Scala：case类构造函数不起作用时的默认值

Question

I'm creating a case class with default-valued constructor:

abstract class Interaction extends Action
case class Visit(val url: String)(val timer: Boolean = false) extends Interaction

But I cannot create any of its instance without using all of its parameters, for example. If I write:

Visit("https://www.linkedin.com/")

The compiler will complain:

missing arguments for method apply in object Visit;
follow this method with `_' if you want to treat it as a partially applied function
[ERROR]     Visit("http://www.google.com")

What do I need to do to fix it?

Answer 1

You need to tell the compiler that this is not a partially applied function, but that you want the default values for the second set of parameters. Just open and close paranthesis...

scala> Visit("https://www.linkedin.com/")()
res1: Visit = Visit(https://www.linkedin.com/)

scala> res1.timer
res2: Boolean = false

EDIT to explain @tribbloid comment.

If you use _, instead of creating a visit you are creating a partially applied function which then can be use to create a Visit object:

val a = Visit("asdsa")_ // a is a function that receives a boolean and creates and Visit
a: Boolean => Visit = <function1> 

scala> val b = a(true) // this is equivalent to val b = Visit("asdsa")(true)
b: Visit = Visit(asdsa)

Answer 2

请更正在case类中指定可选字段的语法，如下所示

case class Visit(val url: String,val timer: Boolean = false) extends Interaction