Package scala.collection.immutable

A non empty list characterized by a head and a tail.

A class for immutable bitsets.

Bitsets are sets of non-negative integers which are represented as variable-size arrays of bits packed into 64-bit words. The memory footprint of a bitset is determined by the largest number stored in it.

A default map which implements the + and - methods of maps. It does so using the default builder for maps defined in the Map object. Instances that inherit from DefaultMap[A, B] still have to define:

def get(key: A): Option[B]
def iterator: Iterator[(A, B)]

It refers back to the original map.

It might also be advisable to override foreach or size if efficient implementations can be found.

This class implements immutable maps using a hash trie.

Note: The builder of this hash map may return specialized representations for small maps.

This class implements immutable sets using a hash trie.

Note: The builder of this hash set may return specialized representations for small sets.

A subtrait of collection.IndexedSeq which represents indexed sequences that are guaranteed immutable.

Indexed sequences support constant-time or near constant-time element access and length computation. They are defined in terms of abstract methods apply for indexing and length.

Indexed sequences do not add any new methods to Seq, but promise efficient implementations of random access patterns.

Specialised immutable map structure for integer keys, based on Fast Mergeable Integer Maps by Okasaki and Gill. Essentially a trie based on binary digits of the integers.

Note: This class is as of 2.8 largely superseded by HashMap.

A base trait for iterable collections that are guaranteed immutable.

This is a base trait for all immutable Scala collections that define an iterator method to step through one-by-one the collection's elements. Implementations of this trait need to provide a concrete method with signature:

def iterator: Iterator[A]

They also need to provide a method newBuilder which creates a builder for collections of the same kind.

This trait implements Iterable's foreach method by stepping through all elements using iterator. Subclasses should re-implement foreach with something more efficient, if possible.

This trait adds methods iterator, sameElements, takeRight, dropRight to the methods inherited from trait `Traversable`.

Note: This trait replaces every method that uses break in TraversableLike by an iterator version.

A subtrait of collection.LinearSeq which represents sequences that are guaranteed immutable.

Linear sequences have reasonably efficient head, tail, and isEmpty methods. If these methods provide the fastest way to traverse the collection, a collection Coll that extends this trait should also extend LinearSeqOptimized[A, Coll[A]].

A class for immutable linked lists representing ordered collections of elements of type A.

This class comes with two implementing case classes scala.Nil and scala.:: that implement the abstract members isEmpty, head and tail.

This class is optimal for last-in-first-out (LIFO), stack-like access patterns. If you need another access pattern, for example, random access or FIFO, consider using a collection more suited to this than List.

Note: Despite being an immutable collection, the implementation uses mutable state internally during construction. These state changes are invisible in single-threaded code but can lead to race conditions in some multi-threaded scenarios. The state of a new collection instance may not have been "published" (in the sense of the Java Memory Model specification), so that an unsynchronized non-volatile read from another thread may observe the object in an invalid state (see scala/bug#7838 for details). Note that such a read is not guaranteed to ever see the written object at all, and should therefore not be used, regardless of this issue. The easiest workaround is to exchange values between threads through a volatile var.

Performance

Time: List has O(1) prepend and head/tail access. Most other operations are O(n) on the number of elements in the list. This includes the index-based lookup of elements, length, append and reverse.

Space: List implements structural sharing of the tail list. This means that many operations are either zero- or constant-memory cost.

val mainList = List(3, 2, 1)
val with4 =    4 :: mainList  // re-uses mainList, costs one :: instance
val with42 =   42 :: mainList // also re-uses mainList, cost one :: instance
val shorter =  mainList.tail  // costs nothing as it uses the same 2::1::Nil instances as mainList

This class implements immutable maps using a list-based data structure. List map iterators and traversal methods visit key-value pairs in the order whey were first inserted.

Entries are stored internally in reversed insertion order, which means the newest key is at the head of the list. As such, methods such as head and tail are O(n), while last and init are O(1). Other operations, such as inserting or removing entries, are also O(n), which makes this collection suitable only for a small number of elements.

Instances of ListMap represent empty maps; they can be either created by calling the constructor directly, or by applying the function ListMap.empty.

This class implements immutable sets using a list-based data structure. List set iterators and traversal methods visit elements in the order whey were first inserted.

