How to use mutable collections in Scala

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I think I may be failing to understand how mutable collections work. I would expect mutable collections to be affected by applying map to them or adding new elements, however:

scala> val s: collection.mutable.Seq[Int] = collection.mutable.Seq(1)
s: scala.collection.mutable.Seq[Int] = ArrayBuffer(1)

scala> s :+ 2 //appended an element
res32: scala.collection.mutable.Seq[Int] = ArrayBuffer(1, 2)

scala> s //the original collection is unchanged
res33: scala.collection.mutable.Seq[Int] = ArrayBuffer(1)

scala> s.map(_.toString) //mapped a function to it
res34: scala.collection.mutable.Seq[java.lang.String] = ArrayBuffer(1)

scala> s //original is unchanged
res35: scala.collection.mutable.Seq[Int] = ArrayBuffer(1)

//maybe mapping a function that changes the type of the collection shouldn't work
//try Int => Int

scala> s.map(_ + 1)
res36: scala.collection.mutable.Seq[Int] = ArrayBuffer(2)

scala> s //original unchanged
res37: scala.collection.mutable.Seq[Int] = ArrayBuffer(1)

This behaviour doesn't seem to be separate from the immutable collections, so when do they behave separately?

mutable scala collections
4 Answers

As others have noted, the map and :+ methods return a modified copy of the collection. More generally, all methods defined in collections from the scala.collection package will never modify a collection in place even when the dynamic type of the collection is from scala.collection.mutable. To modify a collection in place, look for methods defined in scala.collection.mutable._ but not in scala.collection._.

For example, :+ is defined in scala.collection.Seq, so it will never perform in-place modification even when the dynamic type is a scala.collection.mutable.ArrayBuffer. However, +=, which is defined in scala.collection.mutable.ArrayBuffer and not in scala.collection.Seq, will modify the collection in place.

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