Implementing Custom Collections (Scala 2.13)


Martin Odersky, Lex Spoon and Julien Richard-Foy

This article shows how to implement custom collection types on top of the collections framework. It is recommended to first read the article about the architecture of the collections.

What needs to be done if you want to integrate a new collection class, so that it can profit from all predefined operations with the right types? In the next few sections you’ll be walked through three examples that do this, namely capped sequences, sequences of RNA bases and prefix maps implemented with Patricia tries.

Capped sequence

Say you want to create an immutable collection containing at most n elements: if more elements are added then the first elements are removed.

The first task is to find the supertype of our collection: is it Seq, Set, Map or just Iterable? In our case, it is tempting to choose Seq because our collection can contain duplicates and iteration order is determined by insertion order. However, some properties of Seq are not satisfied:

(xs ++ ys).size == xs.size + ys.size

Consequently, the only sensible choice as a base collection type is collection.immutable.Iterable.

First version of Capped class

import scala.collection._

class Capped1[A] private (val capacity: Int, val length: Int, offset: Int, elems: Array[Any])
  extends immutable.Iterable[A] { self =>

  def this(capacity: Int) =
    this(capacity, length = 0, offset = 0, elems = Array.ofDim(capacity))

  def appended[B >: A](elem: B): Capped1[B] = {
    val newElems = Array.ofDim[Any](capacity)
    Array.copy(elems, 0, newElems, 0, capacity)
    val (newOffset, newLength) =
      if (length == capacity) {
        newElems(offset) = elem
        ((offset + 1) % capacity, length)
      } else {
        newElems(length) = elem
        (offset, length + 1)
    new Capped1[B](capacity, newLength, newOffset, newElems)

  @`inline` def :+ [B >: A](elem: B): Capped1[B] = appended(elem)

  def apply(i: Int): A = elems((i + offset) % capacity).asInstanceOf[A]

  def iterator: Iterator[A] = new AbstractIterator[A] {
    private var current = 0
    def hasNext = current < self.length
    def next(): A = {
      val elem = self(current)
      current += 1
  override def className = "Capped1"


The above listing presents the first version of our capped collection implementation. It will be refined later. The class Capped1 has a private constructor that takes the collection capacity, length, offset (first element index) and the underlying array as parameters. The public constructor takes only the capacity of the collection. It sets the length and offset to 0, and uses an empty array of elements.

The appended method defines how elements can be appended to a given Capped1 collection: it creates a new underlying array of elements, copies the current elements and adds the new element. As long as the number of elements does not exceed the capacity, the new element is appended after the previous elements. However, as soon as the maximal capacity has been reached, the new element replaces the first element of the collection (at offset index).

The apply method implements indexed access: it translates the given index into its corresponding index in the underlying array by adding the offset.

These two methods, appended and apply, implement the specific behavior of the Capped1 collection type. In addition to them, we have to implement iterator to make the generic collection operations (such as foldLeft, count, etc.) work on Capped collections. Here we implement it by using indexed access.

Last, we override className to return the name of the collection, “Capped1”. This name is used by the toString operation.

Here are some interactions with the Capped1 collection:

scala> new Capped1(capacity = 4)
res0: Capped1[Nothing] = Capped1()

scala> res0 :+ 1 :+ 2 :+ 3
res1: Capped1[Int] = Capped1(1, 2, 3)

scala> res1.length
res2: Int = 3

scala> res1.lastOption
res3: Option[Int] = Some(3)

scala> res1 :+ 4 :+ 5 :+ 6
res4: Capped1[Int] = Capped1(3, 4, 5, 6)

scala> res4.take(3)
res5: collection.immutable.Iterable[Int] = List(3, 4, 5)

You can see that if we try to grow the collection with more than four elements, the first elements are dropped (see res4). The operations behave as expected except for the last one: after calling take we get back a List instead of the expected Capped1 collection. This is because all that was done in class Capped1 was making Capped1 extend immutable.Iterable. This class has a take method that returns an immutable.Iterable, and that’s implemented in terms of immutable.Iterable’s default implementation, List. So, that’s what you were seeing on the last line of the previous interaction.

Now that you understand why things are the way they are, the next question should be what needs to be done to change them? One way to do this would be to override the take method in class Capped1, maybe like this:

def take(count: Int): Capped1 = …

This would do the job for take. But what about drop, or filter, or init? In fact there are over fifty methods on collections that return again a collection. For consistency, all of these would have to be overridden. This looks less and less like an attractive option. Fortunately, there is a much easier way to achieve the same effect, as shown in the next section.

