Scala Collections

Learn about Lists, Maps, Sets, and other Scala collections.

Immutable Collections

Scala encourages immutable collections by default:

import scala.collection.immutable._

// List
val numbers = List(1, 2, 3, 4, 5)
val doubled = numbers.map(_ * 2)

// Set
val uniqueNumbers = Set(1, 2, 3, 2, 1)  // Becomes Set(1, 2, 3)

// Map
val ages = Map("Alice" -> 25, "Bob" -> 30)
val aliceAge = ages("Alice")  // 25

Mutable Collections

When you need mutability, import mutable collections explicitly:

import scala.collection.mutable

// Mutable ListBuffer
val buffer = mutable.ListBuffer(1, 2, 3)
buffer += 4  // Adds element
buffer -= 2  // Removes element

// Mutable Set
val mutableSet = mutable.Set(1, 2, 3)
mutableSet += 4  // Adds element
mutableSet -= 2  // Removes element

// Mutable Map
val mutableMap = mutable.Map("a" -> 1, "b" -> 2)
mutableMap("c") = 3  // Adds new key-value pair

Common Collection Operations

Operation	Description	Example
map	Transform each element	List(1,2,3).map(_ * 2) → List(2,4,6)
filter	Select elements that satisfy predicate	List(1,2,3).filter(_ > 1) → List(2,3)
fold	Combine elements with binary operation	List(1,2,3).fold(0)(_ + _) → 6
flatMap	Map then flatten	List(1,2).flatMap(x ⇒ List(x,x)) → List(1,1,2,2)
reduce	Combine elements without initial value	List(1,2,3).reduce(_ + _) → 6
groupBy	Group elements by key	List("a", "aa", "b").groupBy(_.length) → Map(1 → List("a", "b"), 2 → List("aa"))

Collection Hierarchy

Scala collections follow a consistent hierarchy:

Traversable
├── Iterable
│   ├── Seq
│   │   ├── IndexedSeq (Vector, Array, String)
│   │   └── LinearSeq (List, Queue, Stack)
│   ├── Set
│   │   ├── SortedSet (TreeSet)
│   │   └── HashSet
│   └── Map
│       ├── SortedMap (TreeMap)
│       └── HashMap

Performance Characteristics

Collection	Operation	Time Complexity
List	head	O(1)
List	tail	O(1)
List	access by index	O(n)
Vector	access by index	O(log32(n))
Set	contains	O(1)
Map	lookup	O(1)

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