Learning Scala

Chapter 7. More Collections

In Chapter 6 we were introduced to the Iterable root type and three of its immutable subtypes: the ordered collection List and the unordered collections Set and Map. These collections were labeled common because they are ubiquitous in modern programming languages, not to imply that they are basic and unadorned. In this chapter we will uncover Scala collections that may not be ubiquitous but are just as important.

We’ll start with mutable collections, which probably can be considered ubiquitous because more languages support them than they do immutable collections. Then we’ll move on to arrays, streams, and other collections.

Mutable Collections

The List, Set, and Map immutable collections we are familiar with cannot be changed after they have been created (see the definition of “immutable”). They can, however, be transformed into new collections. For example, we can create an immutable map, and then transform it by removing one mapping and adding another:

scala> val m = Map("AAPL" -> 597, "MSFT" -> 40) 
m: scala.collection.immutable.Map[String,Int] =
  Map(AAPL -> 597, MSFT -> 40)

scala> val n = m - "AAPL" + ("GOOG" -> 521)     
n: scala.collection.immutable.Map[String,Int] =
  Map(MSFT -> 40, GOOG -> 521)

scala> println(m)                               
Map(AAPL -> 597, MSFT -> 40)