Second version of Capped class

import scala.collection._

class Capped2[A] private (val capacity: Int, val length: Int, offset: Int, elems: Array[Any])
  extends immutable.Iterable[A]
    with IterableOps[A, Capped2, Capped2[A]] { self =>

  def this(capacity: Int) = // as before

  def appended[B >: A](elem: B): Capped2[B] = // as before
  @`inline` def :+ [B >: A](elem: B): Capped2[B] = // as before
  def apply(i: Int): A = // as before

  def iterator: Iterator[A] = // as before

  override def className = "Capped2"
  override val iterableFactory: IterableFactory[Capped2] = new Capped2Factory(capacity)
  override protected def fromSpecific(coll: IterableOnce[A]): Capped2[A] = iterableFactory.from(coll)
  override protected def newSpecificBuilder: mutable.Builder[A, Capped2[A]] = iterableFactory.newBuilder
  override def empty: Capped2[A] = iterableFactory.empty


class Capped2Factory(capacity: Int) extends IterableFactory[Capped2] {

  def from[A](source: IterableOnce[A]): Capped2[A] =
    (newBuilder[A] ++= source).result()

  def empty[A]: Capped2[A] = new Capped2[A](capacity)

  def newBuilder[A]: mutable.Builder[A, Capped2[A]] =
    new mutable.ImmutableBuilder[A, Capped2[A]](empty) {
      def addOne(elem: A): this.type = { elems = elems :+ elem; this }

The Capped class needs to inherit not only from Iterable, but also from its implementation trait IterableOps. This is shown in the above listing of class Capped2. The new implementation differs from the previous one in only two aspects. First, class Capped2 now also extends IterableOps[A, Capped2, Capped2[A]]. Second, its iterableFactory member is overridden to return an IterableFactory[Capped2]. As explained in the previous sections, the IterableOps trait implements all concrete methods of Iterable in a generic way. For instance, the return type of methods like take, drop, filter or init is the third type parameter passed to class IterableOps, i.e., in class Capped2, it is Capped2[A]. Similarly, the return type of methods like map, flatMap or concat is defined by the second type parameter passed to class IterableOps, i.e., in class Capped2, it is Capped2 itself.

Operations returning Capped2[A] collections are implemented in IterableOps in terms of the fromSpecific and newSpecificBuilder operations. The immutable.Iterable[A] parent class implements the fromSpecific and newSpecificBuilder such that they only return immutable.Iterable[A] collections instead of the expected Capped2[A] collections. Consequently, we override the fromSpecific and newSpecificBuilder operations to make them return a Capped2[A] collection. Another inherited operation returning a too general type is empty. We override it to return a Capped2[A] collection too. All these overrides simply forward to the collection factory referred to by the iterableFactory member, whose value is an instance of class Capped2Factory.

The Capped2Factory class provides convenient factory methods to build collections. Eventually, these methods delegate to the empty operation, which builds an empty Capped2 instance, and newBuilder, which uses the appended operation to grow a Capped2 collection.

With the refined implementation of the Capped2 class, the transformation operations work now as expected, and the Capped2Factory class provides seamless conversions from other collections:

scala> object Capped extends Capped2Factory(capacity = 4)
defined object Capped

scala> Capped(1, 2, 3)
res0: Capped2[Int] = Capped2(1, 2, 3)

scala> res0.take(2)
res1: Capped2[Int] = Capped2(1, 2)

scala> res0.filter(x => x % 2 == 1)
res2: Capped2[Int] = Capped2(1, 3)

scala> => x * x)
res3: Capped2[Int] = Capped2(1, 4, 9)

scala> List(1, 2, 3, 4, 5).to(Capped)
res4: Capped2[Int] = Capped2(2, 3, 4, 5)

This implementation now behaves correctly, but we can still improve a few things:

  • since our collection is strict, we can take advantage of the better performance offered by strict implementations of transformation operations,
  • since our fromSpecific, newSpecificBuilder and empty operation just forward to the iterableFactory member, we can use the IterableFactoryDefaults trait that provides such implementations.

Final version of Capped class

import scala.collection._

final class Capped[A] private (val capacity: Int, val length: Int, offset: Int, elems: Array[Any])
  extends immutable.Iterable[A]
    with IterableOps[A, Capped, Capped[A]]
    with IterableFactoryDefaults[A, Capped]
    with StrictOptimizedIterableOps[A, Capped, Capped[A]] { self =>

  def this(capacity: Int) =
    this(capacity, length = 0, offset = 0, elems = Array.ofDim(capacity))

  def appended[B >: A](elem: B): Capped[B] = {
    val newElems = Array.ofDim[Any](capacity)
    Array.copy(elems, 0, newElems, 0, capacity)
    val (newOffset, newLength) =
      if (length == capacity) {
        newElems(offset) = elem
        ((offset + 1) % capacity, length)
      } else {
        newElems(length) = elem
        (offset, length + 1)
    new Capped[B](capacity, newLength, newOffset, newElems)

