Scala for Java Programmers
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This document provides an introduction to Scala for Java developers. It discusses that Scala is a hybrid object-oriented and functional language that runs on the JVM and interoperates well with Java. It highlights several features of Scala that allow for more concise code compared to Java, such as type inference, expressions instead of statements, higher-order functions, and case classes.
val
p
=
MyPair(1,
“foo”)
//
p
is
MyPair[Int,
String]](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-9-320.jpg)
![Expressions, not statements
val
server
=
if
(environment
==
"development”)
{
val
factory
=
new
MBeanServerFactoryBean
factory.aberProper7esSet()
factory.getObject.asInstanceOf[MbeanServer]
}
else
{
val
clazz
=
Class.forName("org.jboss.mx.u7l.MBeanServerLocator")
clazz.getMethod("locateJBoss”).invoke(null)
}
//
server is assigned one of the bold values](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-11-320.jpg)
![Pattern Matching
• Typed patterns
def
generalSize(x:
Any)
=
x
match
{
case
s:
String
=>
s.length
case
m:
Map[_,
_]
=>
m.size
case
_
=>
-‐1
}](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-26-320.jpg)
![Futures
Return a Future immediately, run func in new thread
scala>
future
{
Thread.sleep(10000);
println("hi");
10
}
res2:
scala.actors.Future[Int]
=
<
func7on0>
Use the Future apply()
method to get the result
scala>
res2()
//
blocks
wai7ng
for
sleep()
to
finish
hi
res3:
Int
=
10](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-34-320.jpg)
![JavaConversions
• Add Scala collection API methods to Java
collections
import
collec7on.JavaConversions._
import
collec7on.Iterable
import
java.u7l.{List=>JList}
def
goodStudents(students:
JList[Student]):
Iterable[String]
=
students.filter(_.score
>
5).map(_.name)](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-40-320.jpg)
![Named and Default Params
• Named parameters
def
resize(width:
Int,
height:
Int)
=
{
...
}
resize(width
=
120,
height
=
42)
• Default parameters
def
f(elems:
List[Int],
x:
Int
=
0,
cond:
Boolean
=
true)
f(List(1))
f(Nil,
cond
=
false)](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-41-320.jpg)
![Type Conveniences
• Type Aliases
type
MyMap
=
mutable.HashMap[String,
mutable.HashMap[String,
Int]]
• Import Aliases
import
com.nyx.domain.no7fica7on.{Topic=>DomainTopic}](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-43-320.jpg)
![Duck Typing
type
Closeable
=
{
def
close():
Unit
}
def
using[T
<:
Closeable,
S]
(obj:
T)(func:
T
=>
S):
S
=
{
val
result
=
func
(obj)
obj.close()
result
}
val
fis
=
new
FileInputStream(“data.txt”)
using(fis)
{
f
=>
while
(f.read()
!=
-‐1)
{}
}](https://image.slidesharecdn.com/scalanyjavasig-120709134355-phpapp02/85/Scala-for-Java-Programmers-45-320.jpg)
Scala for Java Programmers
- 1. Intro to Scala for Java Devs Sungard– 4/17/2012
- 2. Intro • Eric Pederson – [email protected] – Twitter @ericacm – Sourcedelica.com/blog – Background in Java, Groovy, Javascript, PHP, etc. • Using Scala for last two years • Two Scala apps developed for NYSE in Production
- 3. Platform at NYSE • Scala 2.9.1 • JDK 1.6 • Tomcat / JBoss • Maven • Using lots of Java libraries – Spring, Hibernate, CXF, Mule, ApacheMQ – Bouncycastle, OpenSAML, Velocity, etc, etc.
