Code Examples for

Programming in Scala

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4 Classes and Objects

Sample run of chapter's interpreter examples

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/ChecksumCalculator3.scala class ChecksumAccumulator { private var sum = 0 def add(b: Byte): Unit = sum += b def checksum(): Int = ~(sum & 0xFF) + 1 }
// In file classes-and-objects/ChecksumCalculator4.scala // In file ChecksumAccumulator.scala class ChecksumAccumulator { private var sum = 0 def add(b: Byte) { sum += b } def checksum(): Int = ~(sum & 0xFF) + 1 }
scala> def f(): Unit = "this String gets lost" f: ()Unit
scala> def g() { "this String gets lost too" } g: ()Unit
scala> def h() = { "this String gets returned!" } h: ()java.lang.String scala> h res0: java.lang.String = this String gets returned!

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/ChecksumCalculator5.scala // In file ChecksumAccumulator.scala 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.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 Application trait


// In file classes-and-objects/FallWinterSpringSummer.scala import ChecksumAccumulator.calculate object FallWinterSpringSummer extends Application { 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() }

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