Scala Equivalent of Java Java.Lang.Class<T> Object

Scala equivalent of Java java.lang.ClassT Object

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 expression T.class.

Using classOf[T] is convenient when you have a type that you want information about, while getClass 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

Convert class parameter from Java to Scala

The equivalent of Java's T.class in Scala would be classOf[T]:

val connection = DriverManager.getConnection("jdbc: ...")
val olapConnection = connection.unwrap(classOf[OlapConnection])

Note: not tested as I'm not sure what are your dependencies.

What is the Java equivalent of a Scala object?

Support for singletons is not on a language level, but the language provides enough facilities to create them without any trouble.

Consider the following code:

public class Singleton {
    private static final Singleton instance = new Singleton();

    // Private constructor prevents instantiation from other classes
    private Singleton() {}

    public static Singleton getInstance() {
        return instance;
    }
}

This is an example from Wikipedia, which explains how a singleton can be made. An instance is kept in a private field, constructor is inaccessible outside the class, the method returns this single instance.

As for constructors: every class by default has a so-called default constructor which takes no arguments and simply calls the no-args constructor of the superclass. If the superclass doesn't have any accessible constructor without arguments, you will have to write an explicit constructor.

So a class must have a constructor, but you don't have to write it if the superclass has a no-args constructor.

What is the Scala equivalent of Java's ClassName.class?

There is a method classOf in scala.Predef that retrieves the runtime representation of a class type.

val stringClass = classOf[String]

You can use the getClass method to get the class object of an instance at runtime in the same manner as Java

scala> val s = "hello world"
s: String = hello world

scala> s.getClass
res0: Class[_ <: String] = class java.lang.String

Calling Java Generic Typed Method from Scala gives a Type mismatch error : Scala

Welcome to SO.

This seems to compile, but I am not sure if it works with the real implementation of JavaClass, but I am pretty sure it should.

import scala.reflect.ClassTag

object ScalaClass {
  def toArray[T <: AnyRef](coll: java.util.Collection[T])(implicit ct: ClassTag[T]): Array[T] =
    JavaClass.toArray[T](ct.runtimeClass.asInstanceOf[Class[T]], coll)
}

To call it from Scala you just need to pass the java collection, it would get the correct class from the type, like this:

val col: java.util.Collection[String] = ???

val array: Array[String] = ScalaClass.toArray(col)

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Edit

If you want to preserve the original signature, you have to do this:

object ScalaClass {
  def toArray[T <: AnyRef](t: java.lang.Class[T], coll: java.util.Collection[T]): Array[T] =
    JavaClass.toArray[T](t, coll)
}

Which you can use like this:

val col: java.util.Collection[String] = ???

val array: Array[String] = ScalaClass.toArray(classOf[String], col)

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In both cases the trick is the <: AnyRef type bound.

IMHO, the first version is more ergonomic to be used on idiomatic Scala code.

How to get an java.lang.Class of a scala object for an annotation argument

classOf[MyObject$] does not work because there is no type called MyObject$.

In fact, the issue did come up before and there is no easy solution. See the discussion on https://lampsvn.epfl.ch/trac/scala/ticket/2453#comment:5

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