Is it possible to create a Class object purely from a generic parameter? For example:
class myclass[T] {
def something(): Class[_ <: T] =
classOf[T] //this doesn't work
}
Since the type will have been erased at runtime, it seems like this a job for manifests, but I haven't found an example that demonstrates this particular usage. I tried the following, but it doesn't work either:
class myclass[T] {
def something()(implicit m: Manifest[T]): Class[_ <: T] =
m.erasure //this doesn't work
}
I suspect this failure is due to, as the API points out, there is no subtype relationship between the type of m.erasure
's result and T
.
EDIT: I'm not really interested in what the type T
is, I just need an object of type Class[_ <: T]
to pass to a method in the hadoop framework.
Any pointers?
To use a generic class, put the type in the square brackets in place of A . Class Apple and Banana both extend Fruit so we can push instances apple and banana onto the stack of Fruit . Note: subtyping of generic types is *invariant*.
Generics in Java is similar to templates in C++. The idea is to allow type (Integer, String, … etc and user defined types) to be a parameter to methods, classes and interfaces. For example, classes like HashSet, ArrayList, HashMap, etc use generics very well. We can use them for any type.
Most Scala generic classes are collections, such as the immutable List, Queue, Set, Map, or their mutable equivalents, and Stack. Collections are containers of zero or more objects. We also have generic containers that aren't so obvious at first.
A classOf[T] is a value of type Class[T] . In other words, classOf[T]: Class[T] . For example: scala> val strClass = classOf[String] strClass: Class[String] = class java. lang. String scala> :t strClass Class[String]
def myClassOf[T:ClassTag] = implicitly[ClassTag[T]].runtimeClass
You can cast the result of m.erasure
to a Class[T]
:
class myclass[T] {
def something()(implicit m: Manifest[T]): Class[T] =
m.erasure.asInstanceOf[Class[T]]
}
This works fine for basic (non-generic) types:
scala> new myclass[String]().something()
res5: Class[String] = class java.lang.String
But note what happens if I use an instantiated type constructor like List[String]
for T
:
scala> new myclass[List[String]]().something()
res6: Class[List[String]] = class scala.collection.immutable.List
Due to erasure, there is only one Class
object for all the possible instantiations of a given type constructor.
I'm not sure why Manifest[T].erasure
returns Class[_]
instead of Class[T]
, but if I had to speculate, I would say it's to discourage you from using the methods on Class
which allow you to compare two classes for equality or a subtype relationship, since those methods will give you wrong answers when the Class
is parameterized with an instantiated generic type.
For example,
scala> classOf[List[String]] == classOf[List[Int]]
res25: Boolean = true
scala> classOf[List[String]].isAssignableFrom(classOf[List[Int]])
res26: Boolean = true
These results might surprise you and/or lead to a bug in your program. Instead of comparing classes this way, you should normally just pass around Manifest
s instead and compare them, since they have more information*:
scala> manifest[List[String]] == manifest[List[Int]]
res27: Boolean = false
scala> manifest[List[String]] >:> manifest[List[Int]]
res28: Boolean = false
As I understand it, Manifest
s are meant to supersede Class
es for most use cases... but of course, if you're using a framework that requires a Class
, there's not much choice. I would suppose that the imposition of casting the result of erasure
is just a sort of "acknowledgement of liability" that you're using an inferior product at your own risk :)
* Note that, as the documentation for Manifest says, these manifest comparison operators "should be considered approximations only, as there are numerous aspects of type conformance which are not yet adequately represented in manifests."
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