- 4. What is Scala? • Hybrid Object-Functional language • Statically typed • Developed by Martin Odersky – Java Generics – Java Compiler (1.3+) • First release in 2003
- 5. What is Scala? • Designed for general purpose programming • Performance on par with Java* • Scalable – Designed to write programs ranging from scripts up to huge systems – You don’t have to use all of the features to be productive * There are some gotchas you have to watch out for
- 6. Why Should I Use Scala? • Allows you to write very concise code – Productivity on the level of Groovy / Ruby • Concurrency-ready • Excellent interoperability with Java code • Lots of other reasons…
- 7. Conciseness • Code size reduced by 2-3x compared to Java • Less code == easier to read • Less code == fewer bugs
- 8. Conciseness • Type Inference • Expressions, not statements • Higher-ordered functions • Case classes • Pattern matching
- 9. Type Inference • Variables val subscrip7onEvents = foo() • Method return types def listOfPeople = List(“Paul”, “Eric”, “John”, “Mar7n”) • Generic type parameters case class MyPair[A, B](x: A, y: B) val p = MyPair(1, “foo”) // p is MyPair[Int, String]
- 10. Type inference • Java HashMap<String, Customer> customers = new HashMap<String, Customer>(); customers.add("id1", new Customer("Eric", "917-‐444-‐1234"); customers.add("id2", new Customer("Paul", "718-‐666-‐9876"); • Scala val customers = HashMap( "id1"-‐>Customer("Eric", "917-‐434-‐1852"), "id2"-‐>Customer("Paul", "718-‐666-‐9876"))
- 11. Expressions, not statements val server = if (environment == "development”) { val factory = new MBeanServerFactoryBean factory.aberProper7esSet() factory.getObject.asInstanceOf[MbeanServer] } else { val clazz = Class.forName("org.jboss.mx.u7l.MBeanServerLocator") clazz.getMethod("locateJBoss”).invoke(null) } // server is assigned one of the bold values
- 12. Expressions, not statements val n = try { userInput.toInt } catch { case _ => 0 } // n is an Int, 0 if unable to parse userInput
- 13. Collections API • Very comprehensive – For example, over 200 methods on List • Higher ordered functions – foreach, map, flatMap, exists, forall, find, findAll, filter, groupBy, par77on, etc. • Concise literals • Immutable and mutable variations
- 14. Collection Literals • val l1 = List(1, 2, 3) • val m1 = Map(“name” -‐> “Eric”, “city” -‐> “NYC”) • val s1 = Set(Car(“Ford”), Car(“Isuzu”), Car(“VW”))
- 15. Collections API • Scala val groups = subscrip7onEvents.groupBy(e => e.subscrip7on.id) • Java Map<String, List<SubscripLonEvent>> groups = new HashMap<String, ArrayList<Subscrip7onEvent>(); for (Subscrip7onEvent se : subscrip7onEvents) { ArrayList<Subscrip7onEvent> seList = groups.get(se.subscrip7on.id) if (seList == null) { seList = new ArrayList<Subscrip7onEvent>(); groups.put(se.subscrip7on.id, seList) } seList.add(se); }
- 16. Count characters in documents Try #1 – Java-esque Scala var total = 0 for (doc <-‐ docs) { total += doc.length } Try #2 – Use higher order functions var total = 0 docs.foreach(doc => total += doc.length)
- 17. Count characters in documents Try #3 – Use fold docs.foldLeb(0)((accum, current) => accum + current.length) Try #4 – Use type-classes (eg. Numeric) docs.map(_.length).sum Try #5 – Use the 'view' method to turn multiple passes into one docs.view.map(_.length).sum
- 18. Variables scala> var i = 0 i: Int = 0 scala> i = 2 i: Int = 2 scala> val j = 0 j: Int = 0 scala> j = 3 <console>:8: error: reassignment to val scala> lazy val l = expensiveComputa7on() l: Double = <lazy>
- 19. Uniform Access Principle scala> object Ints { | var i = 1 | val j = 2 | def k = i + j | } scala> Ints.i res8: Int = 1 scala> Ints.j res9: Int = 2 scala> Ints.k res10: Int = 3
- 20. Uniform Access Principle scala> object M { | private var pm = 0 | def m = pm | def m_=(in: Int) { pm = in } | } scala> import M._ scala> m res5: Int = 0 scala> m = 5 m: Int = 5
- 21. Case Classes • Scala scala> case class Car(make: String, model: String, mpg: Int) defined class Car scala> val c = Car("Honda", "Civic", 40) c: Car = Car(Honda,Civic,40)
- 22. Case Classes • Java public class Car implements scala.Product, scala.Serializable { final private String make, model; final private int mpg; Car(String make, String model, int mpg) { this.make = make; this.model = model; this.mpg = mpg; } public String getMake() { return make; } public String getModel() { return model; } public int getMpg() { return mpg; } public String toString() { return “Car( “ + make + …. } public boolean equals(Object that) { if (that instanceOf Car) && …… } public int hashCode() { return 19 + …… } public Car copy(String make, String model, int mpg) { ….. } // plus 9 other Scala-‐specific methods }
- 23. Case Classes • Case classes can also have mutable fields and methods case class Car(make: String, model: String, mpg: Int, var odometer) { def driveMiles(miles: Int) { odometer += miles } } • In Scala you can define multiple classes per source file
- 24. Pattern Matching • Case Classes // Class hierarchy: trait Expr case class Num(value : int) extends Expr case class Var(name : String) extends Expr case class Mul(left : Expr, right : Expr) extends Expr // Simplification rule: e match { case Mul(x, Num(1)) ⇒ x case _ ⇒ e }
- 25. Pattern Matching • Match on constants def describe(x: Any) = x match { case 5 => "five" case true => "truth" case "hello" => "hi!” case Nil => "the empty list" case _ => "something else” }
- 26. Pattern Matching • Typed patterns def generalSize(x: Any) = x match { case s: String => s.length case m: Map[_, _] => m.size case _ => -‐1 }
- 27. No Checked Exceptions // Look ma, no throws clause! def foo() { throw new java.lang.Excep7on }
- 28. Concurrency-readiness • The future present is many cores • Writing thread-safe code in Java is very difficult – Mostly due to shared, mutable state
- 29. Concurrency-readiness • Scala – Excellent support for immutability – Actors / Futures – Parallel collections
- 30. Immutability • Case classes • Immutable collections are default – Copies of collections share data • val vs. var, val is encouraged • Method parameters are vals
- 31. Actors • Included in standard Scala library • Simplified multithreading and coordination • Based on message passing – Each actor has a mailbox queue of messages • Implementation based on Erlang
- 32. Actors object Coun7ngActor extends Actor { def act() { for (i <-‐ 1 to 10) { println("Number: "+i) Thread.sleep(1000) } } } Coun7ngActor.start()
- 33. Actors import scala.actors.Actor._ val echoActor = actor { while (true) { receive { case msg => println("received: ” + msg) } } } echoActor ! "hello" echoActor ! "world!"
