Introduction to new features in Java 8 and their use in functional programming, such as lambda methods, extension methods, method handles, the new Stream API, parallellism and laziness.

1. !1
RETURNOF
THE AVA
J
JAVA EIGHT

2. Java 8 – what’s new?
Lambdas, Extension Methods, SAMs, Streams, Method
Handles, Oh My!
Nashorn JS engine
JSR-310: Date & time API
No more PermGen– where classloaders go to die
...
!2

3. Java 8 – what’s new?
Lambdas, Extension Methods, SAMs, Streams, Method
Handles, Oh My!
Nashorn JS engine
JSR-310: Date & time API
No more PermGen– where classloaders go to die
...
!3

4. Your Mission
Should you choose to accept it
Sort a list of people
by their names

5. Java 7
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
!5

6. We are not amused
© Fredrik Vraalsen 2012

7. Java 8
Comparator<Person> byName = new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
};
Single Abstract Method
!
(SAM)
!7

8. Single Abstract Method
Interface with only one (non-default) method
Abstract class with only one abstract method
Sound familiar?
Pretty much every event listener, callback mechanism, ...
Can be replaced by a Lambda
!8

9. Lambda
Closure
Anonymous function
!9

10. SAM
Comparator<Person> byName = new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
};
!10

11. SAM
Comparator<Person> byName = new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
};
BodyReturn type
Parameter list
Method name
Type
!11

12. Lambda
Comparator<Person> byName = new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
};
BodyReturn type
Parameter list
Method name
Type
!12

13. Lambda
Comparator<Person> byName =
!
return x.getName().compareTo(y.getName());
};
BodyReturn type?
Parameter list
(Person x, Person y) -> {
!13

14. Lambda
Comparator<Person> byName =
(Person x, Person y) ->
Parameter list
BodyReturn type?
x.getName().compareTo(y.getName());
{
};
return
!14

15. Lambda
Comparator<Person> byName =
Parameter list
BodyReturn type?
(x, y) -> x.getName().compareTo(y.getName());(Person x, Person y)
!15

16. Lambda
Comparator<Person> byName =
(x, y) -> x.getName().compareTo(y.getName());
!
Collections.sort(people, byName);
!16

17. Lambda
Comparator<Person> byName =
(x, y) -> x.getName().compareTo(y.getName());
!
sort(people, byName);
!17

18. Lambda
!
!
!
sort(people, (x, y) -> x.getName().compareTo(y.getName()));
!18

19. Comparators
import static java.util.Comparator.comparing;
…
!
sort(people, comparing((Person p) -> p.getName());
!19

20. Cool!
!
!
!
!
Now onwards... © Fredrik Vraalsen 2013

21. Method handles
Reference to methods
Can be used in place of lambdas
!21

22. Method handles
!
!
!
sort(people, comparing((Person p) -> p.getName()));
!22

23. Method handles
!
!
!
sort(people, comparing(Person::getName));
!23

24. Ok, nice...
What else?
© Fredrik Vraalsen 2012

25. Extension methods
!
!
!
sort(people, comparing(Person::getName));
!25

26. Extension methods
!
!
!
people.sort(comparing(Person::getName));
!26

27. Extension methods
!27
java.util.List:
!
default void sort(Comparator<? super E> c)

28. Extension methods
Defender Method
Default Method
(Virtual) Extension Method
!28
java.util.List:
!
default void sort(Comparator<? super E> c)

29. Extension methods
java.util.List:
!
default void sort(Comparator<? super E> c) {
}
!
Defender Method
Default Method
(Virtual) Extension Method
!29

30. Extension methods
java.util.List:
!
default void sort(Comparator<? super E> c) {
Collections.sort(this, c);
}
!
Defender Method
Default Method
(Virtual) Extension Method
!30

31. Java 8 extension methods
Extend interfaces with new methods
Compatibility
Default implementation
Override
Requires modification of original interface
!31

32. C# extension methods
“Add” methods to existing types
Without modifying original type
Defined as static methods
Called as instance methods
!32

