Container.scala

package ch2

case class Container(group: Set[Char], x: Double)

object Container {
  def apply(key: Char): Container = Container(Set(key), 0)
}

case class World(containers: Map[Char, Container]) {

  def getAmount(key: Char): Double = containers(key).x

  def addWater(key: Char, amount: Double): World = {
    val container = containers(key)
    val y = amount / container.group.size

    World(container.group.foldLeft(containers) { case (containers, memberKey) =>
      val container1 = containers(memberKey)
      containers.updated(memberKey, container1.copy(x = container1.x + y))
    })
  }

  def connectTo(key1: Char, key2: Char): World = {
    val container1 = containers(key1)
    val container2 = containers(key2)

    val group1 = container1.group ++ container2.group
    val z = (container1.x * container1.group.size + container2.x * container2.group.size) / group1.size

    World(group1.foldLeft(containers){ case (containers, memberKey) =>
      val container1 = containers(memberKey)
      containers.updated(memberKey, container1.copy(group = group1, x = z))
    })
  }

  override def toString: String = containers.keys.toList.sorted.map(getAmount).mkString(" ")
}

UseCase.scala

package ch2

object UseCase {
  def main(args: Array[String]): Unit = {
    val world = World(Map(
     'a' -> Container('a'),
      'b' -> Container('b'),
      'c' -> Container('c'),
      'd' -> Container('d'),
    ))

    val world1 = world
      .addWater('a', 12)
      .addWater('d', 8)
      .connectTo('a', 'b')

    println(world1)

    val world2 = world1.connectTo('b', 'c')

    println(world2)

    val world3 = world2.connectTo('b', 'd')

    println(world3)
  }
}