  @`inline` def :+ [B >: A](elem: B): Capped[B] = appended(elem)

  def apply(i: Int): A = elems((i + offset) % capacity).asInstanceOf[A]

  def iterator: Iterator[A] = view.iterator

  override def view: IndexedSeqView[A] = new IndexedSeqView[A] {
    def length: Int = self.length
    def apply(i: Int): A = self(i)

  override def knownSize: Int = length

  override def className = "Capped"

  override val iterableFactory: IterableFactory[Capped] = new CappedFactory(capacity)


class CappedFactory(capacity: Int) extends IterableFactory[Capped] {

  def from[A](source: IterableOnce[A]): Capped[A] =
    source match {
      case capped: Capped[A] if capped.capacity == capacity => capped
      case _ => (newBuilder[A] ++= source).result()

  def empty[A]: Capped[A] = new Capped[A](capacity)

  def newBuilder[A]: mutable.Builder[A, Capped[A]] =
    new mutable.ImmutableBuilder[A, Capped[A]](empty) {
      def addOne(elem: A): this.type = { elems = elems :+ elem; this }


That is it. The final Capped class:

  • extends the StrictOptimizedIterableOps trait, which overrides all transformation operations to take advantage of strict builders,
  • extends the IterableFactoryDefaults trait, which overrides the fromSpecific, newSpecificBuilder and empty operations to forward to the iterableFactory,
  • overrides a few operations for performance: the view now uses indexed access, and the iterator delegates to the view. The knownSize operation is also overridden because the size is always known.

Its implementation requires a little bit of protocol. In essence, you have to inherit from the Ops template trait in addition to just inheriting from a collection type, override the iterableFactory member to return a more specific factory, and finally implement abstract methods (such as iterator in our case), if any.

RNA sequences

To start with the second example, we define the four RNA Bases:

abstract class Base
case object A extends Base
case object U extends Base
case object G extends Base
case object C extends Base

object Base {
  val fromInt: Int => Base = Array(A, U, G, C)
  val toInt: Base => Int = Map(A -> 0, U -> 1, G -> 2, C -> 3)

Say you want to create a new immutable sequence type for RNA strands, which are sequences of bases A (adenine), U (uracil), G (guanine), and C (cytosine). The definitions for bases are easily set up as shown in the listing of RNA bases above.

Every base is defined as a case object that inherits from a common abstract class Base. The Base class has a companion object that defines two functions that map between bases and the integers 0 to 3. You can see in the examples two different ways to use collections to implement these functions. The toInt function is implemented as a Map from Base values to integers. The reverse function, fromInt, is implemented as an array. This makes use of the fact that both maps and arrays are functions because they inherit from the Function1 trait.

The next task is to define a class for strands of RNA. Conceptually, a strand of RNA is simply a Seq[Base]. However, RNA strands can get quite long, so it makes sense to invest some work in a compact representation. Because there are only four bases, a base can be identified with two bits, and you can therefore store sixteen bases as two-bit values in an integer. The idea, then, is to construct a specialized subclass of Seq[Base], which uses this packed representation.

First version of RNA strands class

import collection.mutable
import collection.immutable.{ IndexedSeq, IndexedSeqOps }

final class RNA1 private (
  val groups: Array[Int],
  val length: Int
) extends IndexedSeq[Base]
  with IndexedSeqOps[Base, IndexedSeq, RNA1] {

  import RNA1._

  def apply(idx: Int): Base = {
    if (idx < 0 || length <= idx)
      throw new IndexOutOfBoundsException
    Base.fromInt(groups(idx / N) >> (idx % N * S) & M)

  override protected def fromSpecific(coll: IterableOnce[Base]): RNA1 =
  override protected def newSpecificBuilder: mutable.Builder[Base, RNA1] =
  override def empty: RNA1 = fromSeq(Seq.empty)
  override def className = "RNA1"

object RNA1 {

  // Number of bits necessary to represent group
  private val S = 2            

  // Number of groups that fit in an Int
  private val N = 32 / S       

  // Bitmask to isolate a group
  private val M = (1 << S) - 1

  def fromSeq(buf: collection.Seq[Base]): RNA1 = {
    val groups = new Array[Int]((buf.length + N - 1) / N)
    for (i <- 0 until buf.length)
      groups(i / N) |= Base.toInt(buf(i)) << (i % N * S)
    new RNA1(groups, buf.length)

  def apply(bases: Base*) = fromSeq(bases)

The RNA strands class listing above presents the first version of this class. The class RNA1 has a constructor that takes an array of Ints as its first argument. This array contains the packed RNA data, with sixteen bases in each element, except for the last array element, which might be partially filled. The second argument, length, specifies the total number of bases on the array (and in the sequence). Class RNA1 extends IndexedSeq[Base] and IndexedSeqOps[Base, IndexedSeq, RNA1]. These traits define the following abstract methods:

  • length, automatically implemented by defining a parametric field of the same name,
  • apply (indexing method), implemented by first extracting an integer value from the groups array, then extracting the correct two-bit number from that integer using right shift (>>) and mask (&). The private constants S, N, and M come from the RNA1 companion object. S specifies the size of each packet (i.e., two); N specifies the number of two-bit packets per integer; and M is a bit mask that isolates the lowest S bits in a word.

