Scala-对Java的修正和超越

Java
Scala

2009.10 SD2China

Caoyuan (NetBeans Dream Team Memeber)

http://blogtrader.net/dcaoyuan/

• JVM
123.toByte
"1".toInt
true.toString

• val compare = (x: Int, y: Int) => x > y
compare(1, 2) // result: Boolean = false

• Java sta&c
( ) Scala object
object Dog {
val whatever = "dog" // static field in Java
}
class Dog {
def callWhatever = Dog.whatever
}

• val compare = (x: Int, y: Int) => x > y
list sortWith compare

• class AComparator {
def compare(x: Int, y: Int) = x > y
}
list sortWith (new AComparator).compare

• object annonymous extends scala.Function2[Int, Int, Boolean] {
override def apply(x: Int, y: Int) = x > y
}
list sortWith annonymous

• sta&c object
( )

(1)
• 1.+(1)
”.” ”()”
1 + 1
1.>(0)
1 > 0
(1 > 0).&(2 > 1)
(1 > 0) & 2 > 1
stack.push(10)
stack push 10
stack.pop
stack pop

• stack push {
{...}
val a = 1
val b = 2
a + b
}

• def !@#%^&*-<=>?|~:/ = println("noop")
def √(x: Double) = Math.sqrt(x)
val Π = Math.Pi
val r = √(9*Π)

• ‘<’, ‘>’ ’[’ ‘]’

(2)
• for
for (i <- List(1, 2)) {
println(i)
}
List(1, 2) foreach {i => println(i)}

for (i <- List(1, 2)) yield {
i + 10
}
List(1, 2) map {i => i + 10}

• // synchronized is function call instead of keyword
def check = synchronized {
// isInstanceOf is function call instead of keyword
100.isInstanceOf[String]
}

• stack.pop.asInstanceOf[Int] // (Integer) stack.pop() in Java

• return
val r1 = { // return 3
val a = 1
val b = 2
a + b
}

val r2 = if (true) 1 else 2

val r3 = // return (): Unit
for (i <- List(1, 2)) {
println(i)
}

val r4 = // return List(11, 12)
for (i <- List(1, 2)) yield {
i + 10
}

val r5 = // return java.io.File
try {
val f = new File("afile")
f
} catch {
case ex: IOException => null
}

Scala

( )

=
OO + FP

Scala-对Java的修正和超越

(1)
• Scala Any Any JVM

• JVM (byte, short, char, int, long, ﬂoat, double, boolean) void
• Scala (Byte, Short, Char, Int, Long, Float, Double, Boolean) Unit
AnyVal
• JVM AnyRef (java.lang.Object)

// Scala, Array 8
val arrayOfListOfString = new Array[List[String]](10)
val arrayOfAny: Array[_] = Array(1, 2)
// Java
List<String>[] arrayOfListOfString = new ArrayList<String>[10]

• Scala Null Nothing
• JVM null Null AnyRef
• Nothing Scala ( AnyVal AnyRef)

(2)
-
• (Invariant): C[T], C is invariant on T
Tsub Tsup T C[Tsub] C[Tsup] C[T]

• (Covariant): C[+T], C is covariant on T
Tsub T C[Tsub] C[T]

• (Contrava&ant): C[‐T], C is contravariant on T
Tsup T C[Tsup] C[T]

Java ?
Scala +/-

(2)
class Animal {}
class Dog extends Animal {}
class Fox extends Animal {}

• Animal Animal
class Xian<T> {} // Java class Xian[+T] //

void isAnimalXian(Xian<? extends Animal> in) def isAnimalXian(in: Xian[Animal])

isAnimalXian(new Xian<Dog>()) isAnimalXian(new Xian[Dog])
isAnimalXian(new Xian<Fox>()) isAnimalXian(new Xian[Fox])

• Dog Dog
class Xian<T> {} // Java class Xian[-T] //

void isDogXian(Xian<? super Dog> in) def isDogXian(in: Xian[Dog])

isDogXian(new Xian[Animal]) isDogXian(new Xian[Animal])

vs
•
trait Pair[K, V] { class PairImpl extends Pair[Dog, Fox] {
def get(key: K): V def get(key: Dog): Fox
def set(key: K, value: V) def put(key: Dog, value: Fox)
} }

=>

• + +
trait Pair { class PairImpl extends Pair {
type K // deferred type type K = Dog
type V // deferred type type V = Fox

def get(key: K): V def get(key: K): V
def set(key: K, value: V) def set(key: K, value: V)
} }

=>
• => /
• => type StrToList = HashMap[String, List[String]]

