[英]Scala/IntelliJ: case object X vs case class X vs case class X()
There are case classes in Scala: Scala中有案例类:
case class X(one: One, two: two)
There are case objects which have only one instance (which makes sense): 有些案例对象只有一个实例(这很有意义):
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: 当我声明不带参数的case类时,IntelliJ做正确的事并弃用警告我使其成为case对象:
case class X // deprecated stuff
However it does not warn me about 但是它并没有警告我
case class X()
Which makes me wonder whether there is a difference between case class X
and case class X()
? 这使我想知道
case class X
和case class X()
之间是否有区别?
I don't know what Scala version you are using, but in 2.11 this is not only deprecated but indeed an error: 我不知道您使用的是什么Scala版本,但是在2.11中,这不仅被弃用,而且确实是一个错误:
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. 简而言之,您无法针对无参数列表案例类进行模式匹配,因此它们没有用。
case class Foo()
def test(in: Any) = in match {
case Foo() => "foo"
case _ => "other"
}
When you declare a case object, only a single instance is ever created. 声明案例对象时,只会创建一个实例。 However, you can have multiple different instances of a no-param case class.
但是,无参数案例类可以有多个不同的实例。 Why you might want that is a different matter.
为什么您会想要那是另一回事。
In principle, there is no difference between case class Foo
and case class Foo()
. 原则上,
case class Foo
和case class Foo()
之间没有区别。 I think the former was deprecated because of how easy it is to inadvertently misuse it. 我认为前者已被弃用,因为不经意间滥用它很容易。 This post illustrates the point nicely.
这篇文章很好地说明了这一点。
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