The Scala collections module is a package in the Scala standard library that contains a variety of general-purpose collection types. Scala collections provide a general and easy-to-use way of declaratively manipulating data using functional combinators. For example, in the following program, we use the filter combinator on a range of numbers to return a sequence of palindromes between 0 and 100,000; that is, numbers that are read in the same way in both the forward and reverse direction:

(0 until 100000).filter(x => x.toString == x.toString.reverse)

Scala collections define three basic types of collections: sequences, maps, and sets. Elements stored in sequences are ordered and can be retrieved using the apply method and an integer index. Maps store key-value pairs and can be used to retrieve a value associated with a specific key. Sets can be used to test the element membership with the apply method.

The Scala collection library makes a distinction between...