I want to get the type of a variable at runtime
Solution 1
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 => "Int"
case _: String => "String"
case _ => "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 => "A is a B"
case _ => "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 => "A is a B"
case _ => "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 => "a is the same class as b"
case _ => "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.
Solution 2
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> def manOf[T: Manifest](t: T): Manifest[T] = manifest[T]
manOf: [T](t: T)(implicit evidence$1: Manifest[T])Manifest[T]
scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)
scala> println(manOf(x))
scala.collection.immutable.List[Int]
If you are in repl mode then
scala> :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
Solution 3
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.
Solution 4
i have tested that and it worked
val x = 9
def printType[T](x:T) :Unit = {println(x.getClass.toString())}
Solution 5
instance.getClass
above method return path of instance's Class. to get the exact Class name of the instance, try this:
instance.getClass.getSimpleName
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Comments
-
Incerteza almost 3 years
I want to get the type of a variable at runtime. How do I do this?
-
Readren almost 8 yearsThe example code you wrote after "But this will:" is confusing. It compiles but the result is not the one you show in the comments. Both calls return the same result: "A is a B". Because the value
5
is both an instance ofInt
and an instance ofAny
. Apart from that, your explanation was perfect :) -
Daniel C. Sobral almost 8 years@Readren The value isn't tested for, the class is.
Int
isAny
, butAny
is notInt
. It works on Scala 2.10, and it should work on Scala 2.11, and I don't know why it isn't. -
Readren almost 8 yearsIt scares me to contradict an eminence like you, but the code
a match { case _: B => ...
test the type of the actual value of the variablea
, not the type of the variablea
. You are right in that it returns what you say in scala 2.10.6. But it should be a bug. In scala 2.11.8 the type of the actual value is tested, as it should. -
David Arenburg over 7 yearsYou can also do
name.getClass.getSimpleName
for a more readable output -
marcin_koss about 7 yearsVery nice coverage on differences between ClassTag and TypeTag, just what I was looking for.
-
ChiMo almost 6 yearsIs there a way to null check this?
-
Peter Krauss over 4 yearsfor readers: this is the most useful solution. As in Javascript
typeof x
, heremanOf(x)
say the data type! -
Ravinder Payal about 3 yearsIt doesn't tell the sub types of class. As mentioned above, for both List[String] and List[int] , it will return same thing: List