Scala for Java Developers (Silicon Valley Code Camp 13)

Scala for Java Developers
Ramnivas Laddad
@ramnivas

@ramnivas
•  Author of books and articles
–  AspectJ in Action (1st and 2nd edition)
•  Spring framework, Cloud Foundry
•  Main interests
–  Cloud computing
–  Aspect-oriented programming
–  Scala and functional programming
•  Speaker at many professional conferences
–  JavaOne, JavaPolis, SpringOne, Software Development, No Fluff Just Stuff,
EclipseCon, O’Reilly OSCON etc.
•  Active involvement in AspectJ, Spring, and Cloud Foundry since
their early form

What is Scala
3
“a general purpose programming language designed to
express common programming patterns in a concise,
elegant, and type-safe way. It smoothly integrates features
of object-oriented and functional languages, enabling Java
and other programmers to be more productive.”
http://www.scala-lang.org

Object-oriented
•  Everything is an object
–  No “primitives”
•  Classes
–  Same as Java, but concise
•  Traits
–  Interfaces done right
•  Singletons
–  Language-level concept
4

Statically typed
•  Rich type system (by Java’s standard)
–  Higher-kinded types
–  Implicit conversions
–  Type evidence
•  Expressive type system
•  Inferred types
5

Functional Programming
•  Functions as values
–  May be
•  Saved
•  Passed to other functions (higher-order functions)
 No need to write ugly anonymous classes
–  Advanced pattern matching
–  Expressions return a value
•  if/else, try/catch, match, …
•  Promotes immutability
–  But doesn’t force it
6

Java Interoperability
•  Compiles to Java byte code
–  Jars, wars, …
–  No special operational change
•  Scala calling Java, Java calling Scala code is fine
•  Whole Java eco-system at your service
•  You can write apps using Scala and Spring, today
7

Hello World: Scripting Style
$ scala hello-script.scala
Hello World

println("Hello World")
No compilation

Hello World: Porting of Java Code
$ scalac hello-java.scala
$ scala example.Main
Hello World
// hello-java.scala
package example

object Main {
def main(args: Array[String]) {
}
}
‘static’
Inferred
semicolons

Hello World: Using the App trait
$ scalac hello-app.scala
$ scala example.Main
Hello World
// hello-app.scala
package example

object Main extends App {
}

Simple Class
class Person
val p = new Person
Type
Inferred
Default
access: public
No curly
braces needed
(but allowed)

Simple Class
class Person
val p: Person = new Person
Explicit type
specification

Class with constructor
class Person(firstName: String,
lastName: String)
val p = new Person("Ramnivas", "Laddad")
println(p.firstName) // Error
Primary
constructor
Fields –
accessible in
class body

Class with “getters”
class Person(val firstName: String,
val lastName: String)
println(p.firstName)
Value
(Java ‘final’)

Class with “getters” and “setters”
class Person(var firstName: String,
var lastName: String)
println(p.firstName)
p.firstName = "Ramnivas2”
Variable (Java
non-final)

Extending a class
class Student(firstName: String,
lastName: String,
val grade: Int)
extends Person(firstName, lastName)
val s = new Student("Ramnivas", "Laddad", 1)
println(s.firstName)
println(s.grade)

Defining methods
val lastName: String) {
def name = firstName + " " + lastName

override def toString = name
}
println(p.name) // Ramnivas Laddad
println(p) // Ramnivas Laddad
Not optional

Uniform access principle
val lastName: String) {
val name = firstName + " " + lastName

override def toString = name
}
println(p.name) // Ramnivas Laddad
println(p) // Ramnivas Laddad

Names in Scala
•  Class, method, field names can contain non alpha-
numeric characters
–  ::
–  :::
–  ~>
–  f@#:
•  Valuable if used judiciously
–  DSLs
19

More about methods and fields
•  Declaring abstract methods and fields
–  Just don’t provide definition
def learn(subject: String)
val knowledge
•  Classes with abstract method must be declared abstract
–  Just as in Java
•  Methods can be defined inside methods
•  Methods may be marked @tailrec to check for tail-
recursiveness
20

Finer access control levels
•  Default access level: public
•  Protected: protected
–  Same as Java
•  Private:
–  private
–  private[this] Access only from this instance
–  private[package-name] Access from package and its
subpackages

21

Traits: Interfaces done right
22
trait PartyGoer {
val age: Int
val yearsUntilLegalDrinking =
if (age >= 21) 0 else 21-age
}
class Student(firstName: String, lastName: String,
val age: Int, val grade: Int)
extends Person(firstName, lastName)
with PartyGoer
val s = new Student("a", "b", 17, 12)
s.yearsUntilLegalDrinking // 4