33. Scala implicit classes
“Add” methods or properties to existing types
Without modifying original type
Implicit wrapper object
!33

34. Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
!
vs
!
people.sort(comparing(Person::getName));
!34

35. So far, so good?
Lambdas
Method handles
Extension methods
!35

36. So, what’s the catch?
© Fredrik Vraalsen 2012

37. What’s wrong with using List::sort ?
Modifies existing collection
Others may be using it?
Concurrency issues
Performance
!37

38. Streams
© Fredrik Vraalsen 2008

39. The old fashioned way
!39
List<RoadData> filtered = new ArrayList<>();
int count = 0;
for (Iterator<RoadData> i = roadData.iterator();
i.hasNext() && count < 10; ) {
RoadData data = i.next();
if (data.getName().contains(nameQuery)) {
filtered.add(data);
count++;
}
}

40. Streams
!40
!
roadData.stream()
.filter(r -> r.getName().contains(nameQuery))
.limit(10)
.collect(Collectors.toList());

41. Streams – pipelines
!41
!
roadData.stream()
.filter(r -> r.getName().contains(nameQuery))
.limit(10)
.collect(Collectors.toList());
!
!
cat roadData.txt | grep … | head > output.txt

42. Streams – pipelines
Source
collection, array, generator function, IO channel, ...
Intermediate operations (Stream-producing)
filter, map, ...
Terminal operations (value-producing)
!42

43. Performance
© Fredrik Vraalsen 2012

44. Streams – performance
!
people.stream()
.filter(p -> p.getAge() >= 18)
.map(Person::getName)
.collect(Collectors.toList());
!44

45. This one goes to 11!
!
people.parallelStream()
.filter(p -> p.getAge() >= 18)
.map(Person::getName)
.collect(Collectors.toList());
!45

46. Scala FTW!
!
people.par
.filter(_.age >= 18)
.map(_.name)
!46

47. Streams
java.util.stream
Create new results
Lazy
Parallelizable
!47

48. More cool stuff in Java 8
String join
Collection removeIf (≈ filter)
List sort
Map getOrDefault, putIfAbsent, replace, forEach, etc.
java.util.stream.Collectors
count, sum, average, min, max, groupingBy, etc.
java.nio.file.Files lines
!48

49. What’s missing?
© Fredrik Vraalsen 2012

50. What’s missing?
Immutability
Value types
Data structures (lists, maps, etc.)
Concurrency
!50

51. Some help to be found
Immutable collections
Google Guava, FunctionalJava, clj-ds
Concurrency mechanisms
Akka (Actors, STM)
!51

52. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 37),
new Person("Hedda", 2));
!52

53. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 37),
new Person("Hedda", 2));
!
PersistentVector<Person> morePeople =
people.plus(new Person("Johannes", 4));
!53

54. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 37),
new Person("Hedda", 2));
!
PersistentVector<Person> morePeople =
people.plus(new Person("Johannes", 4));
!
morePeople.stream()
.forEach(p -> System.out.println(p.getName()));
!54

55. Ready to
make the jump!?
© Fredrik Vraalsen 2013

56. Play with it!
Download – http://www.oracle.com/technetwork/java/
… or https://jdk8.java.net/download.html
Whitepapers – http://openjdk.java.net/projects/lambda/
FAQ – http://www.lambdafaq.org/
Supported in IntelliJ IDEA 12 & 13
Eclipse Java 8 beta plugin and NetBeans 8.0 RC !56

57. Why use X instead?
Java 8 just released
Will be a long time before you can use it in enterprise dev!
Clojure and Scala available NOW! (JDK 1.6)
Important things are still missing
!57

58. Want to know more?
^{Oslo "Socially Functional Programmers" #OsloSFP}
“Haskell på godt og vondt” – tonight 6pm @ Teknologihuset
http://www.meetup.com/Oslo-Socially-Functional/
Functional Programming in Java 8
http://2014.flatmap.no/
!58

59. Questions?
© Fredrik Vraalsen 2012
vraalsen@iterate.no / @fredriv