Railroading into Scala

railroading into scala
a really fast guide for Java developers
Inspired by Gilt Tech’s Scala Course

scala basics
● Runs on the JVM
● Fully object-oriented, no primitives
● Fully supports functional programming
● Interpreted or compiled to Java bytecode

syntax
● Less verbose than Java
● No semi-colons (except multi-statement
lines)
● Type declaration after identifier:
○ val myVar: Int = 10
● Unit = void

syntax
● Declarations
○ def for functions
○ val for immutables
○ var for mutables
○ lazy val executes at first access, intended for
expensive operations:
lazy val fib = fibonacci(10)

expressions
● Everything is an expression
● No "return" needed, last statement is the
return value
● Anonymous function:
(parameters) => return_type =
{ argument => return_value }

expressions
(Int) => String = { i => "Number " + i }
val x = if (total == 30) {
"Total is 30"
} else {
"Total is something else"
}

loops
● for-loop
for (arg <- args) {
println(arg)
}
for (i <- 0 to 10 by 2) {
println(i)
}

loops
● for-comprehension
val ints = for (arg <- args) yield {
arg.toInt
}
for {
i <- 0 to 2
j <- 1 to 3
} yield {i * j}

functions
● First-class citizens
● Can be returned from a function
● Can be assigned to a val
● Can be nested
● Can have anonymous functions

options
● To get around nulls, Scala gives us Option
● 2 states: Some and None
● Test with .isDefined and .isEmpty
● get() returns the value if Some, else throws
exception
● orElse() and getOrElse() for None

options
Some(“Hello”).isDefined
// res0: Boolean = true
None.getOrElse(“Nothing!”)
// res0: String = Nothing!

exceptions
● No checked exceptions
● Catch matches exceptions by type

exceptions
try{}
catch{
case nfe: NumberFormatException =>
println("Not a number")
case oom: OutOfMemoryException =>
println("Out of memory")
}
finally{}

REPL (Read-Eval-Print-Loop)
● sbt console or sbt console-quick or
scala
● You can import packages as well
● :paste

classes
● Abstract classes and traits
● Only one primary constructor, ancillary
constructors possible: def this(...)
● Members are public by default
● Nothing vs. Null types
● def can be overridden by val

classes
trait Shape {
def area: Double
}
class Circle(val radius: Double) extends Shape {
override val area = math.Pi * radius
val circumference = 2 * math.Pi * radius
}
val c = new Circle(1)
c.radius // res0: Double = 1.0
c.area // res1: Double = 3.141592653589793
c.circumference // res2: Double = 6.283185307179586

classes
class ModifiableRectangle(var x: Double, var y: Double)
extends Shape {
def this(x: Double) = this(x, x)
override def area = x * y
}
class ModifiableSquare(a: Double) extends
ModifiableRectangle(a, a) {
private val originalArea = a * a
}

traits
● Similar to interfaces, but can have default
implementation
● No constructor
● Multiple inheritance: initialize left to right,
linearize right to left

traits
trait IntStack {
def pop(): Option[Int]
def push(x: Int): Unit
def isEmpty: Boolean
}
class BasicStack extends IntStack {
private val stack = new collection.mutable.Stack[Int]()
override def pop(): Option[Int] = {
if (stack.empty()) { None }
else { Some(stack.pop()) }
}
override def push(x: Int): Unit = stack.push(x)
override def isEmpty = stack.empty
}

traits
trait Doubling extends IntStack {
abstract override def push(x: Int): Unit = super.push(x * 2)
}
trait Incrementing extends IntStack {
abstract override def push(x: int): Unit = super.push(x + 1)
}
class MyStack extends BasicStack with Doubling with Incrementing
class YourStack extends BasicStack with Incrementing with Doubling
val me = new MyStack()
me.push(2)
me.pop
// res0: Option[Int] = Some(6)
val you = new YourStack()
you.push(2)
you.pop
// res0: Option[Int] = Some(5)

objects
● Equivalent to static class
● May extend/implement a class or trait
(singleton)

companion object
● Same name as companion class, same file
● Equivalent to static methods
● May apply() methods

companion object
class MyStack extends BasicStack
object MyStack {
def apply(): MyStack = new MyStack()
def apply(ints: Int*): MyStack = {
val stack = new MyStack()
ints.foreach(stack.push(_))
stack
}
}
val myStack = new MyStack()
val yourStack = MyStack()
val ourStack = MyStack(1, 2, 3, 4)

enumerations
● Extend scala.Enumeration
● Values have inner type Enumeration.Value
object Color extends Enumeration {
val Red, Green, Blue = Value
}
val red = Color.Red

Java Scala
Interface Trait
Abstract Class Trait or Abstract Class
Class Class
Object/Instance Object/Instance
Static Class, Singleton Object
Static Members Companion Object
Enum Enumeration

case classes
● Implements hashCode, equals, and
toString methods
● Add companion object with apply()
● Implements copy()
● Great for pattern matching!

immutability
● val vs. var
● Reduce side effects
● Concurrency issues
● Transform data vs. update data

collections
● Immutable vs. mutable collections

lists
● Construction: List(1, 2, 3), List.empty
[String], List[String] = Nil
● Access: myList(2)
● Concatenation: myList ::: otherList, 0 ::
myList, myList :+ 4
● Update: myList.updated(1, 9)
● isEmpty

● head, tail, init, last
● headOption, lastOption
● take, drop, slice
● toString, mkString
● contains, exists, forall (return Boolean)
● find (returns Option)
lists

sets and maps
● Construction: Set(1, 2, 3)
● Combination:
○ mySet ++ Set(5, 6, 7)
○ myMap ++ Map("three" -> 3)
● Insert:
○ mySet + 9
○ myMap + ("three" -> 3)
● Access: myMap.get("one")

monads
map
def map[B](f: A => B): List[B]
flatMap
def flatMap[B](f: A => List[B]): List[B]
filter
def filter(f: A => Boolean): List[A]

higher order functions
foldLeft
def foldLeft[B](z: B)(op: (B, A) = B): B
List(1, 2, 3, 4).foldLeft(0)({ (a: Int, b: Int) => a + b
})
collect
(1 until 100).toList.collect {
case i if (i % 2 == 0) => "even"
case _ => "odd"
}

higher order functions
groupBy
List("one", "two", "three", "four", "five").
groupBy(a => a.size)
partition
(1 until 10).toList.partition(_ % 2 == 0)

learn more
Scala Docs
http://www.scala-lang.org/documentation/
Twitter’s Scala School
https://twitter.github.io/scala_school/
Coursera
https://www.coursera.org/course/progfun
https://www.coursera.org/course/reactive
Books
Scala for the Impatient
Ninety-Nine Scala Problems
http://aperiodic.net/phil/scala/s-99/

Railroading into Scala

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