Xebicon2013 scala vsjava_final

Is Java 8 a Scala killer?
Urs Peter
upeter@xebia.com
@urs_peter
http://bit.ly/xc_is

Born in 2003 – 10 years old
Sponsored by EPFL
(École Polytechnique Fédérale de Lausanne – Switzerland)
Some Facts about Scala

Also co-designed Generic Java,
and built the current generation of javac.
Creator: Martin Odersky

Scala is fully interoperable with Java

Scala is used in many worldclass companies &
applications

There is a company behind Scala

…that offers a supported Stack

Recently joined Typesafe:

Java
1.7Lightweight Syntax
Strong Typing
FunctionalObject Oriented

May 2013
Thoughtworks Technology Radar
Scala
Play Framework

• JSRs
– JSR 335 Lambda Expressions and Virtual Extension
Methods (in debate since 2008)
– JSR 308 Annotations on Java Types
– JSR 310 Date and Time API
• Schedule:
– 2013/02/21 M7
– 2013/07/05 M8 (Final Release Candidate)
– 2013/09/09 GA (General Availability)
– Spring 2014
Java 8

Java 8
• JSRs
• Schedule:
– 2013/02/21 M7
– Spring 2014

Java
• JSRs
• Schedule:
– 2013/02/21 M7
– Spring 2014
8

Java 8 Lambda’s versus
Scala Functions
What’s so fun about Functional Programming anyway?

List<Integer> numbers = Arrays.asList(1,2,3);
Filter even numbers
Filter odd numbers
DuplicationVariation
List<Integer> res = new ArrayList<>();
for (Integer i : numbers) {
if( i % 2 == 0 ) res.add(i);
}
return res;
if( i % 2 != 0 ) res.add(i);
}
return res;
From Imperative to
Functional in Java

interface Predicate<T> {
public boolean op(T item);
}
The Function: Varying computation
The Loop: Generic Control Structure
public List<T> filter(List<T> items, Predicate<? super T> predicate) {
List<T> res = new ArrayList<>();
for(T item : items) {
if(predicate.op(item)){
res.add(item);
}
}
return res;
}
if( i % 2 == 0 ) res.add(i);
}
return res;
From Imperative to
Functional in Java
Generic Control Structure
==
Higher Order Function

List<Integer> numbers = Arrays.asList(1, 2, 3);
Filter even numbers
Filter odd numbers
List<Integer> result = filter(numbers, new Predicate<Integer>() {
public boolean op(Integer item) {
return item % 2 == 0;
}
});
public boolean op(Integer item) {
return item % 2 != 0;
}
});
From Imperative to
Functional in Java

Take Spring:
Take Google Guava:
jdbcTemplate.queryForObject("select * from student where id = ?",
new Object[]{1212l},
new RowMapper() {
public Object mapRow(ResultSet rs, int rowNum) throws SQLException {
return new Student(rs.getString("name"), rs.getInt("age"));
}
});
Iterables.filter(persons, new Predicate<Person>() {
public boolean apply(Person p) {
return p.getAge() > 18;
}
});
Can you see it?
Is the Functional Concept new in Java?

Before (< Java 8)
public boolean op(Integer i) {
return i % 2 == 0;
}
});
After (>= Java 8)
Lambda Expressions can be used for
‘Functional Interfaces’ (here Predicate)
that have a single method
(here: boolean op(T f)).
List<Integer> result = filter(numbers, (Integer i) -> i % 2 == 0 );
Functional Programming with Lambda’s
Lambda Expression
==
Syntactic sugar

List<Integer> result = filter(numbers, (Integer i) -> i % 2 == 0);
List<Integer> result = filter(numbers, i -> i % 2 == 0);
No need to define the
type since it can be
inferred by the compiler.
Lambda Features
at one Glance
Java 8
Complete Lambda Expression
Lambda Expression with Type Inference