We also override the following members used by transformation operations such as filter and take:

  • fromSpecific, implemented by the fromSeq method of the RNA1 companion object,
  • newSpecificBuilder, implemented by using the default IndexedSeq builder and transforming its result into an RNA1 with the mapResult method.

Note that the constructor of class RNA1 is private. This means that clients cannot create RNA1 sequences by calling new, which makes sense, because it hides the representation of RNA1 sequences in terms of packed arrays from the user. If clients cannot see what the representation details of RNA sequences are, it becomes possible to change these representation details at any point in the future without affecting client code. In other words, this design achieves a good decoupling of the interface of RNA sequences and its implementation. However, if constructing an RNA sequence with new is impossible, there must be some other way to create new RNA sequences, else the whole class would be rather useless. In fact there are two alternatives for RNA sequence creation, both provided by the RNA1 companion object. The first way is method fromSeq, which converts a given sequence of bases (i.e., a value of type Seq[Base]) into an instance of class RNA1. The fromSeq method does this by packing all the bases contained in its argument sequence into an array, then calling RNA1’s private constructor with that array and the length of the original sequence as arguments. This makes use of the fact that a private constructor of a class is visible in the class’s companion object.

The second way to create an RNA1 value is provided by the apply method in the RNA1 object. It takes a variable number of Base arguments and simply forwards them as a sequence to fromSeq. Here are the two creation schemes in action:

scala> val xs = List(A, G, U, A)
xs: List[Base] = List(A, G, U, A)

scala> RNA1.fromSeq(xs)
res1: RNA1 = RNA1(A, G, U, A)

scala> val rna1 = RNA1(A, U, G, G, C)
rna1: RNA1 = RNA1(A, U, G, G, C)

Also note that the type parameters of the IndexedSeqOps trait that we inherit from are: Base, IndexedSeq and RNA1. The first one stands for the type of elements, the second one stands for the type constructor used by transformation operations that return a collection with a different type of elements, and the third one stands for the type used by transformation operations that return a collection with the same type of elements. In our case, it is worth noting that the second one is IndexedSeq whereas the third one is RNA1. This means that operations like map or flatMap return an IndexedSeq, whereas operations like take or filter return an RNA1.

Here is an example showing the usage of take and filter:

scala> rna1.take(3)
res5: RNA1 = RNA1(A, U, G)

scala> rna1.filter(_ != U)
res6: RNA1 = RNA1(A, G, G, C)

Dealing with map and friends

However, transformation operations that return a collection with a different element type always return an IndexedSeq.

How should these methods be adapted to RNA strands? The desired behavior would be to get back an RNA strand when mapping bases to bases or appending two RNA strands with ++:

scala> val rna = RNA(A, U, G, G, C)
rna: RNA = RNA(A, U, G, G, C)

scala> rna map { case A => U case b => b }
res7: RNA = RNA(U, U, G, G, C)

scala> rna ++ rna
res8: RNA = RNA(A, U, G, G, C, A, U, G, G, C)

On the other hand, mapping bases to some other type over an RNA strand cannot yield another RNA strand because the new elements have the wrong type. It has to yield a sequence instead. In the same vein appending elements that are not of type Base to an RNA strand can yield a general sequence, but it cannot yield another RNA strand.

scala> rna map Base.toInt
res2: IndexedSeq[Int] = Vector(0, 1, 2, 2, 3)

scala> rna ++ List("missing", "data")
res3: IndexedSeq[java.lang.Object] =
  Vector(A, U, G, G, C, missing, data)

This is what you’d expect in the ideal case. But this is not what the RNA1 class provides. In fact, all examples will return instances of Vector, not just the last two. If you run the first three commands above with instances of this class you obtain:

scala> val rna1 = RNA1(A, U, G, G, C)
rna1: RNA1 = RNA1(A, U, G, G, C)

scala> rna1 map { case A => U case b => b }
res0: IndexedSeq[Base] = Vector(U, U, G, G, C)

scala> rna1 ++ rna1
res1: IndexedSeq[Base] = Vector(A, U, G, G, C, A, U, G, G, C)

So the result of map and ++ is never an RNA strand, even if the element type of the generated collection is Base. To see how to do better, it pays to have a close look at the signature of the map method (or of ++, which has a similar signature). The map method is originally defined in class scala.collection.IterableOps with the following signature:

def map[B](f: A => B): CC[B]

Here A is the type of elements of the collection, and CC is the type constructor passed as a second parameter to the IterableOps trait.

In our RNA1 implementation, this CC type constructor is IndexedSeq, this is why we always get a Vector as a result.

