A Scala Tutorial for Java programmers

Scala

hello world

$ cat HelloWorld.scala

object HelloWorld {
def main(args: Array[String]) {
println("Hello, world!")
}
}

$ scalac HelloWorld.scala && scala -classpath . HelloWorld
Hello, world!

Interaction with Java

$ cat ChinaDate.scala

import java.util.{Date, Locale}
import java.text.DateFormat
import java.text.DateFormat._

object  ChinaDate {
def main(args: Array[String]) {
val now = new Date
val df = getDateInstance(LONG, Locale.CHINA)
println(df format now)
}
}

$ scalac ChinaDate.scala && scala -classpath . ChinaDate
2016年12月7日
$ sed -i "s/df format now/df.format(now)/" ChinaDate.scala
$ scalac ChinaDate.scala && scala -classpath . ChinaDate
2016年12月7日

Object: Numbers are objects

scala> val x = 5
x: Int = 5

scala> 1 + 2 * 3 / x
res1: Int = 2

scala> (1).+(((2).*(3))./(x))
res2: Int = 2

scala> 1.+(2)
res3: Int = 3

scala> (1).+(2)
res4: Int = 3

Object: Functions are objects

$ cat Timer.scala

object Timer {
def oncePerSecond(callback: () => Unit) {
while (true) { callback(); Thread sleep 1000 }
}

def timeFlies() {
println("time flies like an arrow...")
}
def main(args: Array[String]) {
oncePerSecond(timeFlies)
}
}

$ scalac Timer.scala && scala -classpath . Timer
warning: there was one deprecation warning (since 2.12.0); re-run with -deprecation for details
one warning found
time flies like an arrow...
time flies like an arrow...
time flies like an arrow...
^C

Object: Anonymous functions

$ cat TimerAnonymous.scala

object TimerAnonymous {
def oncePerSecond(callback: () => Unit) {
while (true) { callback(); Thread sleep 1000 }
}
def main(args: Array[String]) {
oncePerSecond(() =>
println("time flies like an arrow..."))
}
}

$ scalac TimerAnonymous.scala && scala -classpath . TimerAnonymous
time flies like an arrow...
time flies like an arrow...
time flies like an arrow...
^C

Classes: Methods without arguments

$ cat ComplexNumbers.scala

class Complex(real: Double, imaginary: Double) {
def re() = real
def im() = imaginary
}

object ComplexNumbers {
def main(args: Array[String]) {
val c = new Complex(1.2, 3.4)
println("imaginary part: " + c.im())
}
}

$ scalac ComplexNumbers.scala && scala -classpath . ComplexNumbers
imaginary part: 3.4

Classes: Fields

$ cat ComplexNumbers.scala

class Complex(real: Double, imaginary: Double) {
def re = real
def im = imaginary
}

object ComplexNumbers {
def main(args: Array[String]) {
val c = new Complex(1.2, 3.4)
println("real part: " + c.re)
}
}

$ scalac ComplexNumbers.scala && scala -classpath . ComplexNumbers
real part: 1.2

Classes: Inheritance and overriding (scala.AnyRef)

$ cat ComplexNumbers.scala

class Complex(real: Double, imaginary: Double) {
def re = real
def im = imaginary
override def toString() =
"" + re + (if (im < 0) "" else "+") + im + "i"
}

object ComplexNumbers {
def main(args: Array[String]) {
val c = new Complex(1.2, 3.4)
println("complex: " + c)
}
}

$ scalac ComplexNumbers.scala && scala -classpath . ComplexNumbers
complex: 1.2+3.4i

Case classes and pattern matching

$ cat Fomulate.scala

abstract class Tree
case class Sum(l: Tree, r: Tree) extends Tree
case class Var(n: String) extends Tree
case class Const(v: Int) extends Tree

object Fomulate {
type Environment = String => Int

def eval(t: Tree, env: Environment): Int = t match {
case Sum(l, r) => eval(l, env) + eval(r, env)
case Var(n) => env(n)
case Const(v) => v
}

def derive(t: Tree, v: String): Tree = t match {
case Sum(l, r) => Sum(derive(l, v), derive(r, v))
case Var(n) if (v == n) => Const(1)
case _ => Const(0)
}

def main(args: Array[String]) {
val exp: Tree = Sum(Sum(Var("x"),Var("x")),Sum(Const(7),Var("y")))
val env: Environment = { case "x" => 5 case "y" => 7 }
println("Expression: " + exp)
println("Evaluation with x=5, y=7: " + eval(exp, env))
println("Derivative relative to x:\n " + derive(exp, "x"))
println("Derivative relative to y:\n " + derive(exp, "y"))
}
}

$ scalac Fomulate.scala && scala -classpath . Fomulate
Expression: Sum(Sum(Var(x),Var(x)),Sum(Const(7),Var(y)))
Evaluation with x=5, y=7: 24
Derivative relative to x:
Sum(Sum(Const(1),Const(1)),Sum(Const(0),Const(0)))
Derivative relative to y:
Sum(Sum(Const(0),Const(0)),Sum(Const(0),Const(1)))

Traits

$ cat Date.scala

trait Ord {
def < (that: Any): Boolean
def <=(that: Any): Boolean = (this < that) || (this == that)
def > (that: Any): Boolean = !(this <= that)
def >=(that: Any): Boolean = !(this < that)
}

class Date(y: Int, m: Int, d: Int) extends Ord {
def year = y
def month = m
def day = d

override def toString(): String = year + "-" + month + "-" + day

override def equals(that: Any): Boolean =
that.isInstanceOf[Date] && {
val o = that.asInstanceOf[Date]
o.day == day && o.month == month && o.year == year
}

def <(that: Any): Boolean = {
if (!that.isInstanceOf[Date])
sys.error("cannot compare " + that + " and a Date")
val o = that.asInstanceOf[Date]
(year < o.year) ||
(year == o.year && (month < o.month ||
(month == o.month && day < o.day)))
}
}

$ scalac Date.scala

Genericity

$ cat IntegerReference.scala

class Reference[T] {
private var contents: T = _
def set(value: T) { contents = value }
def get: T = contents
}

object IntegerReference {
def main(args: Array[String]) {
val cell = new Reference[Int]
cell.set(13)
println("Reference contains the half of " + (cell.get * 2))
}
}

$ scalac IntegerReference.scala && scala -classpath . IntegerReference
Reference contains the half of 26