Mutable HashMap with a mutable default value doesn't keep the changes

Question

Suppose that I want a mutable HashMap[Int, HashSet[Int]] that has

• integers as keys
• mutable hash sets of integers as values

I want that an empty mutable HashSet is created by default whenever a value for a new key is accessed or updated.

Here is what I tried:

import collection.mutable.{HashMap, HashSet}

val hm = HashMap
  .empty[Int, HashSet[Int]]
  .withDefault(_ => HashSet.empty[Int])

hm(42) += 1234

println(hm)

The unexpected result is an empty HashMap. I expected a hash map with (42 -> HashSet(1234)) key-value pair.

Why doesn't the HashMap save the default mutable HashSets, and how do I fix this?

Answer 1

The statement

hm(42) += 1234

will create the default value (an empty HashSet), then update it by adding 1234 to it, then throw it away.

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If you want to update the HashMap itself, then remove the withDefault part from the definition, and use getOrElseUpdate instead:

hm.getOrElseUpdate(42, HashSet.empty[Int]) += 1234

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Alternatively, you can leave the withDefault as-is, but update your hash map as follows:

hm(42) = (hm(42) += 1234)