The question is best explained by an example:
In Java for a JPA EntityManager, I can do the following(Account is my Entity class):
Account result = manager.find(Account.class, primaryKey);
In Scala, my naive attempt is:
val result = manager.find(Account.class, primaryKey)
But when I try to use Account.class
in Scala, it seems to not like this. How can I specify the java.lang.Class object for the Account class in Scala?
According to "The Scala Type System",
val c = new C
val clazz = c.getClass // method from java.lang.Object
val clazz2 = classOf[C] // Scala method: classOf[C] ~ C.class
val methods = clazz.getMethods // method from java.lang.Class<T>
The
classOf[T]
method returns the runtime representation for a Scala type. It is analogous to the Java expressionT.class
.
UsingclassOf[T]
is convenient when you have a type that you want information about, whilegetClass
is convenient for retrieving the same information from an instance of the type.
However, classOf[T]
and getClass
return slightly different values, reflecting the effect of type erasure on the JVM, in the case of getClass.
scala> classOf[C]
res0: java.lang.Class[C] = class C
scala> c.getClass
res1: java.lang.Class[_] = class C
That is why the following will not work:
val xClass: Class[X] = new X().getClass //it returns Class[_], nor Class[X]
val integerClass: Class[Integer] = new Integer(5).getClass //similar error
There is a ticket regarding the return type of getClass
.
(James Moore reports that the ticket is "now", ie Nov. 2011, two years later, fixed.
In 2.9.1, getClass
now does:
scala> "foo".getClass
res0: java.lang.Class[_ <: java.lang.String] = class java.lang.String
)
Back in 2009:
It would be useful if Scala were to treat the return from getClass() as a java.lang.Class[T] forSome { val T : C } where C is something like the erasure of the static type of the expression on which getClass is called
It would let me do something like the following where I want to introspect on a class but shouldn't need a class instance.
I also want to limit the types of classes I want to introspect on, so I use Class[_ <: Foo]. But this prevents me from passing in a Foo class by using Foo.getClass() without a cast.
Note: regarding getClass
, a possible workaround would be:
class NiceObject[T <: AnyRef](x : T) {
def niceClass : Class[_ <: T] = x.getClass.asInstanceOf[Class[T]]
}
implicit def toNiceObject[T <: AnyRef](x : T) = new NiceObject(x)
scala> "Hello world".niceClass
res11: java.lang.Class[_ <: java.lang.String] = class java.lang.String