Elements are stored internally in reversed insertion order, which means the newest element is at the head of the list. As such, methods such as head and tail are O(n), while last and init are O(1). Other operations, such as inserting or removing entries, are also O(n), which makes this collection suitable only for a small number of elements.

Instances of ListSet represent empty sets; they can be either created by calling the constructor directly, or by applying the function ListSet.empty.

Specialised immutable map structure for long keys, based on Fast Mergeable Long Maps by Okasaki and Gill. Essentially a trie based on binary digits of the integers.

Note: This class is as of 2.8 largely superseded by HashMap.

A generic trait for immutable maps. Concrete classes have to provide functionality for the abstract methods in Map:

def get(key: K): Option[V]
def iterator: Iterator[(K, V)]
def + [V1 >: V](kv: (K, V1)): Map[K, V1]
def -(key: K): Map[K, V]

A generic template for immutable maps from keys of type K to values of type V. To implement a concrete map, you need to provide implementations of the following methods (where This is the type of the actual map implementation):

def get(key: K): Option[V]
def iterator: Iterator[(K, V)]
def + [V1 >: V](kv: (K, V)): Map[K, V1]
def - (key: K): This

If you wish that transformer methods like take, drop, filter return the same kind of map, you should also override:

def empty: This

It is also good idea to override methods foreach and size for efficiency.

NumericRange is a more generic version of the Range class which works with arbitrary types. It must be supplied with an Integral implementation of the range type.

Factories for likely types include Range.BigInt, Range.Long, and Range.BigDecimal. Range.Int exists for completeness, but the Int-based scala.Range should be more performant.

val r1 = new Range(0, 100, 1)
val veryBig = Int.MaxValue.toLong + 1
val r2 = Range.Long(veryBig, veryBig + 100, 1)
assert(r1 sameElements r2.map(_ - veryBig))

Queue objects implement data structures that allow to insert and retrieve elements in a first-in-first-out (FIFO) manner.

Queue is implemented as a pair of Lists, one containing the in elements and the other the out elements. Elements are added to the in list and removed from the out list. When the out list runs dry, the queue is pivoted by replacing the out list by in.reverse, and in by Nil.

Adding items to the queue always has cost O(1). Removing items has cost O(1), except in the case where a pivot is required, in which case, a cost of O(n) is incurred, where n is the number of elements in the queue. When this happens, n remove operations with O(1) cost are guaranteed. Removing an item is on average O(1).

The Range class represents integer values in range [start;end) with non-zero step value step. It's a special case of an indexed sequence. For example:

val r1 = 0 until 10
val r2 = r1.start until r1.end by r1.step + 1
println(r2.length) // = 5

Ranges that contain more than Int.MaxValue elements can be created, but these overfull ranges have only limited capabilities. Any method that could require a collection of over Int.MaxValue length to be created, or could be asked to index beyond Int.MaxValue elements will throw an exception. Overfull ranges can safely be reduced in size by changing the step size (e.g. by 3) or taking/dropping elements. contains, equals, and access to the ends of the range (head, last, tail, init) are also permitted on overfull ranges.

A subtrait of collection.Seq which represents sequences that are guaranteed immutable.

Sequences are special cases of iterable collections of class Iterable. Unlike iterables, sequences always have a defined order of elements. Sequences provide a method apply for indexing. Indices range from 0 up to the length of a sequence. Sequences support a number of methods to find occurrences of elements or subsequences, including segmentLength, prefixLength, indexWhere, indexOf, lastIndexWhere, lastIndexOf, startsWith, endsWith, indexOfSlice.

Another way to see a sequence is as a PartialFunction from Int values to the element type of the sequence. The isDefinedAt method of a sequence returns true for the interval from 0 until length.

Sequences can be accessed in reverse order of their elements, using methods reverse and reverseIterator.

Sequences have two principal subtraits, IndexedSeq and LinearSeq, which give different guarantees for performance. An IndexedSeq provides fast random-access of elements and a fast length operation. A LinearSeq provides fast access only to the first element via head, but also has a fast tail operation.

A generic trait for immutable sets.

A set is a collection that contains no duplicate elements.

