Scala collections

In Scala, collections are automatically mutable or immutable depending on your usage. All collections in scala.collections.immutable are immutable, and vice versa for scala.collections.immutable. Scala picks immutable collections by default, so your code will then draw automatically from the mutable collections:

var List mylist;

This happens unless you prefix your variable with immutable:

var mylist immutable.List;

We can see this in this small amount of code, for example:

var mutableList = List(1, 2, 3);
var immutableList = scala.collection.immutable.List(4, 5, 6);
mutableList.updated(1,400);
immutableList.updated(1,700);

As you can see in this screenshot of the notebook:

Note that Scala cheated a little here; it created a new collection when we updated immutableList, as you can see, with the variable name as real_3 instead.