Sample collection implementation with Scala 2.13

MyList.scala

package com.tiqwab.example.step4

import scala.collection.mutable.ListBuffer
import scala.collection.{immutable, mutable, SeqFactory, StrictOptimizedLinearSeqOps}

class MyList[+A] private (elems: List[A])
  extends immutable.LinearSeq[A] with immutable.LinearSeqOps[A, MyList, MyList[A]]
  with StrictOptimizedLinearSeqOps[A, MyList, MyList[A]] {
  // To be overridden in implementations, as said in scala.collection.LinearSeq
  override def isEmpty: Boolean = elems.isEmpty
  override def head: A = elems.head
  override def tail: MyList[A] = new MyList(elems.tail)

  override def iterator: Iterator[A] = elems.iterator

  // To be overridden for LinearSeqOps
  override def iterableFactory: SeqFactory[MyList] = MyList

  override def toString: String = s"MyList(${elems.mkString(",")})"
}

object MyList extends SeqFactory[MyList] {
  override def from[A](source: IterableOnce[A]): MyList[A] =
    newBuilder[A].addAll(source).result()
  override def empty[A]: MyList[A] =
    new MyList(List.empty)
  override def newBuilder[A]: mutable.Builder[A, MyList[A]] =
    new ListBuffer[A].mapResult(elems => new MyList(elems))
}

MyMap.scala

package com.tiqwab.example.step4

import scala.collection.mutable.ListBuffer
import scala.collection.{immutable, mutable, MapFactory, StrictOptimizedMapOps}

class MyMap[K, +V] private (elems: Map[K, V])
  extends immutable.Map[K, V] with immutable.MapOps[K, V, MyMap, MyMap[K, V]]
  with StrictOptimizedMapOps[K, V, MyMap, MyMap[K, V]] {
  // Should be implemented for immutable.Map
  override def remove(key: K): MyMap[K, V] =
    new MyMap(elems.remove(key))
  override def updated[V1 >: V](key: K, value: V1): MyMap[K, V1] =
    new MyMap(elems.updated(key, value))
  override def get(key: K): Option[V] =
    elems.get(key)
  override def iterator: Iterator[(K, V)] =
    elems.iterator

  override def mapFactory: MapFactory[MapCC] = MyMap

  override def toString: String = s"MyMap(${elems.mkString(",")})"
}

object MyMap extends MapFactory[MyMap] {
  override def from[K, V](it: IterableOnce[(K, V)]): MyMap[K, V] = newBuilder[K, V].addAll(it).result()
  override def empty[K, V]: MyMap[K, V] = new MyMap(Map.empty)
  override def newBuilder[K, V]: mutable.Builder[(K, V), MyMap[K, V]] =
    new ListBuffer[(K, V)]().mapResult(elems => new MyMap(elems.toMap))
}

MySet.scala

package com.tiqwab.example.step4

import scala.collection.mutable.ListBuffer
import scala.collection.{immutable, mutable, IterableFactory, StrictOptimizedSetOps}

class MySet[A] private (elems: Set[A])
  extends immutable.Set[A] with immutable.SetOps[A, MySet, MySet[A]] with StrictOptimizedSetOps[A, MySet, MySet[A]] {
  // Should be implemented for immutable.Set
  override def incl(elem: A): MySet[A] = new MySet(elems.incl(elem))
  override def excl(elem: A): MySet[A] = new MySet(elems.excl(elem))
  override def contains(elem: A): Boolean = elems.contains(elem)
  override def iterator: Iterator[A] = elems.iterator

  override def iterableFactory: IterableFactory[MySet] = MySet

  override def toString: String = s"MySet(${elems.mkString(",")})"
}

object MySet extends IterableFactory[MySet] {
  override def from[A](source: IterableOnce[A]): MySet[A] = newBuilder[A].addAll(source).result()
  override def empty[A]: MySet[A] = new MySet(Set.empty)
  override def newBuilder[A]: mutable.Builder[A, MySet[A]] =
    new ListBuffer[A]().mapResult(elems => new MySet(elems.toSet))
}