Question

I’m reading ‘Programming in scala’ and in one of the example when I try to compile it in Eclipse I receive error : ‘not found: type ChecksumAccumulator’ The type is declared as below. Is the code below correct ?

import scala.collection.mutable.Map

object ChecksumAccumulator {

  private val cache = Map[String, Int]()

  def calculate(s: String): Int = 
    if(cache.contains(s))
      cache(s)
    else {
      val acc = new ChecksumAccumulator
      for(c <- s)
        acc.add(c.toBye)
      val cs = acc.checksum()
      cache += (s -> cs)
      cs
    }
}

Answer 1

From Programming in Scala:

The singleton object in this figure is named ChecksumAccumulator,
the same name as the class in the previous example. When a singleton
object shares the same name with a class, it is called that class’s
companion object. You must define both the class and its companion
object in the same source file. The class is called the companion
class of the singleton object.

If you try to compile this code alone, without the ChecksumAccumulator class, you’ll get a compiler error because you can’t create an instance of a singleton object with new.
The book does a great job of explaining how companion objects and classes work together, and since you’re already reading it I won’t bother adding any further summary here.