Return to chapter index

4 Classes and Objects

Sample run of chapter's interpreter examples

• 4.1 Classes, fields, and methods
• 4.2 Semicolon inference
• 4.3 Singleton objects
• 4.4 A Scala application
• 4.5 The App trait
• 4.6 Conclusion

4.1 Classes, fields, and methods

  class ChecksumAccumulator {
    // class definition goes here
  }


  new ChecksumAccumulator


// In file classes-and-objects/ChecksumCalculator1.scala

  class ChecksumAccumulator {
    var sum = 0
  }


  val acc = new ChecksumAccumulator
  val csa = new ChecksumAccumulator


  acc.sum = 3


  // Won't compile, because acc is a val
  acc = new ChecksumAccumulator


  class ChecksumAccumulator {
    private var sum = 0
  }


  val acc = new ChecksumAccumulator
  acc.sum = 5 // Won't compile, because sum is private


// In file classes-and-objects/ChecksumCalculator2.scala

  class ChecksumAccumulator {

    private var sum = 0

    def add(b: Byte): Unit = {
      sum += b
    }

    def checksum(): Int = {
      return ~(sum & 0xFF) + 1
    }
  }


  def add(b: Byte): Unit = {
    b = 1     // This won't compile, because b is a val
    sum += b
  }


// In file classes-and-objects/ChecksumAccumulator.scala

  class ChecksumAccumulator {
    private var sum = 0
    def add(b: Byte) = sum += b
    def checksum() = ~(sum & 0xFF) + 1
  }


  // In file ChecksumAccumulator.scala
  class ChecksumAccumulator {
    private var sum = 0
    def add(b: Byte): Unit = { sum += b }
    def checksum(): Int = ~(sum & 0xFF) + 1
  }


  

4.2 Semicolon inference

// In file classes-and-objects/SemiInference.scala

  val s = "hello"; println(s)


// In file classes-and-objects/SemiInference.scala

  if (x < 2)
    println("too small")
  else
    println("ok")


  x
  + y


  (x
  + y)


  x +
  y +
  z


  

4.3 Singleton objects

// In file classes-and-objects/ChecksumAccumulator.scala

  // In file ChecksumAccumulator.scala
  import scala.collection.mutable

  object ChecksumAccumulator {

    private val cache = mutable.Map.empty[String, Int]

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


  ChecksumAccumulator.calculate("Every value is an object.")


  

4.4 A Scala application

// In file classes-and-objects/Summer.scala

  // In file Summer.scala
  import ChecksumAccumulator.calculate

  object Summer {
    def main(args: Array[String]) = {
      for (arg <- args)
        println(arg + ": " + calculate(arg))
    }
  }


  $ scalac ChecksumAccumulator.scala Summer.scala


  $ fsc ChecksumAccumulator.scala Summer.scala


  $ scala Summer of love


  of: -213
  love: -182


  

4.5 The App trait

// In file classes-and-objects/FallWinterSpringSummer.scala

import ChecksumAccumulator.calculate

object FallWinterSpringSummer extends App {

  for (season <- List("fall", "winter", "spring"))
    println(season + ": " + calculate(season))
}


  

4.6 Conclusion

object EchoArgs {

  def main(args: Array[String]) = {
    for (arg <- args)
      print(arg + " ")
    println()
  }