Scala Cookbook

10.6. Understanding Mutable Variables with Immutable Collections

Problem

You may have seen that mixing a mutable variable (var) with an immutable collection causes surprising behavior. For instance, when you create an immutable Vector as a var, it appears you can somehow add new elements to it:

scala> var sisters = Vector("Melinda")
sisters: collection.immutable.Vector[String] = Vector(Melinda)

scala> sisters = sisters :+ "Melissa"
sisters: collection.immutable.Vector[String] = Vector(Melinda, Melissa)

scala> sisters = sisters :+ "Marisa"
sisters: collection.immutable.Vector[String] = Vector(Melinda, Melissa, Marisa)

scala> sisters.foreach(println)
Melinda
Melissa
Marisa

How can this be?

Solution

Though it looks like you’re mutating an immutable collection, what’s really happening is that the sisters variable points to a new collection each time you use the :+ method. The sisters variable is mutable—like a non-final field in Java—so it’s actually being reassigned to a new collection during each step. The end result is similar to these lines of code:

var sisters = Vector("Melinda")
sisters = Vector("Melinda", "Melissa")
sisters = Vector("Melinda", "Melissa", "Marisa")

In the second and third lines of code, the sisters reference has been changed to point to a new collection.

You can demonstrate that the vector itself is immutable. Attempting to mutate one of its elements—which doesn’t involve reassigning the variable—results in an error:

scala> sisters(0) = "Molly" <console>:12: error: value ...