Case Class default apply method

问题

Assuming we have the following case class:

case class CasePerson(firstName: String)

And we also define a companion object for it:

object CasePerson {
 def apply() = new CasePerson( "XYZ" )
}

Notice that in the example above I explicitly defined a companion object with an apply method, without defining the the default apply method:

// This "default" apply has the same argument as the primary constructor of the case class
def apply(firstName : String) = new CasePerson(firstName)

Q: So where does Scala gets this "default" apply? I explicitly defined the companion object here without the default apply and the compiler still knows how to execute this:

val casePerson = CasePerson("PQR")

How does this work?

回答1:

Case classes are implicitly accompanied with a companion object with an apply() that has the same arguments as the primary constructor of the case class.

That is:

case class CasePerson(firstName: String)

Will be accompanied by

object CasePerson{
   def apply(firstName: String) = new CasePerson(firstName)
}

Now if you also explicitly define a companion object, you can think of it as appending to the implicitly defined one.

For instance, in your example you added one new apply to the companion object:

object CasePerson{
   def apply() = new CasePerson("XYZ")
}

This statement, you can think of it as if it is creating an combined companion object:

object CasePerson{
   def apply() = new CasePerson("XYZ") // the one manually added
   def apply(firstName: String) = new CasePerson(firstName) // this one is automatically added
}

Now, if you decide to add your own version of the apply that has the same arguments as the primary constructor, then this will overshadow the default behavior of the case class.

object CasePerson{
   def apply() = new CasePerson("XYZ")
   def apply(s: String) = Seq(new CasePerson(s), new CasePerson(s)) // will replace the default factory for the case class
}

Now, if you call CasePerson("hi") it will instead generate:

List(CasePerson("hi"), CasePerson("hi"))

回答2:

Scala case classes are syntactic sugar. When you create a case class the Scala compiler will create a companion object with an apply and an unapply method for you, which you can then use as if it simply exists. Here is a link to more in depth information on case classes.

回答3:

You may also give your firstName class parameter a default value. So you can create an instance with passing no parameter.

Example:

case class CasePerson(firstName : String = "XYZ")
val person = CasePerson()

来源：https://stackoverflow.com/questions/34533539/case-class-default-apply-method

标签

scala

case-class