- 34. Futures Return a Future immediately, run func in new thread scala> future { Thread.sleep(10000); println("hi"); 10 } res2: scala.actors.Future[Int] = < func7on0> Use the Future apply() method to get the result scala> res2() // blocks wai7ng for sleep() to finish hi res3: Int = 10
- 35. Actors / Futures / STM • Akka provides more robust Actors and Futures • Also provides – Distributed (Remote) Actors – Software Transactional Memory – Java API
- 36. Parallel Collections • Add .par to collection to get parallel version • Uses JDK7 fork-join framework • Example: myData.par.filter(_.expensiveTest()).map(_.expensiveComputa7on()) – Filter is run in parallel, results are collected, then map is run in parallel
- 37. Interoperability with Java • Scala classes are Java classes • You can pass Scala objects to Java methods and vice-versa • For the most part, seamless interop – Cannot use Scala-only features from Java
- 38. Java Interop Example @En7ty class Subscrip7onEvent { @Id @GeneratedValue var id: Long = _ @ManyToOne(op7onal=false) var subscrip7on: Subscrip7on = _ var address: String = _ @Index(name="Subscrip7onEventStatus") private var status: String = _ def deliveryStatus = DeliveryStatus.withName(status) def deliveryStatus_=(s: DeliveryStatus) { status = s.toString } }
- 39. Java Interop Example @Controller @RequestMapping(Array("/report")) class ReportController { class MessageDto(message: Message) { @BeanProperty val id = message.id @BeanProperty val address = message.address // … } @RequestMapping(Array("/messages")) def messages(@RequestParam(value="fromDate”) from: String, map: ExtendedModelMap): String = { //… map.put(“messages”, asJavaCollec7on(messageDtos)) “report/messages” }
- 40. JavaConversions • Add Scala collection API methods to Java collections import collec7on.JavaConversions._ import collec7on.Iterable import java.u7l.{List=>JList} def goodStudents(students: JList[Student]): Iterable[String] = students.filter(_.score > 5).map(_.name)
- 41. Named and Default Params • Named parameters def resize(width: Int, height: Int) = { ... } resize(width = 120, height = 42) • Default parameters def f(elems: List[Int], x: Int = 0, cond: Boolean = true) f(List(1)) f(Nil, cond = false)
- 42. By-name Parameters • Method parameters can be lazily evaluated class Logger { def debug(msg: => String) { if (isDebug) doLog(DEBUG, msg) } } log.debug(“this “ + “ is “ + “expensive”)
- 43. Type Conveniences • Type Aliases type MyMap = mutable.HashMap[String, mutable.HashMap[String, Int]] • Import Aliases import com.nyx.domain.no7fica7on.{Topic=>DomainTopic}
- 44. Mixins • Multiple implementation inheritance trait UserIden7fierCmd extends ApiKeyCmd { var userId: String = _ def getUser = {…} } trait RoleIdCmd extends ApiKeyCmd { var… def… } object cmd extends UserIden7fierCmd with RoleIdCmd {..}
- 45. Duck Typing type Closeable = { def close(): Unit } def using[T <: Closeable, S] (obj: T)(func: T => S): S = { val result = func (obj) obj.close() result } val fis = new FileInputStream(“data.txt”) using(fis) { f => while (f.read() != -‐1) {} }
- 46. More Information • My Scala Links gist – https://gist.github.com/1249298