Turn an existing static method
with the same signature as
the functional interface into a
closure, using ::
Method Reference
at one Glance
Java 8
Boolean::booleanValue
refers to the instance that
the filter is iterating through.
Static Method References
Instance Method References
List<Boolean> trueOrFalse = Arrays.asList(true, false);
List<Boolean> trueOnly = filter(trueOrFalse, Boolean::booleanValue );
public class NumUtils {
public static boolean isEven(Integer i) {
return i % 2 == 0;
}
}
List<Integer> result = filter(numbers, NumUtils::isEven );
(Integer i) -> NumUtils.isEven(i)
(Boolean b) -> b.booleanValue()

Complete Lambda Expression
Java 8
Scala
Java 8 Lambda Expressions
versus Scala Functions
val result = filter(numbers, (i:Int) => i % 2 == 0)
List<Integer> result = filter(numbers, (Integer i) -> i % 2 == 0);

List<Integer> result = filter(numbers, i -> i % 2 == 0);
Lambda with type inference
Java 8
Scala
val result = filter(numbers, i => i % 2 == 0)

List<Integer> result = filter(numbers, NumUtils::isEven);
Static Method References
Java 8
val result = filter(numbers, NumUtils.isEven)
Scala Every method that is not
called with its arguments is
automatically turned into a
Function.
Methods == Functions

List<Boolean> trueOnly = filter(trueOrFalse, Boolean::booleanValue);
Instance Method References
Java 8
Scala
Scala uses the _ (underscore) to
refer to the current instance that
is iterated through
val trueOnly = filter(trueOrFalse, _.booleanValue )

So, what’s the difference?
trait Seq[T] {
def filter(f:T => Boolean):Seq[T]
...
Higher Order Functions reveal the difference:
interface Predicate<T> {
public boolean op(T t);
}
Java 8
Scala
interface Collection<T> {
public Collection<T> filter(Predicate<T> p);
...
Scala has a
function syntax.
Every Higher Order Function needs a
specific Functional Interface in Java 8.
The compiler transforms this syntax to a
generic Function object. Therefore, no
Functional Interface needs to be defined.
class Function1[I, R] {
def apply(i:I): R
}

Detailed Comparison
Syntax is limited to calling Higher
Order Functions
Functions are interfaces with one
method
Lambda’s are Syntactic Sugar for
Functional Interfaces
Scala Functions
Have a syntax for defining and
calling Higher Order Functions
Functions are objects with methods
Rich Function Features:
– Function Composition
– Currying
– Call-by-name arguments
– PartialFunctions
– Pattern matching
Functions are First Class Citizens
fully supporting the Functional
Programming Paradigm

Taking off with Functional
Programming

Iterations in Java are ‘external’ by
default…
What is going on here?
List<Student> students = ...;
List<Student> topGrades = new ArrayList<Student>();
List<String> result = new ArrayList<String>();
for(Student student : students){
if(student.getGrade() >= 9) {
topGrades.add(student);
}
}
}
Collections.sort(topGrades, new Comparator<Student>() {
public int compare(Student student1, Student student2) {
return student1.getGrade().compareTo(student2.getGrade());
}
});
for(Student student : topGrades){
result.add(student.getFirstName() + " " + student.getLastName());
}
Java

Advantages:
• Re-use of generic, higher level control structures
• Less error prone
• More expressive and readable
• Chainable
Internal Iteration: When
Lambda’s/Functions start to shine
Ah, now I get it:
List<Student> students = ...
List<String> topGrades =
students.filter(s -> s.getScore() >= 9)
.sortedBy(Student::getLastName)
.map(s -> s.getFirstName() + " " + s.getLastName())
.into(new ArrayList<>());
Java 8
Not yet
available

Internal Iteration: When
Lambda’s/Functions start to shine
val students = ...
val topGrades =
students.filter(_.score >= 9)
.sortBy(_.lastName)
.map(s => s.firstName + " " + s.lastName)
Java 8
Scala
No need to use the into()
method. This happens in
Scala ‘automagically’
List<Student> students = ...
List<String> topGrades =
students.filter(s -> s.getScore() >= 9)
.sortedBy(Student::getLastName)
.map(s -> s.getFirstName() + " " + s.getLastName())
.into(new ArrayList<>());
Not yet
available