Collections
val people = List("John", "Jacob",
"Mike")
val firstPerson = people(0)
println(firstPerson) // John

Collections
val people = Array("John", "Jacob",
"Mike")
val firstPerson = people(0)
println(firstPerson) // John

Working with collections: for comprehension
25
for (person <- people) {
println(person)
}

26
for (person <- people if person startsWith "J") {
println("""Lucky one to start name in "J" """ + person)
}

// You are lucky one to start name in "J" John
// You are lucky one to start name in "J" Jacob

27
for (person <- people if person startsWith "J") {
println(s"""Lucky one to start name in "J" $person""")
}

// You are lucky one to start name in "J" John
// You are lucky one to start name in "J" Jacob

Working with collections: filter
val student1 = new Student("first1", "last1", 1)
val students = List(student1, student2,
student3, student4)
val firstGraders = students.filter(
s => s.grade == 1)
println(firstGraders)
// List(first1 last1, first2 last2)

Working with collections: filter
student3, student4)
val inFirstGrade: Student => Boolean
= s => s.grade == 1

val firstGraders = students.filter(inFirstGrade)

Working with collections: using _
student3, student4)
val firstGraders
= students.filter(_.grade == 1)
println(firstGraders)

Working with collections: passing method as function
student3, student4)
def inFirstGrade(s: Student) : Boolean
= s.grade == 1
val firstGraders = students.filter(inFirstGrade)

Working with collections: partition
student3, student4)
val (elementarySchoolers, middleSchoolers)
= students.partition(_.grade < 6)
println(elementarySchoolers)
println(middleSchoolers)
//List(first1 last1, first2 last2, first3 last3)
//List(first4 last4)
Tuple

Working with collections: transforming
student3, student4)
val rollCall = students.map(_.firstName)
println(rollCall)
// List(first1, first2, first3, first4)

Map
// student1, student2, student3, student4
val studentSchools = Map(student1 -> "Miller",
student2 -> "Lawson",
student3 -> "Lawson",
student4 -> "Miller")
println(studentSchools(student1)) // Miller

More collections
•  Set
•  IndexedSeq
•  Vector (good one!)
•  Range
–  1 to 100
–  1 until 100
–  2 until 100 by 2
•  …
•  Mutable versions
•  Parallel versions
35

Putting it together: Quicksort
def quicksort[T](input: Traversable[T])
(ordering: Ordering[T]) : Traversable[T] =
if (input.isEmpty) {
input
} else {
val (low, high)
= input.tail.partition(ordering.lt(_, input.head))
quicksort(low)(ordering) ++ List(input.head)
++ quicksort(high)(ordering)
}
println(quicksort(List(1, 3, 4, 5, 1))(Ordering.Int))

Putting it together: Quicksort
(implicit ordering: Ordering[T])
: Traversable[T] =
if (input.isEmpty) {
input
} else {
val (low, high)
= input.tail.partition(ordering.lt(_, input.head))
quicksort(low) ++ List(input.head) ++ quicksort(high)
}
println(quicksort(List(1, 3, 4, 5, 1)))

Pattern matching: Basics
val a:Any = "foo"

a match {
case str: String => println("A string: " + str)
case i: Int => println("An int: " + i)
case _ => println("Something else")
}

Pattern matching: with collections
val l = List("a", "b", "c")

l match {
case Nil => println("Empty!")
case head :: Nil =>
println("Only one item " + head)
case head :: tail =>
println("Item " + head +
" followed by " + tail)
}

Quicksort with pattern matching
(implicit ordering: Ordering[T]) : Traversable[T] =
input match {
case head :: tail =>
val (low, high) = tail.partition(ordering.lt(_, head))
quicksort(low) ++ List(head) ++ quicksort(high)
case _ => input
}
println(quicksort(List(1, 3, 4, 5, 1)))

Destutter using Pattern matching
41
def destutter[A](lst: List[A]): List[A] = lst match {
case h1 :: h2 :: tail if (h1 == h2)
=> destutter(h2 :: tail)
case h1 :: h2 :: tail
=> h1 :: destutter(h2 :: tail)
case _
=> lst
}
// destutter(List(1,1,1,1,1,1)) => List(1)
// destutter(List(1,1,4,3,3,2)) => List(1,4,3,2)
// destutter(List())=> List()

Case classes
•  Useful in pattern matching
–  “case”
•  Offer many useful common methods
–  equals()
–  hashCode()
–  toString
–  copy()
42