Second version of RNA strands class

import scala.collection.{ View, mutable }
import scala.collection.immutable.{ IndexedSeq, IndexedSeqOps }

final class RNA2 private (val groups: Array[Int], val length: Int)
  extends IndexedSeq[Base] with IndexedSeqOps[Base, IndexedSeq, RNA2] {

  import RNA2._

  def apply(idx: Int): Base = // as before
  override protected def fromSpecific(coll: IterableOnce[Base]): RNA2 = // as before
  override protected def newSpecificBuilder: mutable.Builder[Base, RNA2] = // as before
  // Overloading of `appended`, `prepended`, `appendedAll`,
  // `prependedAll`, `map`, `flatMap` and `concat` to return an `RNA2`
  // when possible
  def concat(suffix: IterableOnce[Base]): RNA2 =
    fromSpecific(iterator ++ suffix.iterator)
  // symbolic alias for `concat`
  @inline final def ++ (suffix: IterableOnce[Base]): RNA2 = concat(suffix)
  def appended(base: Base): RNA2 =
    fromSpecific(new View.Append(this, base))
  def appendedAll(suffix: IterableOnce[Base]): RNA2 =
  def prepended(base: Base): RNA2 = 
    fromSpecific(new View.Prepend(base, this))
  def prependedAll(prefix: IterableOnce[Base]): RNA2 =
    fromSpecific(prefix.iterator ++ iterator)
  def map(f: Base => Base): RNA2 =
    fromSpecific(new View.Map(this, f))
  def flatMap(f: Base => IterableOnce[Base]): RNA2 =
    fromSpecific(new View.FlatMap(this, f))

To address this shortcoming, you need to overload the methods that return an IndexedSeq[B] for the case where B is known to be Base, to return an RNA2 instead.

Compared to class RNA1 we added overloads for methods concat, appended, appendedAll, prepended, prependedAll, map and flatMap.

This implementation now behaves correctly, but we can still improve a few things. Since our collection is strict, we could take advantage of the better performance offered by strict builders in transformation operations. Also, if we try to convert an Iterable[Base] into an RNA2 it fails:

scala> val bases: Iterable[Base] = List(A, U, C, C)
bases: Iterable[Base] = List(A, U, C, C)

       error: type mismatch;
        found   : RNA2.type
        required: scala.collection.Factory[Base,?]

Final version of RNA strands class

import scala.collection.{ AbstractIterator, SpecificIterableFactory, StrictOptimizedSeqOps, View, mutable }
import scala.collection.immutable.{ IndexedSeq, IndexedSeqOps }

final class RNA private (
  val groups: Array[Int],
  val length: Int
) extends IndexedSeq[Base]
    with IndexedSeqOps[Base, IndexedSeq, RNA]
    with StrictOptimizedSeqOps[Base, IndexedSeq, RNA] { rna =>

  import RNA._

  // Mandatory implementation of `apply` in `IndexedSeqOps`
  def apply(idx: Int): Base = {
    if (idx < 0 || length <= idx)
      throw new IndexOutOfBoundsException
    Base.fromInt(groups(idx / N) >> (idx % N * S) & M)

  // Mandatory overrides of `fromSpecific`, `newSpecificBuilder`,
  // and `empty`, from `IterableOps`
  override protected def fromSpecific(coll: IterableOnce[Base]): RNA =
  override protected def newSpecificBuilder: mutable.Builder[Base, RNA] =
  override def empty: RNA = RNA.empty

  // Overloading of `appended`, `prepended`, `appendedAll`, `prependedAll`,
  // `map`, `flatMap` and `concat` to return an `RNA` when possible
  def concat(suffix: IterableOnce[Base]): RNA =
    strictOptimizedConcat(suffix, newSpecificBuilder)
  @inline final def ++ (suffix: IterableOnce[Base]): RNA = concat(suffix)
  def appended(base: Base): RNA =
    (newSpecificBuilder ++= this += base).result()
  def appendedAll(suffix: Iterable[Base]): RNA =
    strictOptimizedConcat(suffix, newSpecificBuilder)
  def prepended(base: Base): RNA =
    (newSpecificBuilder += base ++= this).result()
  def prependedAll(prefix: Iterable[Base]): RNA =
    (newSpecificBuilder ++= prefix ++= this).result()
  def map(f: Base => Base): RNA =
    strictOptimizedMap(newSpecificBuilder, f)
  def flatMap(f: Base => IterableOnce[Base]): RNA =
    strictOptimizedFlatMap(newSpecificBuilder, f)

  // Optional re-implementation of iterator,
  // to make it more efficient.
  override def iterator: Iterator[Base] = new AbstractIterator[Base] {
    private var i = 0
    private var b = 0
    def hasNext: Boolean = i < rna.length
    def next(): Base = {
      b = if (i % N == 0) groups(i / N) else b >>> S
      i += 1
      Base.fromInt(b & M)

  override def className = "RNA"

object RNA extends SpecificIterableFactory[Base, RNA] {

  private val S = 2            // number of bits in group
  private val M = (1 << S) - 1 // bitmask to isolate a group
  private val N = 32 / S       // number of groups in an Int

  def fromSeq(buf: collection.Seq[Base]): RNA = {
    val groups = new Array[Int]((buf.length + N - 1) / N)
    for (i <- 0 until buf.length)
      groups(i / N) |= Base.toInt(buf(i)) << (i % N * S)
    new RNA(groups, buf.length)

