val AB = classOf[Array[Byte]]
val BL = classOf[Boolean]
val TS = classOf[Timestamp]
val DT = classOf[Date]

x match {
  case I => IntegerType
  case S => StringType
  case D => DoubleType
  case L => LongType
  case SH => ShortType
  case B => ByteType
  case F => FloatType
  case AB => BinaryType
  case BL => BooleanType
  case TS => TimestampType
  case DT => DateType
  case _ => StringType
}}

The second way worked well and return the StringType as expected. I have no idea what's wrong in the first implementation?

Thanks a lot.

Answer 1

Matching : Class[Double] is exactly the same as matching : Class[_] because the generic type parameters aren't available. The compiler actually should warn you about it:

 warning: non variable type-argument Double in type pattern java.lang.Class[Double] is unchecked since it is eliminated by erasure

This also happens for any other generic type except Array s. There are a lot of Stack Overflow questions and articles about this, search for "type erasure".

In the second case, you are doing something entirely different: comparing values instead of checking types. This doesn't run into the above problem.