Case classes
43
case class Human(name: String)
case class SuperHero(name: String, power: String)
val characters = List(Human("Programmer"),
SuperHero("Customer", "money"),
SuperHero("QA", "testing"))

Case classes and pattern matching
44
val actions = for (character <- characters)
yield character match {
case Human(name) =>
name + " needs to be saved"
case SuperHero(name, power) =>
name + " will save using " + power
}

actions.foreach(println)

Pattern matching and extracting just enough
45
val actions = for (character <- characters)
yield character match {
case Human(name) =>
name + " needs to be saved"
case SuperHero(_, power) =>
"Could be saved using " + power
}

actions.foreach(println)

// Programmer needs to be saved
// Could be saved using money
// Could be saved using testing

Regular expressions
46
val text = "Ramnivas Laddad"

val Name = """(w+)s+(w+)""".r

val person = text match {
case Name(first, last) =>
Some(new Person(first, last))
case _ =>
None
}

println(person) // Some(Ramnivas Laddad)

Options
47
val texts =
List("Ramnivas Laddad", "foo", "Martin Odersky")

val peopleOptions = texts.map {
_ match {
case _ => None
}
}

println(peopleOptions)
// List(Some(Ramnivas Laddad),
None,
Some(Martin Odersky))

Options: flattening
48
val texts =

val peopleOptions = texts.map {
_ match {
case _ => None
}
}

println(peopleOptions.flatten)
// List(Ramnivas Laddad, Martin Odersky)

Options: flatMap
49
val texts =

val people = texts.flatMap {
_ match {
case _ => None
}
}

println(people)
// List(Ramnivas Laddad, Martin Odersky)

Higher order functions
def process() : Unit = {
retry(5) {
...
}
}
def retry[T](maxRetry: Int)(thunk: => T) = {
def loop(thunk: => T, attempt: Int): T = {
try {
thunk
} catch {
case ex if (attempt < maxRetry) =>
loop(thunk, attempt + 1)
}
}
loop(thunk, 0)
}
Thunk

Caching using Scala
def getQuoteGraph(stock: Stock,
days: Int) : Array[Byte] = {
cached("chart", stock.ticker + ":" + days) {
!
... Expensive calculation !
!
}
}

Caching higher-order function
abstract class Caching(val cacheManager: CacheManager) {
def cached[T](region: String, key: Any)
(thunk: => T): T = {
val cache = ...

if (cache.containsKey(key)) {
cache.get(key).asInstanceOf[T]
} else {
val thunkVal: T = thunk
cache.put(key, thunkVal)
thunkVal
}
}
}

Transaction management
def findOrder(orderId: Long) : Order = {
transactional(readOnly=true) {
//...
}
}

def updateOrder(order: Order) {
transactional() {
//...
}
}
53

Transaction management function
def transactional[T](propgation: Propagation = Propagation.REQUIRED,
isolation: Isolation = Isolation.DEFAULT,
readOnly: Boolean = false,
timeout: Int =TransactionDefinition.TIMEOUT_DEFAULT,
rollbackFor: List[Throwable] = List(),
noRollbackFor: List[Throwable] = List())
(thunk: => T) : T

Transaction management implementation
abstract class TransactionManagement(val txManager: PlatformTransactionManager) {

def transactional[T](...)(thunk: => T) : T = {
val txAttribute = new TransactionAttributeWithRollbackRules(...)

val status = txManager.getTransaction(txAttribute)

try {
val ret = thunk
txManager.commit(status)
ret
} catch {
case ex => {
if (txAttribute.rollbackOn(ex)) {
txManager.rollback(status)
} else {
txManager.commit(status)
}
throw ex
}
}
}
}

There is more… a lot more
•  Methods/functions
–  Default parameters
–  Named parameters
–  Curried parameters
–  Partial, partially-applied
functions
•  Type system
–  Higher-kinded types
–  Bounded types
–  Implicit type conversion
–  Type parameter
evidence
–  Type aliasing
•  Lazy values
•  Partial imports
•  Actors
•  Extractors
•  Scala ecosystem
•  Combinator/parser
•  Continuations
•  Compiler plugin
•  …
56

Learning Scala
•  Read a Scala book
–  Get the whole picture
•  May feel complex at first
–  Java-style Scala may serve best during initial exploration
–  Over time you will appreciate its simplicity
–  Will affect your non-Scala programming deeply
57
Be ready to be humbled
Be ready to have fun!

Scala for Java Developers (Silicon Valley Code Camp 13)

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