Scala/IntelliJ: case object X vs case class X vs case class X()

Question

There are case classes in Scala:

case class X(one: One, two: two)

There are case objects which have only one instance (which makes sense):

case object X

When I declare a case class without parameters, IntelliJ does the right thing and deprecation-warns me to make it a case object:

case class X // deprecated stuff

However it does not warn me about

case class X()

Which makes me wonder whether there is a difference between case class X and case class X() ?

Answer 1

I don't know what Scala version you are using, but in 2.11 this is not only deprecated but indeed an error:

 Error:(34, 17) case classes without a parameter list are not allowed;
 use either case objects or case classes with an explicit `()' 
 as a parameter list.
   case class Foo
                 ^

Simply, you cannot pattern match against a no-parameter-list case class, so there is no use for them.

case class Foo()

def test(in: Any) = in match {
  case Foo() => "foo"
  case _     => "other"
}

Answer 2

When you declare a case object, only a single instance is ever created. However, you can have multiple different instances of a no-param case class. Why you might want that is a different matter.

In principle, there is no difference between case class Foo and case class Foo() . I think the former was deprecated because of how easy it is to inadvertently misuse it. This post illustrates the point nicely.