Higher Order Functions
Parallelism
Automatic collection conversion
Fast immutable data structures
Other
Detailed Comparison:
Collections & Lambda’s/Functions
Scala
Collections
Automatic conversion
from Java to Scala
collections
Java 8
Collections

How is it possible to add Higher Order Functions
(e.g. filter map foreach etc.) to the java.util.Collection interface without
breaking backwards compatibility?
public Collection<T> filter(Predicate<T> p);
public <R> Collection<R> map(Mapper<T, R> m);
...
‘Old’ Java code (prior Java 8) running in
JRE 8 would have to implement all the
new methods that offer ‘internal
iteration’ with Lambda’s.
Back to Java 8:
How to make it fit?
Java 8

…with default implementations for interfaces
aka Virtual Extension Methods!
public default Collection<T> filter(Predicate<? super T> p) {
Collection<T> res = new ArrayList<>();
for(T item : this) {
if(p.test(item)) {
res.add(item);
}
}
return res;
}
...
Back to Java 8:
How to make it fit?
Java 8

A closer look at Java 8’
Virtual Extension Methods
• Primary motivator is API evolution
(Backwards compatibility for Lambda’s in
Collections)
• Useful mechanism by itself:
Enables Multiple Inheritance of behavior
• Inspired by Scala traits (among others)

Let’s model the domain for a ‘futuristic’ game
Gun
fireAt(s:Ship)
Shield
hit() //50%
Medic
repair(s:Ship)
Ship
health:Integer
hit()
Possible characteristics of a Ship:
s.health = 10
repaired += 1
health - 1s.hit()
health - 2
Do we need Multiple Inheritance?

Possible Ship implementations:
Base MechanicFighterCommander
The Limits of
Single inheritance

Implementation Challenge: Single inheritance won’t work
Gun Shield Medic
Base
Commander
Fighter
Mechanic
The Limits of
Single inheritance

Adding Behavior to a Ship (Scala)
class Ship(var health:Int = 0) {
def hit() = health -= 2
}
trait Gun {
def fireAt(s:Ship) = s.hit
}
A trait is an interface with
an implementation
(behavior and/or state)
val goodFighter = new Ship(10) with Gun
val evilFighter = new Ship(10) with Gun
goodFighter.fireAt(evilFighter)
println(evilFighter.health)
> 8
It can be ‘mixed-in’ with a
class using the keyword:
with
Scala
Multiple Inheritance of Behavior

Adding Behavior to a Ship (Java 8)
class Ship {
int health = 0;
//constructor, getters, setters, hit method
}
interface Gun {
default void fireAt(Ship s) { s.hit(); }
}
class Fighter extends Ship implements Gun {…}
Fighter goodFighter = new Fighter(10);
Fighter evilFighter = new Fighter(10);
goodFighter.fireAt(evilFighter);
println(evilFighter.getHealth());
> 8
Works with Virtual
Extension Methods
Java 8

Change Behavior of Ship (Scala)
}
trait Shield {
self:Ship =>
override def hit() = self.health -= 1
}
val commander = new Ship(10) with Gun with Shield
evilFighter.fireAt(commander)
println(commander.health)
> 9
Trait’s can have a self-
type. The self-type tells
with which class this trait
can be ‘mixed’.
The self-type provides
access to the ‘mixed-in’
class
Traits can override
methods of the
‘mixed-in’ class
Scala

Change Behavior of Ship (Java 8)
val commander = new Ship(10) with Gun with Shield
evilFighter.fireAt(commander)
println(commander.health)
> 9
}
trait Shield {
self:Ship =>
override def hit() = self.health -= 1
}
Does not work with Virtual
Extension Methods (VEM)
Why?
• VEM cannot override methods of
a class (they are overridden)
• VEM have no self-types
Java 8

