Immutablity of collection Map

问题

In immutable collection of SCALA a new object is created when length of collection changes Let us suppose I create a immutable Map and then perform concatenation. CODE=>

object Dcoder extends App
{
    var map=Map("abc"-> 1,"xyz"->2)
    var change =map++Map("change of object"+>3)
}

Now my Question is
a) Does the new object gets created because of ++ ??
b) Since I'm using a IMMUTABLE COLLECTION and length of Immutable collection has changed so new object is created ??

回答1:

From the documentation of ++ immutable.Map.++:

Adds a number of elements provided by a traversable object and returns a new collection with the added elements. 

So indeed, it creates a new collection, remaining the old map unmodified:

scala> var map = Map("abc" -> 1, "xyz" -> 2)
map: scala.collection.immutable.Map[String,Int] = Map(abc -> 1, xyz -> 2)

scala> var change = map ++ Map("change of object" -> 3)
change: scala.collection.immutable.Map[String,Int] = Map(abc -> 1, xyz -> 2, change of object -> 3)

scala> change
res9: scala.collection.immutable.Map[String,Int] = Map(abc -> 1, xyz -> 2, change of object -> 3)

scala> map
res10: scala.collection.immutable.Map[String,Int] = Map(abc -> 1, xyz -> 2)

You can use += to "modify" your map likewise:

scala> map += "lol" -> 3

scala> map
res12: scala.collection.immutable.Map[String,Int] = Map(abc -> 1, xyz -> 2, lol -> 3)

Realize that I just put "modify", because this returns another map (since it's immutable) object and assigns it to your map variable.

回答2:

Yes to both questions. In fact, according to scala docs:

ms ++ kvs

creates map containing all mappings of ms as well as all key/value pairs of kvs.

More info here.

来源：https://stackoverflow.com/questions/50180300/immutablity-of-collection-map