Trait Structural Type
• Trait: + =>
• Structural Type:
Type
trait Subject {
type Observer = {def update(subject: Subject)} // type member

private var observers = List[Observer]() // field
def addObserver(observer: Observer) = observers ::= observer
def notifyObservers = observers foreach (_.update(this))
}

class Price(var value: Double) {def setValue(v: Double) = value = v}

Java Interface
Trait
val observablePrice = new Price(10) with Subject {
override def setValue(v: Double) = {
super.setValue(v); notifyObservers
}
}

observablePrice.addObserver(new AnyRef { update(Subject) Observer
def update(subject: Subject) {
println("price changed")
}
})

Trait Self Type
•
• Class Trait: OilPrice extends Price with Subject with Market with ....
•
• (Self Type)

trait Subject {self: Price =>
type Observer = {def update(subject: Price)} // type member

private var observers = List[Observer]() // field
this Subject
def addObserver(observer: Observer) = observers ::= observer
def notifyObservers = observers foreach (_.update(this))
}

observablePrice.addObserver(new AnyRef { // only need a update(Any) method
def update(subject: Price) {
println("price changed to " + subject.value)
}
})

• Self Type:
• Trait
• class trait
• self Trait super this self self
• Trait
•
• class trait ( Class )

apply
• apply <instance>(args) <instance>.apply(args)
• apply Class
class Price(var value: Int) {
def apply() = value // ‘def apply = value’, without ’()’ is bad idea
}

object Price {
def apply(v: Int) = new Price(v)
}

val p = Price(10)
p // Price@565f0e7d
p() // 10
Price(10)() // 10

• apply Scala
val list = List(1, 2, 3) // List.apply[A](xs: A*)
val array = Array("a", "b", "c") // Array.apply[T](xs: T*)
val map = Map("a" -> 1,
"b" -> 2,
"c" -> 3) // MapFactory.apply[A, B](elems: (A, B)*)

array(0) // array.apply(index): "a"
map("a") // map.apply(key): 1

• Java Switch ( Java7) Scala
val str = "abcdef"
str.toSeq match {
case Seq('a', 'b', rest@_*) => println(rest) // cdef
case _ => println("no match")
}

var any: Any = _
any match {
case "scala" | "java" => "string"
case i: Int if i > 10 => "int > 10"
case `str` => "abcdef"
case _: String => "string type"
case hd :: tail => "List"
case _ => "other"
}

case class Address(coutry: String, city: String, street: String, no: Int)
case class Person(name: String, age: Int, address: Address)

val p = Person("Wang wu", 12, Address("China", "Beijing", "Chang'an", 10))

p match {
case Person(name, age, Address(coutry, city, street, no)) =>
println(name + "|" + coutry + "|" + city + "|" + street + "|" + no)
}

Scala
• Existential type ( Java )
• Implicit type * ( )
• val, var, lazy val ( )
• Partial function * ( )
• Primary constructor ( apply)
• Accessor properties (Why getter, setter? )
• XML Process ( )

* DSL

(DSL )
• Actors
class Ping(count: Int, pong: Actor) extends Actor {
def act() {
var pingsLeft = count - 1
pong ! Ping
while (true) {
receive { {...}
case Pong =>
if (pingsLeft % 1000 == 0)
println("Pong”)
if (pingsLeft > 0) {
pong ! Ping
pingsLeft -= 1
} else {
pong ! Stop
exit()
}}}}}
• Bill Venners scalatest
class StackSpec extends FlatSpec with ShouldMatchers {

"A Stack" should "pop values in last-in-first-out order" in {
val stack = new Stack[Int]
stack.push(1)
stack.push(2)
stack.pop() should equal (2) should
stack.pop() should equal (1)
}

it should "throw NoSuchElementException if an empty stack is popped" in {
val emptyStack = new Stack[String]
evaluating { emptyStack.pop() } should produce [NoSuchElementException]
}
}

Scala Java

Scala Java
• Scala Java
• Array
• Scala Java Java
• object
• Scala
• Actors Packrat Parser

Scala

Scala Java

• Scala
• Java Scala( nio )
• Scala ( liftweb)
• IDE

IDE

• IntelliJ Idea
•5
•

• Eclipse
• Sean McDirmid
• 1 (Miles Sabin)
• Scala Martin IDE

• NetBeans
• (dcaoyuan)
• 2007 LL(K)
• 2008 PEGs
• 2008 Scala Hacking NetBeans Innovation Grant
• 2009 8 Scala Scala Martin IDE

IDE -NetBeans
•
•
•
•
•
•
•
•
•
•
• HTML
•
•
• Java/Scala
•
• import
•
•
• Maven
•
• NetBeans 6.7.x 7000
• NetBeans 6.8m2 Scala-2.8.0 beta
http://wiki.netbeans.org/Scala68v1

Scala-对Java的修正和超越

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