Adding State and Behavior to Ship (Scala)
trait Medic {
var repaired = 0
def repair(s:Ship) = {
s.health = 10
repaired += 1
}
}
val medic = new Ship(10) with Shield with Medic
val brokenShip = new Ship(1) with Gun
medic.repair(brokenShip)
println(brokenShip.health)
> 10
println(medic.repaired)
> 1
A trait can have state.
Scala
Multiple Inheritance of State

val medic = new Ship(10) with Shield with Medic
val brokenShip = new Ship(1) with Gun
medic.repair(brokenShip)
println(brokenShip.health)
> 10
println(medic.repaired)
> 1
Adding State and Behavior to Ship (Java 8)
trait Medic {
var repaired = 0
def repair(s:Ship) = {
s.health = 10
repaired += 1
}
}
Does not work with Virtual
Extension Methods (VEM)
Why?
• VEM cannot have state
Java 8
Multiple Inheritance of State

Have methods
Have fields
Access implementing
class in a typesafe way
Override methods
Stackable
(call to super is linearized)
Intent
Scala
Traits
Multiple inheritance
‘without the issues’
Java 8
Virtual Extension Methods
Grow the
language
Scala Traits vs
Java 8 Virtual Extension Methods

Is there no better way to
“Grow the language”?
• What we really want is ‘a mechanism’ that adds ‘new
methods’ to ‘existing classes’
• Virtual Extension Methods solve this problem on the
interface level:
– When we extend the interface with defaults, all classes inheriting
from this interface gain the functionality
– E.g. Arrays.asList(1,2,3).forEach(i -> println(i));
• Drawback: limited to interfaces
• But how about: I want to extend existing
classes by myself too!
"You win %2.2f%n".format(333333.3333);
new java.util.Date().toString("yyyy-MM-dd");

Welcome to Scala Implicits
…or the ‘pimp my library pattern’

Implicits in Practice
implicit class RichDate(val date:Date) extends AnyVal {
def toString(pat:String):String = new SimpleDateFormat(pat).format(date)
}
new java.util.Date().toString("yyyy-MM-dd")
Add a new method to an existing class (pimp my library):
Scala
//compiler turns this into:
RichDate.methods$.toString(new Date(), "yyyy-MM-dd")
Scala

Lambda’s for Collections:
Scala solved it before…
import collection.JavaConversions._
java.util.Arrays.asList(1,2,3).foreach(i => println(i))
java.util.Arrays.asList(1,2,3).filter(_ % 2 == 0)
Import JavaConversions and you are done:
Now we have all Higher
Order Functions like
foreach, filter, map etc.
available
The JavaConversions object
contains implicits that
convert a java.util.Collection
into a Scala’s counterpart.
Scala

If Java 8 had chosen for implicits…
• A proven mechanism was added to add new
methods to existing classes without being limited to
interfaces
• API designers AND API users would be able to add
new methods to existing classes
• Bridging of API’s – like Google Guava to Java
Collections – would be possible
• Last but not least: the implicit mechanism is not new
in Java: How about: Autoboxing, Lambda’s?

Lambda’s/Functions
Multiple Inheritance
Is Java 8 a Scala killer?
ScalaJava 8
Syntactic sugar for
Functional Interfaces
Functions as First
Class Citizens
Of Behaviour Of Behaviour and
State ‘without the
issues’
Extend existing classes - Implicits
• The features of JSR 335 (Lambda’s and Virtual Extension Methods)
are definitely an improvement;
• Choosing Implicits above Virtual Extension Methods for preserving
backwards compatibility would have made Java 8 more powerful;
• Java 8’ new language constructs are not as features rich and
complete as in Scala.

Q & A
def ask(question: Any) = question match {
case "?" => "Another layer of abstraction"
case "???" => "It depends"
case _ => 42
}
Evalutation
http://bit.ly/xc_is

Xebicon2013 scala vsjava_final

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