Slides from a workshop on Functional programming in Java 8, held at the flatMap Oslo conference 2014.

1. Functional Programming in Java 8
!
flatMap 2014
!
!
Fredrik Vraalsen – @fredriv
1

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());
}
};
BodyReturn type
Parameter list
Method name
Type
10

11. 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
11

12. Lambda
Comparator<Person> byName =
!
return x.getName().compareTo(y.getName());
};
BodyReturn type?
Parameter list
(Person x, Person y) -> {
12

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

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

15. Lambda
Comparator<Person> byName =
(x, y) -> x.getName().compareTo(y.getName());
!
Collections.sort(people, byName);
15

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

17. Lambda
!
!
!
sort(people, (x, y) -> x.getName().compareTo(y.getName()));
17

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

19. Lambda
!
!
!
sort(people, (x, y) -> x.getName().compareTo(y.getName()));
!
!
!
(Person, Person) -> int
19

20. Lambda
!
!
!
sort(dogs, (x, y) -> x.getName().compareTo(y.getName()));
!
!
!
(Dog, Dog) -> int
20

21. Lambda
!
!
!
sort(people, (x, y) -> x.getName().compareTo(y.getName()));
21

22. Comparators
import static java.util.Comparator.comparing;
…
!
sort(people, comparing((Person p) -> p.getName());
22

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

24. Method handles
Reference to methods
Can be used in place of lambdas
24

25. Method handles
!
!
!
sort(people, comparing((Person p) -> p.getName()));
25

26. Method handles
!
!
!
sort(people, comparing((Person p) -> p.getName()));
!
!
(Person) -> String
26

27. Method handles
!
!
!
sort(people, comparing(Person::getName));
!
!
(Person) -> String
27

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

29. Extension methods
!
!
!
sort(people, comparing(Person::getName));
29

30. Extension methods
!
!
!
people.sort(comparing(Person::getName));
30

31. Extension methods
Defender Method
Default Method
(Virtual) Extension Method
31
java.util.List:
!
default void sort(Comparator<? super E> c)

32. Extension methods
java.util.List:
!
default void sort(Comparator<? super E> c) {
}
!
Defender Method
Default Method
(Virtual) Extension Method
32

33. Extension methods
java.util.List:
!
default void sort(Comparator<? super E> c) {
Collections.sort(this, c);
}
!
Defender Method
Default Method
(Virtual) Extension Method
33

34. Java 8 extension methods
Extend interfaces with new methods
Compatibility
Default implementation
Override
Requires modification of original interface
34

35. C# extension methods
“Add” methods to existing types
Without modifying original type
Defined as static methods
Called as instance methods
35

36. Scala Traits
“Interfaces on steroids”
Method implementations
Variables
Everything a class can have – except constructors
Can mix in multiple traits
36

37. Scala implicit classes
“Add” methods or properties to existing types
Without modifying original type
Implicit wrapper object
37

38. 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));
38

39. So far, so good?
Lambdas
Method handles
Extension methods
39

40. Exercise!
git clone https://github.com/fredriv/fp-java8
open Exercise_1_Lambda_Expressions_Test.java
in your favourite editor
mvn test
40

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

42. What’s wrong with using List::sort ?
Modifies existing collection
Others may be using it?
Concurrency issues
Performance
42

43. Streams
© Fredrik Vraalsen 2008

44. The old fashioned way
44
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++;
}
}

45. Streams
45
!
roadData.stream()
.filter(r -> r.getName().contains(nameQuery))
.limit(10)
.collect(Collectors.toList());

46. Streams – pipelines
46
!
roadData.stream()
.filter(r -> r.getName().contains(nameQuery))
.limit(10)
.collect(Collectors.toList());
!
!
cat roadData.txt | grep … | head > output.txt

47. Streams – pipelines
Source
collection, array, generator function, IO channel, ...
Intermediate operations (Stream-producing)
filter, map, ...
Terminal operations (value-producing)
47

48. Examples
48
Java 7:
!
Map<String, List<Article>> relatedArticles = new HashMap<>();
!
for (ArticleTransport transport : articleTransports) {
Article article = convertToArticle(transport);
String category = getCategory(article);
!
List<Article> articles = relatedArticles.get(category);
if (articles == null) {
articles = new ArrayList<>();
relatedArticles.put(category, articles);
}
articles.add(article);
}

49. Examples
49
Java 8:
!
Map<String, List<Article>> relatedArticles =
articleTransports.stream()
.map(transport -> convertToArticle(transport))
.collect(Collectors.groupingBy(article -> getCategory(article)));

50. Examples
50
Java 7:
!
for (Map.Entry<K, List<Article>> e : relatedArticles.entrySet()) {
result.put(e.getKey(), convertToTeasers(e.getValue()));
}

51. Examples
51
Java 7:
!
for (Map.Entry<K, List<Article>> e : relatedArticles.entrySet()) {
result.put(e.getKey(), convertToTeasers(e.getValue()));
}
!
!
Java 8:
!
relatedArticles.forEach((k, v) -> result.put(k, convertToTeasers(v)));

52. Performance
© Fredrik Vraalsen 2012

53. Streams – performance
!
List<String> namesOfAdults = people.stream()
.filter(p -> p.getAge() >= 18)
.map(Person::getName)
.collect(Collectors.toList());
53

54. This one goes to 11!
!
List<String> namesOfAdults = people.parallelStream()
.filter(p -> p.getAge() >= 18)
.map(Person::getName)
.collect(Collectors.toList());
54

55. Streams
java.util.stream
Create new results
Lazy
Parallelizable
55

56. More cool stuff in Java 8
String join
Collection removeIf (≈ filter)
Map getOrDefault, putIfAbsent, replace, forEach, etc.
java.util.Optional
java.util.stream.Collectors
count, sum, average, min, max, groupingBy, etc.
java.nio.file.Files lines
56

57. Exercise!
Exercise_2_Streams_Test.java
57

58. What’s missing?
© Fredrik Vraalsen 2012

59. What’s missing?
Immutability
Value types
Data structures (lists, maps, etc.)
Concurrency
59

60. What’s the big deal?
Functional programming is all about values!
And transformations (functions) computing new values
Parallellism vs. Concurrency
Robustness
Testability
60

61. Some help to be found
Immutable collections
Google Guava, FunctionalJava, clj-ds
Concurrency mechanisms
Akka (Actors, STM)
61

62. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 37),
new Person("Hedda", 2));
62

63. 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));
63

64. 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()));
64

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

66. 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 66

67. 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!
Important things are still missing
67

68. Want to know more?
^{Oslo "Socially Functional Programmers" #OsloSFP}
http://www.meetup.com/Oslo-Socially-Functional/
68

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