  // Mandatory factory methods: `empty`, `newBuilder`
  // and `fromSpecific`
  def empty: RNA = fromSeq(Seq.empty)

  def newBuilder: mutable.Builder[Base, RNA] =

  def fromSpecific(it: IterableOnce[Base]): RNA = it match {
    case seq: collection.Seq[Base] => fromSeq(seq)
    case _ => fromSeq(mutable.ArrayBuffer.from(it))

The final RNA class:

  • extends the StrictOptimizedSeqOps trait, which overrides all transformation operations to take advantage of strict builders,
  • uses utility operations provided by the StrictOptimizedSeqOps trait such as strictOptimizedConcat to implement overload of transformation operations that return an RNA collection,
  • has a companion object that extends SpecificIterableFactory[Base, RNA], which makes it possible to use it as a parameter of a to call (to convert any collection of bases to an RNA, e.g. List(U, A, G, C).to(RNA)),
  • moves the newSpecificBuilder and fromSpecific implementations to the companion object.

The discussion so far centered on the minimal amount of definitions needed to define new sequences with methods that obey certain types. But in practice you might also want to add new functionality to your sequences or to override existing methods for better efficiency. An example of this is the overridden iterator method in class RNA. iterator is an important method in its own right because it implements loops over collections. Furthermore, many other collection methods are implemented in terms of iterator. So it makes sense to invest some effort optimizing the method’s implementation. The standard implementation of iterator in IndexedSeq will simply select every i‘th element of the collection using apply, where i ranges from 0 to the collection’s length minus one. So this standard implementation selects an array element and unpacks a base from it once for every element in an RNA strand. The overriding iterator in class RNA is smarter than that. For every selected array element it immediately applies the given function to all bases contained in it. So the effort for array selection and bit unpacking is much reduced.

Prefix map

As a third example you’ll learn how to integrate a new kind of mutable map into the collection framework. The idea is to implement a mutable map with String as the type of keys by a “Patricia trie”. The term Patricia is in fact an abbreviation for “Practical Algorithm to Retrieve Information Coded in Alphanumeric” and trie comes from retrieval (a trie is also called a radix tree or prefix tree). The idea is to store a set or a map as a tree where subsequent characters in a search key uniquely determine a path through the tree. For instance a Patricia trie storing the strings “abc”, “abd”, “al”, “all” and “xy” would look like this:

A sample patricia trie:

To find the node corresponding to the string “abc” in this trie, simply follow the subtree labeled “a”, proceed from there to the subtree labelled “b”, to finally reach its subtree labelled “c”. If the Patricia trie is used as a map, the value that’s associated with a key is stored in the nodes that can be reached by the key. If it is a set, you simply store a marker saying that the node is present in the set.

Patricia tries support very efficient lookups and updates. Another nice feature is that they support selecting a subcollection by giving a prefix. For instance, in the patricia tree above you can obtain the sub-collection of all keys that start with an “a” simply by following the “a” link from the root of the tree.

Based on these ideas we will now walk you through the implementation of a map that’s implemented as a Patricia trie. We call the map a PrefixMap, which means that it provides a method withPrefix that selects a submap of all keys starting with a given prefix. We’ll first define a prefix map with the keys shown in the running example:

scala> val m = PrefixMap("abc" -> 0, "abd" -> 1, "al" -> 2,
  "all" -> 3, "xy" -> 4)
m: PrefixMap[Int] = PrefixMap((abc,0), (abd,1), (al,2), (all,3), (xy,4))

Then calling withPrefix on m will yield another prefix map:

scala> m withPrefix "a"
res14: PrefixMap[Int] = PrefixMap((bc,0), (bd,1), (l,2), (ll,3))

Patricia trie implementation

import scala.collection._
import scala.collection.mutable.{ GrowableBuilder, Builder }

class PrefixMap[A]
  extends mutable.Map[String, A]
    with mutable.MapOps[String, A, mutable.Map, PrefixMap[A]]
    with StrictOptimizedIterableOps[(String, A), mutable.Iterable, PrefixMap[A]] {

  private var suffixes: immutable.Map[Char, PrefixMap[A]] = immutable.Map.empty
  private var value: Option[A] = None

  def get(s: String): Option[A] =
    if (s.isEmpty) value
    else suffixes get (s(0)) flatMap (_.get(s substring 1))

  def withPrefix(s: String): PrefixMap[A] =
    if (s.isEmpty) this
    else {
      val leading = s(0)
      suffixes get leading match {
        case None =>
          suffixes = suffixes + (leading -> empty)
        case _ =>
      suffixes(leading) withPrefix (s substring 1)

  def iterator: Iterator[(String, A)] =
    (for (v <- value.iterator) yield ("", v)) ++
      (for ((chr, m) <- suffixes.iterator;
            (s, v) <- m.iterator) yield (chr +: s, v))

  def addOne(kv: (String, A)): this.type = {
    withPrefix(kv._1).value = Some(kv._2)

  def subtractOne(s: String): this.type  = {
    if (s.isEmpty) { val prev = value; value = None; prev }
    else suffixes get (s(0)) flatMap (_.remove(s substring 1))

