Scala terms explained for Java developer, immutability, implicits, pattern matching, futures

1. Scala taxonomy explained
from Java programmer
Radim.Pavlicek@gmail.com

2. Agenda
● OO Features
● Pattern matching
● Functional Programming
● Actors
● Futures
● Implicits

3. Data Transfer Objets
class Person {
private String firstName;
String getFirstName() {
return firstName;
}
void setFirstName(String aFirstName) {
this.firstName = aFirstName;
}
}

4. toString
@Override
public String toString() {
return "Person{" +
"firstName='" + firstName + ''' +
", lastName='" + lastName + ''' +
'}';
}

5. equals
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Person)) return false;
Person person = (Person) o;
if (!firstName.equals(person.firstName))
return false;
if (!lastName.equals(person.lastName))
return false;
return true
}

6. hashCode
@Override
public int hashCode() {
int result = firstName.hashCode();
result = 31 * result + lastName.hashCode();
return result;
}

7. Builder
static class Builder {
String aName = "Radim";
Builder withFirstName(String aName) {
this.name = aName; return this;
}
Person build() {
Person p = new Person; p.setName(aName);
return p;
}

8. Case Classes
case class Person(firstName: String =
"Bill", lastName :String = "Gates")
val me = Person(lastName = "Crosby")
● Immutable
● toString, equals, hashCode
● person.copy(lastName="Malkin")

9. Laziness in Java
Integer cached = null;
Integer getExpensiveComputation() {
if (null == cached) {
cached = doCount();
}
return cached;
}

10. Lazy Definition
lazy val count = doCount()

11. Imports
import scala.collection.immutable.Map
object HelloWorld {
def main(args: Array[String]) {
import scala.collection.immutable.
{Map=>MobNumbers}
val my : MobNumbers[String, String] =
MobileNumbers("radim" -> "6767982408")
Console.print( my("radim") )
}
}

12. Java: Singletons
public class Singleton {
private Singleton INSTANCE = null;
private Singleton() {}
public Singleton getInstance() {
if (null == INSTANCE) {
INSTANCE = new Singleton()
}
}

13. Objects
class CompanionObjects(val id: Long, val
description: String = "")
object CompanionObjects{
def zero = new CompanionObjects(id = 0,
description = "zero")
def apply(id:Int, description:String)= new
CompanionObjets(id,description)
def one = CompanionObjets(1, "one")
val errorMessage = "ID should be positive"
}

14. Pattern matching
In Java:
Switch statements with String cases have
been implemented in Java SE 7, at least 16
years after they were first requested.

15. Pattern matching
object Matching extends App {
val name :AnyRef= ...
name match {
case "Radim" => println("me")
case "crosby" | "sidney" => println("NHL")
case Seq("sidney", _) => println("sidney with
someone")
case Seq("sidney", _*) => println("sidney with group")
case x: Int if x < 0 => println("negative number")
case y => println("Unknown " + y)
}
}

16. scala.annotation.switch
(ch: @switch) match {
case 'a' if eof => println("a with oef")
case 'b' => println("b")
case 'c' => println("c")
}

17. Functional programming

18. Immutability
Java
String, BigDecimal
Scala
val immutable = 1
var mutable = 2
mutable = 3

19. Immutability cont.
val n = "123".reverse
val m = n.reverse
println(n, m) // 321, 123
val o = new StringBuffer("123").reverse()
val p = o.reverse()
println(o, p) // 123, 123

20. Higher order functions
object Collections extends App{
val c = 4 to 8
val s: IndexedSeq[String] = c map(_+"th")
s map (_.toUpperCase)
println(s) //4th, 5th, 6th, 7th, 8th
}

21. Parallel collection
object Parralel extends App{
def time[R](block: => R): R = {
val t0 = System.nanoTime()
val result = block // call-by-name
val t1 = System.nanoTime()
println("Elapsed time: " + (t1 - t0)/1000000 + " ms")
result
}
val c = 1 to 10000000
time { c map (_ + 1) } //4935 ms
time { c.par map (_ + 1) } //3723 ms
}

22. Currying
object Curry extends App{
def product(i: Int)(j: Int) = i * j
def times2 = product(2)_
def times3 = product(3)_
println(times2 (4)) //8
println(times3 (4)) //12
}

23. Partial functions
val sample = 1 to 10
val isEven: PartialFunction[Int, String] =
{case x if x % 2 == 0 => x+" is even"}
// the method collect can use isDefinedAt
to select which members to collect
val evenNumbers = sample collect isEven
println(evenNumbers)

24. Actors

25. Futures
import ExecutionContext.Implicits.global
object Futures extends App {
@volatile var totalA = 0
val text = Future {
Thread.sleep(1000); val a = "a" * 16
}
text onSuccess {
case txt => totalA += txt.count(_ == 'a')
}
println(totalA) // 0
Thread.sleep(1000); println(totalA) //0
Thread.sleep(1000); println(totalA) //16
}

26. Implicit conversions
Java:
String.valueOf(int i),
Integer.parseInt(String s)
Scala:
(1 to 4).foreach(println)

27. Implicit parameters
object ImplicitParam extends App{
def max[T](a: T, b: T)(implicit $ev1: Ordering[T]): T =
..
class Timeout(val milis: Int)
object Timeout {
def apply(milis: Int) = new Timeout(milis)
implicit val t: Timeout = Timeout(10)
}
def query(i: Int)(implicit t: Timeout) {
println(t.milis)
}
query(1)

28. To be continued...