Scala for Java Developers

2. 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

3. Object-oriented <ul><li>Everything is an object </li></ul><ul><ul><li>No “primitives” </li></ul></ul><ul><li>Classes </li></ul><ul><ul><li>Same as Java, but concise </li></ul></ul><ul><li>Traits </li></ul><ul><ul><li>Interfaces done right </li></ul></ul><ul><li>Singletons </li></ul><ul><ul><li>Language-level concept </li></ul></ul>

4. Statically typed <ul><li>Rich type system (by Java’s standard) </li></ul><ul><ul><li>Higher-kinded types </li></ul></ul><ul><ul><li>Implicit conversions </li></ul></ul><ul><ul><li>Type evidence </li></ul></ul><ul><li>Expressive type system </li></ul><ul><li>Inferred types </li></ul>

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

6. Java Interoperability <ul><li>Compiles to Java byte code </li></ul><ul><ul><li>Jars, wars, … </li></ul></ul><ul><ul><li>No special operational change </li></ul></ul><ul><li>Scala calling Java, Java calling Scala code is fine </li></ul><ul><li>Whole Java eco-system at your service </li></ul><ul><li>You can write apps using Scala and Spring, today </li></ul>

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

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

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

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

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

12. 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

13. Class with “getters” class Person( val firstName: String, val lastName: String) val p = new Person( "Ramnivas" , "Laddad" ) println(p.firstName) Value (Java ‘final’)

14. 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)

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

16. 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

17. 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

18. Names in Scala <ul><li>Class, method, field names can contain non alpha-numeric characters </li></ul><ul><ul><li>:: </li></ul></ul><ul><ul><li>::: </li></ul></ul><ul><ul><li>~> </li></ul></ul><ul><ul><li>f@#: </li></ul></ul><ul><li>Valuable if used judiciously </li></ul><ul><ul><li>DSLs </li></ul></ul>

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

20. Finer access control levels <ul><li>Default access level: public </li></ul><ul><li>Protected: protected </li></ul><ul><ul><li>Same as Java </li></ul></ul><ul><li>Private: </li></ul><ul><ul><li>private </li></ul></ul><ul><ul><li>private [ this ] Access only from this instance </li></ul></ul><ul><ul><li>private [package-name] Access from package and its subpackages </li></ul></ul>

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

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

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

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

25. 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

26. 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)

27. 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)

28. 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)

29. 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)

30. 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

31. 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)

32. Map println(studentSchools(student1)) // Miller

33. More collections <ul><li>Set </li></ul><ul><li>IndexedSeq </li></ul><ul><li>Vector (good one!) </li></ul><ul><li>Range </li></ul><ul><ul><li>1 to 100 </li></ul></ul><ul><ul><li>1 until 100 </li></ul></ul><ul><ul><li>2 until 100 by 2 </li></ul></ul><ul><li>… </li></ul><ul><li>Mutable versions </li></ul><ul><li>Parallel versions </li></ul>

34. 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))

35. Putting it together: Quicksort println(quicksort(List(1, 3, 4, 5, 1)))

36. 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" ) }

37. 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) }

38. 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)))

39. 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()

40. Case classes <ul><li>Useful in pattern matching </li></ul><ul><ul><li>“ case” </li></ul></ul><ul><li>Offer many useful common methods </li></ul><ul><ul><li>equals() </li></ul></ul><ul><ul><li>hashCode() </li></ul></ul><ul><ul><li>toString </li></ul></ul><ul><ul><li>copy() </li></ul></ul>

41. 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" ))

42. 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)

43. 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

44. Regular expressions val text = "Ramnivas Laddad" val Name = """(+)+(+)""" .r val person = text match { case Name(first, last) => Some( new Person(first, last)) case _ => None } println(person) // Some(Ramnivas Laddad)

45. 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))

46. 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)

47. 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)

48. 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

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

50. Caching higher-order function <ul><li>abstract class Caching( val cacheManager: CacheManager) { </li></ul><ul><li>def cached[T](region: String, key: Any) </li></ul><ul><li>(thunk: => T): T = { </li></ul><ul><li>val cache = ... </li></ul><ul><li>if (cache.containsKey(key)) { </li></ul><ul><li>cache.get(key).asInstanceOf[T] </li></ul><ul><li>} else { </li></ul><ul><li>val thunkVal: T = thunk </li></ul><ul><li>cache.put(key, thunkVal) </li></ul><ul><li>thunkVal </li></ul><ul><li>} </li></ul><ul><li>} </li></ul><ul><li>} </li></ul>

51. Transaction management <ul><li>def findOrder(orderId: Long) : Order = { </li></ul><ul><li>transactional(readOnly= true ) { </li></ul><ul><li>//... </li></ul><ul><li>} </li></ul><ul><li>} </li></ul><ul><li>def updateOrder(order: Order) { </li></ul><ul><li>transactional() { </li></ul><ul><li>//... </li></ul><ul><li>} </li></ul><ul><li>} </li></ul>

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

53. Transaction management implementation <ul><li>abstract class TransactionManagement( val txManager: PlatformTransactionManager) { </li></ul><ul><li>def transactional[T](...)(thunk: => T) : T = { </li></ul><ul><li>val txAttribute = new TransactionAttributeWithRollbackRules(...) </li></ul><ul><li>val status = txManager.getTransaction(txAttribute) </li></ul><ul><li>try { </li></ul><ul><li>val ret = thunk </li></ul><ul><li>txManager.commit(status) </li></ul><ul><li>ret </li></ul><ul><li>} catch { </li></ul><ul><li>case ex => { </li></ul><ul><li>if (txAttribute.rollbackOn(ex)) { </li></ul><ul><li>txManager.rollback(status) </li></ul><ul><li>} else { </li></ul><ul><li>txManager.commit(status) </li></ul><ul><li>} </li></ul><ul><li>throw ex </li></ul><ul><li>} </li></ul><ul><li>} </li></ul><ul><li>} </li></ul><ul><li>} </li></ul>

54. There is more… a lot more <ul><li>Methods/functions </li></ul><ul><ul><li>Default parameters </li></ul></ul><ul><ul><li>Named parameters </li></ul></ul><ul><ul><li>Curried parameters </li></ul></ul><ul><ul><li>Partial functions </li></ul></ul><ul><li>Type system </li></ul><ul><ul><li>Higher-kinded types </li></ul></ul><ul><ul><li>Bounded types </li></ul></ul><ul><ul><li>Implicit type conversion </li></ul></ul><ul><ul><li>Type parameter evidence </li></ul></ul><ul><ul><li>Type aliasing </li></ul></ul><ul><li>Lazy values </li></ul><ul><li>Partial imports </li></ul><ul><li>Actors </li></ul><ul><li>Extractors </li></ul><ul><li>Scala ecosystem </li></ul><ul><li>Combinator/parser </li></ul><ul><li>Continuations </li></ul><ul><li>Compiler plugin </li></ul><ul><li>… </li></ul>

55. Learning Scala <ul><li>Read a Scala book </li></ul><ul><ul><li>Get the whole picture </li></ul></ul><ul><li>May feel complex at first </li></ul><ul><ul><li>Java-style Scala may serve best during initial exploration </li></ul></ul><ul><ul><li>Over time you will appreciate its simplicity </li></ul></ul><ul><li>Learn Haskell first! </li></ul><ul><ul><li>Might help to break from the Java way of thinking </li></ul></ul>Be ready to be humbled

Scala for Java Developers

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Scala for Java Developers

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