I need only one instance of a class, so I have to use an object instead of a class. I also need to set some initial value chosen by a client, so I need to a constructor for an object, something like this:
object Object1(val initValue: Int){
//.....
}
I can't use this exact code in Scala. How do I deal with that then?
You have a couple of choices:
Make it a class, have the client construct it, give the value in the parameter
Pro: Preserves immutability
Con: Having only a single instance might be hard to manage
Add a variable for the param to the object, add a setter.
Pro: You still have a singleton
Con: There is mutable state now
Implement a multiton
Pro: Gives you (apparent) immutability and singleton (per param)
Con: More code to implement
You could implement a multiton like this in scala:
class Object1 private (val initValue: Int) {
// ...
}
object Object1 {
val insts = mutable.Map.empty[Int, Object1]
def apply(initV: Int) =
insts.getOrElseUpdate(initV, new Object1(initV))
}
UPDATE You could also turn this into a "singleton with parameter":
object Object1 {
var inst: Option[(Int, Object1)] = None
def apply(initV: Int) = inst match {
case Some((`initV`, i)) => i
case Some(_) =>
sys.error("Object1 already instantiated with different param")
case None =>
val i = new Object1(initV)
inst = Some((initV, i))
i
}
}
apply() in your "singleton with parameter" is not thread-safe.
var insts, and initializing it later because anyways mutable kind of stuff is in use.
this.The object isn't created until you reference it, so you could do something like the following:
object Test1 extends App {
var x = Console.readLine
println(Object1.initVal)
}
object Object1 {
val initVal:String = Test1.x
}
This is rather convention over configuration approach. But it is thread safe and adds obligatory initialisation requirement though a bit verbose:
abstract class NetworkPusher() {
private var networkAddress: String = ""
def setNetworkAddr(addr: String) = networkAddress = addr
def getNetworkAddr() = networkAddress
}
object NetworkPusherFactory {
private var initOnceReady: NetworkPusher => Unit = _
private lazy val initializedInstance = {
val pusher = new NetworkPusher(){}
initOnceReady(pusher)
pusher
}
def create(init: NetworkPusher => Unit): NetworkPusher = {
initOnceReady = init
initializedInstance
}
}
object Main {
def main(args: Array[String]): Unit = {
val networkAddr = "A.D.D.R" // args[0]
val onePusher = NetworkPusherFactory.create(
instance => instance.setNetworkAddr(networkAddr)
)
println(s"using ${onePusher.getNetworkAddr}")
}
}
