How can a Map or List be immutable when we can add or remove elements from them?

Question

Below is a scala code to declare a immutable Map

var m:Map[Int,String] = Map(1->"hi",2->"hello")
println(m)
// Result: Map(1->"hi",2->"hello")

Here we are able to add or change the content of Map, then how can we say a map or list in scala are immutable

m=m+(3->"hey") 
println(m)
// Result: Map(1->"hi",2->"hello",3->"hey")**

Answer 1

Map is immutable, but you used a mutable variable m (because you declare it as var).

This line m=m+(3->"hey") actually creates a new map and assigned it to your variable m.

Try to declare m as val and see that you will get a compilation error.

But - if you will use mutable map:

val m = scala.collection.mutable.Map[Int,String]

You will be able to update this map (while you can't do this with immutable map) -

m(3) = "hey"

or

m.put(3,"hey")

This is how you will update the content of the map, without recreating it or changing the variable m (like you did before with m = m + ...), because here m is declared as val, which makes it immutable, but the map is mutable.

You still can't do m = m + .. when it's declared as val.

Please refer to this answer about the differences between var and val.