apache spark sql - Scala extraction/pattern matching using a companion object -

the below code taken spark sql. performs extraction, casttype companion object - 1 call typecast.castto("abc", stringtype). please explain how pattern matching works companion objects under hood?

private[csv] def castto(datum: string, casttype: datatype): = {                                    casttype match {                                                                                       case _: bytetype => datum.tobyte                                                                     case _: shorttype => datum.toshort     case _: integertype => datum.toint     case _: longtype => datum.tolong     case _: floattype => datum.tofloat     case _: doubletype => datum.todouble                                                                 case _: booleantype => datum.toboolean                                                               case _: decimaltype => new bigdecimal(datum.replaceall(",", ""))                                     case _: timestamptype => timestamp.valueof(datum)                                                    case _: datetype => date.valueof(datum)                                                              case _: stringtype => datum     case _ => throw new runtimeexception(s"unsupported type: ${casttype.typename}")                    }  } 

added: based on understanding extractin/pattern matching implemented unapply method of companion object. example of how case classes implemented under hood

 trait user {   def name: string } class freeuser(val name: string) extends user class premiumuser(val name: string) extends user  object freeuser {   def unapply(user: freeuser): option[string] = some(user.name) } object premiumuser {   def unapply(user: premiumuser): option[string] = some(user.name) } 

i don't understand how same thing can done companion classes

i have never worked spark, cannot answer specific spark knowledge.

but pattern matching in example seems ordinary type based pattern matching.

casttype instance of type datatype has several subclasses e.g. bytetype. pattern matching tests whether casttype object belongs specific class (e.g. bytetype).

because there singleton instances of several sub-types, matching not need provide matches instance @ all. why match uses placeholder _.