What is the syntax for adding an element to a scala.collection.mutable.Map ?

Question

What is the syntax for adding an element to a scala.collection.mutable.Map ?

Here are some failed attempts:

val map = scala.collection.mutable.Map

map("mykey") = "myval"

map += "mykey" -> "myval"

map.put("mykey","myval")

Answer 1

When you say

val map = scala.collection.mutable.Map

you are not creating a map instance, but instead aliasing the Map type.

map: collection.mutable.Map.type = scala.collection.mutable.Map$@fae93e

Try instead the following:

scala> val map = scala.collection.mutable.Map[String, Int]()
map: scala.collection.mutable.Map[String,Int] = Map()

scala> map("asdf") = 9

scala> map
res6: scala.collection.mutable.Map[String,Int] = Map((asdf,9))

Answer 2

var test = scala.collection.mutable.Map.empty[String, String]
test("myKey") = "myValue"

Answer 3

The point is that the first line of your codes is not what you expected. You should use

val map = scala.collection.mutable.Map[A,B]()

or

val map = new scala.collection.mutable.Map[A,B]()

instead.

scala> val map = scala.collection.mutable.Map[String,String]()
map: scala.collection.mutable.Map[String,String] = Map()

scala> map("k1") = "v1"

scala> map
res1: scala.collection.mutable.Map[String,String] = Map((k1,v1))

scala> map += "k2" -> "v2"
res2: map.type = Map((k1,v1), (k2,v2))

scala> map.put("k3","v3")
res3: Option[String] = None

scala> map
res4: scala.collection.mutable.Map[String,String] = Map((k3,v3), (k1,v1), (k2,v2))

Answer 4

As always, you should question whether you truly need a mutable map.

Immutable maps are trivial to build:

val map = Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

Mutable maps are no different when first being built:

val map = collection.mutable.Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

map += "nextkey" -> "nextval"

In both of these cases, inference will be used to determine the correct type parameters for the Map instance.

You can also hold an immutable map in a var, the variable will then be updated with a new immutable map instance every time you perform an "update"

var map = Map(
  "mykey" -> "myval",
  "myotherkey" -> "otherval"
)

map += "nextkey" -> "nextval"

If you don't have any initial values, you can use Map.empty:

val map : Map[String, String] = Map.empty //immutable
val map = Map.empty[String,String] //immutable
val map = collection.mutable.Map.empty[String,String] //mutable