To implement a concrete set, you need to provide implementations of the following methods:

def contains(key: A): Boolean
def iterator: Iterator[A]
def +(elem: A): This
def -(elem: A): This

If you wish that methods like take, drop, filter return the same kind of set, you should also override:

def empty: This

It is also good idea to override methods foreach and size for efficiency.

A map whose keys are sorted.

A subtrait of collection.SortedSet which represents sorted sets which cannot be mutated.

The class Stream implements lazy lists where elements are only evaluated when they are needed. Here is an example:

import scala.math.BigInt
object Main extends App {

  lazy val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(fibs.tail).map { n => n._1 + n._2 }

  fibs take 5 foreach println
}

// prints
//
// 0
// 1
// 1
// 2
// 3

The Stream class also employs memoization such that previously computed values are converted from Stream elements to concrete values of type A. To illustrate, we will alter body of the fibs value above and take some more values:

import scala.math.BigInt
object Main extends App {

  lazy val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(
    fibs.tail).map(n => {
      println("Adding %d and %d".format(n._1, n._2))
      n._1 + n._2
    })

  fibs take 5 foreach println
  fibs take 6 foreach println
}

// prints
//
// 0
// 1
// Adding 0 and 1
// 1
// Adding 1 and 1
// 2
// Adding 1 and 2
// 3

// And then prints
//
// 0
// 1
// 1
// 2
// 3
// Adding 2 and 3
// 5

There are a number of subtle points to the above example.

The definition of fibs is a val not a method. The memoization of the Stream requires us to have somewhere to store the information and a val allows us to do that.While the Stream is actually being modified during access, this does not change the notion of its immutability. Once the values are memoized they do not change and values that have yet to be memoized still "exist", they simply haven't been realized yet.One must be cautious of memoization; you can very quickly eat up large amounts of memory if you're not careful. The reason for this is that the memoization of the Stream creates a structure much like scala.collection.immutable.List. So long as something is holding on to the head, the head holds on to the tail, and so it continues recursively. If, on the other hand, there is nothing holding on to the head (e.g. we used def to define the Stream) then once it is no longer being used directly, it disappears.Note that some operations, including drop, dropWhile, flatMap or collect may process a large number of intermediate elements before returning. These necessarily hold onto the head, since they are methods on Stream, and a stream holds its own head. For computations of this sort where memoization is not desired, use Iterator when possible.

// For example, let's build the natural numbers and do some silly iteration
// over them.

// We'll start with a silly iteration
def loop(s: String, i: Int, iter: Iterator[Int]): Unit = {
  // Stop after 200,000
  if (i < 200001) {
    if (i % 50000 == 0) println(s + i)
    loop(s, iter.next, iter)
  }
}

// Our first Stream definition will be a val definition
val stream1: Stream[Int] = {
  def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
  loop(0)
}

// Because stream1 is a val, everything that the iterator produces is held
// by virtue of the fact that the head of the Stream is held in stream1
val it1 = stream1.iterator
loop("Iterator1: ", it1.next, it1)

// We can redefine this Stream such that all we have is the Iterator left
// and allow the Stream to be garbage collected as required.  Using a def
// to provide the Stream ensures that no val is holding onto the head as
// is the case with stream1
def stream2: Stream[Int] = {
  def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
  loop(0)
}
val it2 = stream2.iterator
loop("Iterator2: ", it2.next, it2)

// And, of course, we don't actually need a Stream at all for such a simple
// problem.  There's no reason to use a Stream if you don't actually need
// one.
val it3 = new Iterator[Int] {
  var i = -1
  def hasNext = true
  def next(): Int = { i += 1; i }
}
loop("Iterator3: ", it3.next, it3)

The fact that tail works at all is of interest. In the definition of fibs we have an initial (0, 1, Stream(...)) so tail is deterministic. If we defined fibs such that only 0 were concretely known then the act of determining tail would require the evaluation of tail which would cause an infinite recursion and stack overflow. If we define a definition where the tail is not initially computable then we're going to have an infinite recursion:

// The first time we try to access the tail we're going to need more
// information which will require us to recurse, which will require us to
// recurse, which...
lazy val sov: Stream[Vector[Int]] = Vector(0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }

The definition of fibs above creates a larger number of objects than necessary depending on how you might want to implement it. The following implementation provides a more "cost effective" implementation due to the fact that it has a more direct route to the numbers themselves:

lazy val fib: Stream[Int] = {
  def loop(h: Int, n: Int): Stream[Int] = h #:: loop(n, h + n)
  loop(1, 1)
}

Note that mkString forces evaluation of a Stream, but addString does not. In both cases, a Stream that is or ends in a cycle (e.g. lazy val s: Stream[Int] = 0 #:: s) will convert additional trips through the cycle to .... Additionally, addString will display an un-memoized tail as ?.