  // Overloading of transformation methods that should return a PrefixMap
  def map[B](f: ((String, A)) => (String, B)): PrefixMap[B] =
    strictOptimizedMap(PrefixMap.newBuilder, f)
  def flatMap[B](f: ((String, A)) => IterableOnce[(String, B)]): PrefixMap[B] =
    strictOptimizedFlatMap(PrefixMap.newBuilder, f)

  // Override `concat` and `empty` methods to refine their return type
  override def concat[B >: A](suffix: IterableOnce[(String, B)]): PrefixMap[B] =
    strictOptimizedConcat(suffix, PrefixMap.newBuilder)
  override def empty: PrefixMap[A] = new PrefixMap

  // Members declared in scala.collection.mutable.Clearable
  override def clear(): Unit = suffixes = immutable.Map.empty
  // Members declared in scala.collection.IterableOps
  override protected def fromSpecific(coll: IterableOnce[(String, A)]): PrefixMap[A] = PrefixMap.from(coll)
  override protected def newSpecificBuilder: mutable.Builder[(String, A), PrefixMap[A]] = PrefixMap.newBuilder
  override def className = "PrefixMap"

object PrefixMap {
  def empty[A] = new PrefixMap[A]

  def from[A](source: IterableOnce[(String, A)]): PrefixMap[A] =
    source match {
      case pm: PrefixMap[A] => pm
      case _ => (newBuilder ++= source).result()

  def apply[A](kvs: (String, A)*): PrefixMap[A] = from(kvs)

  def newBuilder[A]: mutable.Builder[(String, A), PrefixMap[A]] =
    new mutable.GrowableBuilder[(String, A), PrefixMap[A]](empty)

  import scala.language.implicitConversions

  implicit def toFactory[A](self: this.type): Factory[(String, A), PrefixMap[A]] =
    new Factory[(String, A), PrefixMap[A]] {
      def fromSpecific(it: IterableOnce[(String, A)]): PrefixMap[A] = self.from(it)
      def newBuilder: mutable.Builder[(String, A), PrefixMap[A]] = self.newBuilder


The previous listing shows the definition of PrefixMap. The map has keys of type String and the values are of parametric type A. It extends mutable.Map[String, A] and mutable.MapOps[String, A, mutable.Map, PrefixMap[A]]. You have seen this pattern already for sequences in the RNA strand example; then as now inheriting an implementation class such as MapOps serves to get the right result type for transformations such as filter.

A prefix map node has two mutable fields: suffixes and value. The value field contains an optional value that’s associated with the node. It is initialized to None. The suffixes field contains a map from characters to PrefixMap values. It is initialized to the empty map.

You might ask why we picked an immutable map as the implementation type for suffixes? Would not a mutable map have been more standard, since PrefixMap as a whole is also mutable? The answer is that immutable maps that contain only a few elements are very efficient in both space and execution time. For instance, maps that contain fewer than 5 elements are represented as a single object. By contrast, the standard mutable map is a HashMap, which typically occupies around 80 bytes, even if it is empty. So if small collections are common, it’s better to pick immutable over mutable. In the case of Patricia tries, we’d expect that most nodes except the ones at the very top of the tree would contain only a few successors. So storing these successors in an immutable map is likely to be more efficient.

Now have a look at the first method that needs to be implemented for a map: get. The algorithm is as follows: To get the value associated with the empty string in a prefix map, simply select the optional value stored in the root of the tree (the current map). Otherwise, if the key string is not empty, try to select the submap corresponding to the first character of the string. If that yields a map, follow up by looking up the remainder of the key string after its first character in that map. If the selection fails, the key is not stored in the map, so return with None. The combined selection over an option value opt is elegantly expressed using opt.flatMap(x => f(x)). When applied to an optional value that is None, it returns None. Otherwise opt is Some(x) and the function f is applied to the encapsulated value x, yielding a new option, which is returned by the flatmap.

The next two methods to implement for a mutable map are addOne and subtractOne.

The subtractOne method is very similar to get, except that before returning any associated value, the field containing that value is set to None. The addOne method first calls withPrefix to navigate to the tree node that needs to be updated, then sets the value field of that node to the given value. The withPrefix method navigates through the tree, creating sub-maps as necessary if some prefix of characters is not yet contained as a path in the tree.

