I want to get the type of a variable at runtime

Question

I want to get the type of a variable at runtime  How do I do this

User · Answer

I think the question is incomplete  if you meant that you wish to get the type information of some typeclass then below    If you wish to print as you have specified then   scala gt   def manOf T  Manifest  t  T   Manifest T    manifest T  manOf   T  t  T  implicit evidence 1  Manifest T  Manifest T   scala gt  val x   List 1 2 3  x  List Int    List 1  2  3   scala gt  println manOf x   scala collection immutable List Int    If you are in repl mode then   scala gt   type List 1 2 3  List Int    Or if you just wish to know what the class type then as  monkjack explains  string  getClass might solve the purpose

User · Answer

If by the type of a variable you mean the runtime class of the object that the variable points to  then you can get this through the class reference that all objects have   val name    sam   name  java lang String   sam name getClass res0  java lang Class      class java lang String   If you however mean the type that the variable was declared as  then you cannot get that  Eg  if you say   val name  Object    sam    then you will still get a String back from the above code

User · Answer

So  strictly speaking  the  type of a variable  is always present  and can be passed around as a type parameter  For example   val x   5 def f T  v  T    v f x     T is Int  the type of x   But depending on what you want to do  that won t help you  For instance  may want not to know what is the type of the variable  but to know if the type of the value is some specific type  such as this   val x  Any   5 def f T  v  T    v match     case    Int      gt   Int    case    String   gt   String    case             gt   Unknown    f x    Here it doesn t matter what is the type of the variable  Any  What matters  what is checked is the type of 5  the value  In fact  T is useless -- you might as well have written it def f v  Any  instead  Also  this uses either ClassTag or a value s Class  which are explained below  and cannot check the type parameters of a type  you can check whether something is a List     List of something   but not whether it is  for example  a List Int  or List String    Another possibility is that you want to reify the type of the variable  That is  you want to convert the type into a value  so you can store it  pass it around  etc  This involves reflection  and you ll be using either ClassTag or a TypeTag  For example   val x  Any   5 import scala reflect ClassTag def f T  v  T  implicit ev  ClassTag T     ev toString f x     returns the string  Any    A ClassTag will also let you use type parameters you received on match  This won t work   def f A  B  a  A  b  B    a match     case    B   gt   A is a B    case     gt   A is not a B      But this will   val x    c  val y   5 val z  Any   5 import scala reflect ClassTag def f A  B  ClassTag  a  A  b  B    a match     case    B   gt   A is a B    case     gt   A is not a B    f x  y     A  Char  is not a B  Int  f x  z     A  Char  is a B  Any    Here I m using the context bounds syntax  B   ClassTag  which works just like the implicit parameter in the previous ClassTag example  but uses an anonymous variable   One can also get a ClassTag from a value s Class  like this   val x  Any   5 val y   5 import scala reflect ClassTag def f a  Any  b  Any        val B   ClassTag b getClass    ClassTag a getClass  match       case B   gt   a is the same class as b      case     gt   a is not the same class as b        f x  y     f y  x     true  a is the same class as b   A ClassTag is limited in that it only covers the base class  but not its type parameters  That is  the ClassTag for List Int  and List String  is the same  List  If you need type parameters  then you must use a TypeTag instead  A TypeTag however  cannot be obtained from a value  nor can it be used on a pattern match  due to JVM s erasure   Examples with TypeTag can get quite complex -- not even comparing two type tags is not exactly simple  as can be seen below   import scala reflect runtime universe TypeTag def f A  B  a  A  b  B  implicit evA  TypeTag A   evB  TypeTag B     evA    evB type X   Int val x  X   5 val y   5 f x  y     false  X is not the same type as Int   Of course  there are ways to make that comparison return true  but it would require a few book chapters to really cover TypeTag  so I ll stop here   Finally  maybe you don t care about the type of the variable at all  Maybe you just want to know what is the class of a value  in which case the answer is rather simple   val x   5 x getClass    int -- technically  an Int cannot be a class  but Scala fakes it   It would be better  however  to be more specific about what you want to accomplish  so that the answer can be more to the point

User · Answer

i have tested that and it worked  val x   9 def printType T  x T   Unit    println x getClass toString

[scala] I want to get the type of a variable at runtime

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