A trait describing stringlike collections.

This class serves as a wrapper providing scala.Predef.Strings with all the operations found in indexed sequences. Where needed, Strings are implicitly converted into instances of this class.

The difference between this class and WrappedString is that calling transformer methods such as filter and map will yield a String object, whereas a WrappedString will remain a WrappedString.

A trait for traversable collections that are guaranteed immutable.

This is a base trait of all kinds of immutable Scala collections. It implements the behavior common to all collections, in terms of a method foreach with signature:

def foreach[U](f: Elem => U): Unit

Collection classes mixing in this trait provide a concrete foreach method which traverses all the elements contained in the collection, applying a given function to each. They also need to provide a method newBuilder which creates a builder for collections of the same kind.

A traversable class might or might not have two properties: strictness and orderedness. Neither is represented as a type.

The instances of a strict collection class have all their elements computed before they can be used as values. By contrast, instances of a non-strict collection class may defer computation of some of their elements until after the instance is available as a value. A typical example of a non-strict collection class is a scala.collection.immutable.Stream. A more general class of examples are TraversableViews.

If a collection is an instance of an ordered collection class, traversing its elements with foreach will always visit elements in the same order, even for different runs of the program. If the class is not ordered, foreach can visit elements in different orders for different runs (but it will keep the same order in the same run).'

A typical example of a collection class which is not ordered is a HashMap of objects. The traversal order for hash maps will depend on the hash codes of its elements, and these hash codes might differ from one run to the next. By contrast, a LinkedHashMap is ordered because its foreach method visits elements in the order they were inserted into the HashMap.

This class implements immutable maps using a tree.

This class implements immutable sets using a tree.

Vector is a general-purpose, immutable data structure. It provides random access and updates in effectively constant time, as well as very fast append and prepend. Because vectors strike a good balance between fast random selections and fast random functional updates, they are currently the default implementation of immutable indexed sequences. It is backed by a little endian bit-mapped vector trie with a branching factor of 32. Locality is very good, but not contiguous, which is good for very large sequences.

Note: Despite being an immutable collection, the implementation uses mutable state internally during construction. These state changes are invisible in single-threaded code but can lead to race conditions in some multi-threaded scenarios. The state of a new collection instance may not have been "published" (in the sense of the Java Memory Model specification), so that an unsynchronized non-volatile read from another thread may observe the object in an invalid state (see scala/bug#7838 for details). Note that such a read is not guaranteed to ever see the written object at all, and should therefore not be used, regardless of this issue. The easiest workaround is to exchange values between threads through a volatile var.

A class to build instances of Vector. This builder is reusable.

This class serves as a wrapper augmenting Strings with all the operations found in indexed sequences.

The difference between this class and StringOps is that calling transformer methods such as filter and map will yield an object of type WrappedString rather than a String.

This is a simple wrapper class for `scala.collection.immutable.Map`.

It is most useful for assembling customized map abstractions dynamically using object composition and forwarding.

An implementation of lazily computed sequences, where elements are stored in "pages", i.e. arrays of fixed size.

A paged sequence is constructed from a function that produces more elements when asked. The producer function - more, is similar to the read method in java.io.Reader. The more function takes three parameters: an array of elements, a start index, and an end index. It should try to fill the array between start and end indices (excluding end index). It returns the number of elements produced, or -1 if end of logical input stream was reached before reading any element.

This is a simple wrapper class for scala.collection.immutable.Set.

It is most useful for assembling customized set abstractions dynamically using object composition and forwarding.

This class implements immutable stacks using a list-based data structure.

Note: This class exists only for historical reason and as an analogue of mutable stacks. Instead of an immutable stack you can just use a list.