The last abstract method to implement for a mutable map is iterator. This method needs to produce an iterator that yields all key/value pairs stored in the map. For any given prefix map this iterator is composed of the following parts: First, if the map contains a defined value, Some(x), in the value field at its root, then ("", x) is the first element returned from the iterator. Furthermore, the iterator needs to traverse the iterators of all submaps stored in the suffixes field, but it needs to add a character in front of every key string returned by those iterators. More precisely, if m is the submap reached from the root through a character chr, and (s, v) is an element returned from m.iterator, then the root’s iterator will return (chr +: s, v) instead. This logic is implemented quite concisely as a concatenation of two for expressions in the implementation of the iterator method in PrefixMap. The first for expression iterates over value.iterator. This makes use of the fact that Option values define an iterator method that returns either no element, if the option value is None, or exactly one element x, if the option value is Some(x).

However, in all these cases, to build the right kind of collection you need to start with an empty collection of that kind. This is provided by the empty method, which simply returns a fresh PrefixMap.

We’ll now turn to the companion object PrefixMap. In fact it is not strictly necessary to define this companion object, as class PrefixMap can stand well on its own. The main purpose of object PrefixMap is to define some convenience factory methods. It also defines an implicit conversion to Factory for a better interoperability with other collections. This conversion is triggered when one writes, for instance, List("foo" -> 3).to(PrefixMap). The to operation takes a Factory as parameter but the PrefixMap companion object does not extend Factory (and it can not because a Factory fixes the type of collection elements, whereas PrefixMap has a polymorphic type of values).

The two convenience methods are empty and apply. The same methods are present for all other collections in Scala’s collection framework so it makes sense to define them here, too. With the two methods, you can write PrefixMap literals like you do for any other collection:

scala> PrefixMap("hello" -> 5, "hi" -> 2)
res0: PrefixMap[Int] = PrefixMap(hello -> 5, hi -> 2)

scala> res0 += "foo" -> 3
res1: res0.type = PrefixMap(hello -> 5, hi -> 2, foo -> 3)


To summarize, if you want to fully integrate a new collection class into the framework you need to pay attention to the following points:

  1. Decide whether the collection should be mutable or immutable.
  2. Pick the right base traits for the collection.
  3. Inherit from the right implementation trait to implement most collection operations.
  4. Overload desired operations that do not return, by default, a collection as specific as they could. A complete list of such operations is given as an appendix.

You have now seen how Scala’s collections are built and how you can add new kinds of collections. Because of Scala’s rich support for abstraction, each new collection type has a large number of methods without having to reimplement them all over again.


This page contains material adapted from the book Programming in Scala by Odersky, Spoon and Venners. We thank Artima for graciously agreeing to its publication.

Appendix: Methods to overload to support the “same result type” principle

You want to add overloads to specialize transformation operations such that they return a more specific result type. Examples are:

  • map, on StringOps, when the mapping function returns a Char, should return a String (instead of an IndexedSeq),
  • map, on Map, when the mapping function returns a pair, should return a Map (instead of an Iterable),
  • map, on SortedSet, when an implicit Ordering is available for the resulting element type, should return a SortedSet (instead of a Set).

Typically, this happens when the collection fixes some type parameter of its template trait. For instance in the case of the RNA collection type, we fix the element type to Base, and in the case of the PrefixMap[A] collection type, we fix the type of keys to String.

The following table lists transformation operations that might return an undesirably wide type. You might want to overload these operations to return a more specific type.

Collection Operations
Iterable map, flatMap, collect, scanLeft, scanRight, groupMap, concat, zip, zipAll, unzip
Seq prepended, appended, prependedAll, appendedAll, padTo, patch
immutable.Seq updated
SortedSet map, flatMap, collect, zip
Map map, flatMap, collect, concat
immutable.Map updated, transform
SortedMap map, flatMap, collect, concat
immutable.SortedMap updated

Appendix: Cross-building custom collections

Since the new internal API of the Scala 2.13 collections is very different from the previous collections API, authors of custom collection types should use separate source directories (per Scala version) to define them.

With sbt you can achieve this by adding the following setting to your project:

// Adds a `src/main/scala-2.13+` source directory for Scala 2.13 and newer
// and a `src/main/scala-2.13-` source directory for Scala version older than 2.13
unmanagedSourceDirectories in Compile += {
  val sourceDir = (sourceDirectory in Compile).value
  CrossVersion.partialVersion(scalaVersion.value) match {
    case Some((2, n)) if n >= 13 => sourceDir / "scala-2.13+"
    case _                       => sourceDir / "scala-2.13-"

And then you can define a Scala 2.13 compatible implementation of your collection in the src/main/scala-2.13+ source directory, and an implementation for the previous Scala versions in the src/main/scala-2.13- source directory.

You can see how this has been put in practice in scalacheck and scalaz.

Contributors to this page: