Scala for Java Developers

What is Scala “ 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

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

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 Scala doesn’t force it

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

Hello World: Scripting Style $ scala hello-script.scala Hello World No compilation

Hello World: Porting of Java Code $ scalac hello-java.scala $ scala example.Main Hello World ‘ static’ Inferred semicolons

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

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) val p = new Person( "Ramnivas" , "Laddad" ) println(p.firstName) Value (Java ‘final’)

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

Extending a class val s = new Student( "Ramnivas" , "Laddad" , 1) println(s.firstName) println(s.grade)

Defining methods class Person( val firstName: String, val lastName: String) { def name = firstName + " " + lastName override def toString = name } val p = new Person( "Ramnivas" , "Laddad" ) println(p.name) // Ramnivas Laddad println(p) // Ramnivas Laddad Not optional

Uniform access principle class Person( val firstName: String, val lastName: String) { val name = firstName + " " + lastName override def toString = name } val p = new Person( "Ramnivas" , "Laddad" ) 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

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

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

Traits: Interfaces done right trait PartyGoer { val age: Int val yearsUntilLegalDrinking = if (age >= 18) 0 else 18-age }

Collections val people = List( "John" , "Jacob" , "Mike" )

Collections val people = Array( "John" , "Jacob" , "Mike" )

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

Working with collections: for comprehension 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

Working with collections: filter val student1 = new Student( "first1" , "last1" , 1) val student2 = new Student( "first2" , "last2" , 1) val student3 = new Student( "first3" , "last3" , 2) val student4 = new Student( "first4" , "last4" , 6) val students = List(student1, student2, student3, student4)

Working with collections: using _ val student1 = new Student( "first1" , "last1" , 1) val student2 = new Student( "first2" , "last2" , 1) val student3 = new Student( "first3" , "last3" , 2) val student4 = new Student( "first4" , "last4" , 6) val students = List(student1, student2, student3, student4)

Working with collections: passing method as function val student1 = new Student( "first1" , "last1" , 1) val student2 = new Student( "first2" , "last2" , 1) val student3 = new Student( "first3" , "last3" , 2) val student4 = new Student( "first4" , "last4" , 6) val students = List(student1, student2, student3, student4)

Working with collections: partition val student1 = new Student( "first1" , "last1" , 1) val student2 = new Student( "first2" , "last2" , 1) val student3 = new Student( "first3" , "last3" , 2) val student4 = new Student( "first4" , "last4" , 6) val students = List(student1, student2, 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 val student1 = new Student( "first1" , "last1" , 1) val student2 = new Student( "first2" , "last2" , 1) val student3 = new Student( "first3" , "last3" , 2) val student4 = new Student( "first4" , "last4" , 6) val students = List(student1, student2, student3, student4)

Map 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

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 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 def quicksort[T](input: Traversable[T]) ( 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 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()

Case classes 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 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 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 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 val texts = List( "Ramnivas Laddad" , "foo" , "Scott Andrews" ) val peopleOptions = texts.map { _ match { case Name(first, last) => Some( new Person(first, last)) case _ => None } } println(peopleOptions) // List(Some(Ramnivas Laddad), None, Some(Scott Andrews))

Options: flattening val texts = List( "Ramnivas Laddad" , "foo" , "Scott Andrews" ) val peopleOptions = texts.map { _ match { case Name(first, last) => Some( new Person(first, last)) case _ => None } } println(peopleOptions.flatten) // List(Ramnivas Laddad, Scott Andrews)

Options: flatMap val texts = List( "Ramnivas Laddad" , "foo" , "Scott Andrews" ) val people = texts.flatMap { _ match { case Name(first, last) => Some( new Person(first, last)) case _ => None } } println(people) // List(Ramnivas Laddad, Scott Andrews)

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

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 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 …

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 Learn Haskell first! Might help to break from the Java way of thinking Be ready to be humbled

Scala for Java Developers

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