Is it possible and how to have var that can only be set once?

Question

Is there a native way to make sure that a variable can only be set once?

Currently, I am using this approach

class SetOnceVariable[T]
{
  private var value : T = _

  private var initialized = false

  def apply(_v : T = _) : T =
  {
    if (value != null && !initialized) {
      value = _v
      initialized = true
    }
    value
  }
}

class ClientInfo
{
  val userIP : SetOnceVariable[String] = new SetOnceVariable[String]
}

Can you use a `lazy val`? When is your variable set and used? — The Archetypal Paul, Dec 21 '14 at 09:26

score 4 · Answer 1 · answered Dec 21 '14 at 04:29

There's no such language construct, but I think I can clean up your code, at least.

class SetOnce[A](var toOption: Option[A] = None) {
    def set(a: A): Unit = if (toOption.isEmpty) toOption = Some(a)
    def get: A = toOption.get
}

Usage:

val x = new SetOnce[Int]
x.toOption // None

x.set(1)
x.get // 1

x.set(2)
x.get // 1

I omitted the null consideration because idiomatic Scala code tends to not use or consider null outside of Java compatibility. We mostly pretend that it doesn't exist.

user5102379 · Answer 2 · 2014-12-21T10:36:34.283

Approach using lazy:

  class SetOnceVariable[T] {

    private var v: T = _
    private lazy val value: T = v

    def apply(_v: T = ???): T = {
      v = _v
      value
    }
  }

val info = new ClientInfo
println(info.userIP("IP")) // IP
println(info.userIP("IP2")) // IP
println(info.userIP("IP3")) // IP

To make it threadsafe you can use:

def apply(_v: T = ???): T =
  synchronized {
    v = _v
    value
  }

score 0 · Answer 3 · edited May 23 '17 at 11:43

0

You can create a constant variable by using a val. For instance:

val a = 0; // Cannot be changed
var b = 0; // Can be changed

See this answer for more details: https://stackoverflow.com/a/1792207/4380308

Edit:

A val can be declared and then initialized later as well.

val a;
a = 0;

edited May 23 '17 at 11:43

Community

1
1

answered Dec 21 '14 at 04:09

John Cipponeri

882
8
12

3

I think what Andrey wants is something that can have its value set at a later time. – Chris Martin Dec 21 '14 at 04:12
@ChrisMartin I will edit my answer, however a val can still be declared and then initialized later. – John Cipponeri Dec 21 '14 at 04:14
I mean... *later*. Like, not in that block. – Chris Martin Dec 21 '14 at 04:18
That's still achievable given the scope of the variable. – John Cipponeri Dec 21 '14 at 04:19
From a completely different file. e.g. `clientInfo.userIP.set(...)` – Chris Martin Dec 21 '14 at 04:20

nitishagar · Answer 4 · 2014-12-21T04:29:21.483

0

You can try the following:

class varSingleton[A >: Null <: AnyRef] {
  private[this] var _a = null: A
  def :=(a: A) { if (_a eq null) _a = a else throw new IllegalStateException }
  def apply() = if (_a eq null) throw new IllegalStateException else _a
}

You can use this further like:

var singleVal = new varSingleton[Integer]
singleVal := 12
singleVal() // returns 12
singleVal := 13 //IllegalStateException

edited Dec 21 '14 at 04:29

answered Dec 21 '14 at 04:14

nitishagar

9,038
3
28
40

score 0 · Answer 5 · answered Dec 21 '14 at 22:14

you can use simple getter and setter:

class ClientInfo {
  private var _userIP: Option[String] = None
  def userIP: String = _userIP.get
  def userIP_=(value: String): Unit = {
    _userIP = _userIP.orElse(Option(value))
  }
}

val clientInfo = new ClientInfo()                       //> clientInfo : controllers.stackoverflow.ClientInfo controllers.stackoverflow$Clien
                                                        //| tInfo@4fccd51b
clientInfo.userIP = "first"
clientInfo.userIP                                       //> res0: String = first

clientInfo.userIP = "second"
clientInfo.userIP                                       //> res1: String = first

I prefer to use Option that the value directly to prevent nulls and NPE. You can of course add what ever logic you need in the setter.

Is it possible and how to have var that can only be set once?

5 Answers5