Equality

The collection libraries have a uniform approach to equality and hashing. The idea is, first, to divide collections into sets, maps, and sequences. Collections in different categories are always unequal. For instance, Set(1, 2, 3) is unequal to List(1, 2, 3) even though they contain the same elements. On the other hand, within the same category, collections are equal if and only if they have the same elements (for sequences: the same elements in the same order). For example, List(1, 2, 3) == Vector(1, 2, 3), and HashSet(1, 2) == TreeSet(2, 1).

It does not matter for the equality check whether a collection is mutable or immutable. For a mutable collection one simply considers its current elements at the time the equality test is performed. This means that a mutable collection might be equal to different collections at different times, depending on what elements are added or removed. This is a potential trap when using a mutable collection as a key in a hashmap. Example:

scala> import collection.mutable.{HashMap, ArrayBuffer}
import collection.mutable.{HashMap, ArrayBuffer}

scala> val buf = ArrayBuffer(1, 2, 3)
val buf: scala.collection.mutable.ArrayBuffer[Int] =
  ArrayBuffer(1, 2, 3)

scala> val map = HashMap(buf -> 3)
val map: scala.collection.mutable.HashMap[scala.collection.
  mutable.ArrayBuffer[Int],Int] = Map((ArrayBuffer(1, 2, 3),3))

scala> map(buf)
val res13: Int = 3

scala> buf(0) += 1

scala> map(buf)
  java.util.NoSuchElementException: key not found:
    ArrayBuffer(2, 2, 3)

In this example, the selection in the last line will most likely fail because the hash-code of the array buf has changed in the second-to-last line. Therefore, the hash-code-based lookup will look at a different place than the one where buf was stored.