CODEWITHSUNDEEP
scaladictionary
Jerome ROBERT
Jerome ROBERT

Reputation: 337

apply method on Map object?

First of all let me apologize in advance for what is my very first question posted on stack overflow and probably a quite stupid one.

Since a Map in scala is instantiated using the following syntax:

val myMap=Map(1->”value1”,2->”value2”)

I was expecting the Map object from scala.collection.immutable to provide an apply method with a signature roughly looking like:

def apply[A,B](entries :(A,B)*):Map[A,B]

Obviously I should be blind, but I can’t find such a method. Where is it defined ?

Furthermore, could someone give me information about the purpose of the Map1, Map2, Map3, Map4 classes defined in the Map object ? Should they be used by the developer or are they only used internally by the language and/or the compiler ? Are they related somehow to the map instantiation scheme i was asking about above ?

Thanks in advance for your help.

Upvotes: 4

Views: 1727

Answers (1)

Debilski
Debilski

Reputation: 67848

apply looks like it is defined on scala.collection.generic.GenMapFactory, a superclass of scala.collection.immutable.Map. For some reason, Scaladoc simply ignores this method for 2.9.2. (As Rogach notes, it was there in 2.9.1.)

Map1…Map4 (together with EmptyMap, which is private) are simply optimisations. These are defined inside Map.scala and really just hold up to four keys and values directly without any further indirection. If one tries to add to a Map4, a HashMap will automatically be created.

You normally do not need to create a Map[1-4] manually:

scala> Map('a -> 1)
res0: scala.collection.immutable.Map[Symbol,Int] = Map('a -> 1)

scala> res0.isInstanceOf[scala.collection.immutable.Map.Map1[_,_]]
res1: Boolean = true

Upvotes: 6

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