Scala Constructor Parameters
Solution 1
Yes, there are two important differences. First for the easy one: constructor parameters without the var
or val
keywords are not mutable variables—their values can't be changed in the body of the class.
Even if we restrict ourselves to the val
keyword, though, there's still a difference between private val
and keyword-less parameters. Consider the following:
class Person(private val firstName: String, lastName: String)
If we look at the compiled class with javap -v Person
, we'll see that it only has one field, for firstName
. lastName
is just a constructor parameter, which means it may be garbage-collected after the class is initialized, etc.
The compiler is smart enough to know when the value of lastName
will be needed after initialization, and it will create a field for it in that case. Consider the following variation:
class Person(private val firstName: String, lastName: String) {
def fullName = firstName + " " + lastName
}
The compiler can tell that it may need the value of lastName
later, and if we check javap
again we'll see that the class has two fields (note that if we'd defined fullName
as a val
instead of a def
, it'd only have one field).
Lastly, note that if we make firstName
object-private instead of class-private, it works exactly like a plain old keyword-less constructor parameter:
class Person(private[this] val firstName: String, lastName: String)
This works even with var
instead of val
:
class Person(private[this] var firstName: String, lastName: String)
Both of these classes will have no fields. See section 5.2 of the language specification for more details about object-private access.
Solution 2
as a supplement, if your class is a case class, all of the constructor parameters will be automatically public fields.
The compiler will complain about the private keyword if it exists, and for the parameters without val/var, no matter they are used or not in any defs, there will be public fields generated for them.
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user2456976
Updated on June 05, 2022Comments
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user2456976 almost 2 years
What is the difference between a private var constructor parameter and a constructor parameter without val/var? Are they same in terms of scope/visibility?
Ex:
class Person(private var firstName:String, lastName:String)
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danielkza over 10 yearsCheck out this question: stackoverflow.com/questions/15639078/…
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lcn over 10 yearsThis has a better explanation: stackoverflow.com/questions/14694712/…
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RussAbbott over 10 yearsAre you saying that in situations such as the example with
def fullName
there is no difference between aprivate val
and a keyword-less parameter? -
Dmytro Mitin almost 4 years@TravisBrown In 2.13.2 both
class Person(private[this] val firstName: String)
andclass Person(firstName: String)
generate fieldprivate[this] val firstName: String
stackoverflow.com/a/62686327/5249621