@Devoxx 2016 This talk will introduce you to Javaslang, a library that borrows heavily from Scala and applies Scala's ideas to plain old Java. We will cover some of the productivity enhancements and syntactic sugar that Javaslang offers, such as pattern matching aka. structural composition, value types and functional data types. Javaslang can be a building block for functional business code, that is easy to maintain and understand.

1. #Devoxx #Javaslang @koenighotze
JΛVΛSLΛNGFunctional Sugar For Java

2. #Devoxx #Javaslang @koenighotze
Senacor Technologies
Programmer!
That’s me coding Scala
David Schmitz / @koenighotze
Senacor Technologies
Programmer!
That’s me coding Scala

3. #Devoxx #Javaslang @koenighotze
What’s in it for you?
Functional programming is hip
How Javaslang helped us
Examples, Code!

4. #Devoxx #Javaslang @koenighotze
Functors,
applicatives,
monads and friends
stay home

5. #Devoxx #Javaslang @koenighotze
Code that is easier to
reason about

6. #Devoxx #Javaslang @koenighotze
Side-effects are evil
try {
int i = 1/0;
} catch (Throwable t) {
…
}
Exceptions are goto-statements
:(

7. #Devoxx #Javaslang @koenighotze
Referential Transparency
Math.random();
Math.max(1, 2);
Math.random();
Math.max(1, 2);
Pure functions are a Good ThingTm
:(

8. #Devoxx #Javaslang @koenighotze
Thinking in Values
Immutability
Performance
Safety

9. #Devoxx #Javaslang @koenighotze
In a nutshell, think about
“what to code“
not
“how to code”

10. #Devoxx #Javaslang @koenighotze
Enter Java 8
(a) -> a + 2
list.stream().filter…
Excitement…

11. #Devoxx #Javaslang @koenighotze
“Try filtering all invalid users
from a list of users”

12. #Devoxx #Javaslang @koenighotze
Excitement?…until
users.stream()
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

13. #Devoxx #Javaslang @koenighotzePainting by Gustav Courbet

14. #Devoxx #Javaslang @koenighotze
import static javaslang.API.*;
+

15. #Devoxx #Javaslang @koenighotze
What we’ll cover
Immutable collections
Some functional sugar
Pattern matching

16. #Devoxx #Javaslang @koenighotze
Before we get to the details…
Let’s fix that ugly code

17. #Devoxx #Javaslang @koenighotze
Fixing Things….
users.stream()
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

18. #Devoxx #Javaslang @koenighotze
Functional collections
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

19. #Devoxx #Javaslang @koenighotze
No need for Collectors
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}})
.collect(Collectors.toList());

20. #Devoxx #Javaslang @koenighotze
No need for Collectors
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}}); .collect(Collectors.toLis
t());

21. #Devoxx #Javaslang @koenighotze
Wrapping Exceptions
List.ofAll(users)
.filter(user -> {
try {
return user.validate();
} catch (Exception ex) {
return false;
}}); .collect(Collectors.toLis
t());

22. #Devoxx #Javaslang @koenighotze
Wrapping Exceptions
List.ofAll(users)
.filter(user ->
Try.of(user::validateAddress)
.getOrElse(false)
); } catch (IllegalStateException
ex) { return
); .collect(Collectors.toList();

23. #Devoxx #Javaslang @koenighotze
List.ofAll(users)
List.filter(user ->
Try.of(user::validateAddress)
.getOrElse(false));
“Try filtering all invalid users
from a list of users”

24. #Devoxx #Javaslang @koenighotze
Immutable Collections
Image by Douglas Muth, https://flic.kr/p/acFwxG

25. #Devoxx #Javaslang @koenighotze
Mutable Collections are Evil
Returning void == Side-Effect!
interface Collection<E> {
…
void clear();
}
interface Collection<E> {
…
void clear();
}

26. #Devoxx #Javaslang @koenighotze
Easy solution? Collections
Collections
.unmodifiableList(list);
.add("BUMM");

27. #Devoxx #Javaslang @koenighotze
Easy solution? Collections
Collections
.unmodifiableList(list)
.add("BUMM");

28. #Devoxx #Javaslang @koenighotze

29. #Devoxx #Javaslang @koenighotze
Javaslang Collections
List<~> t = List.of(“F95”);
List<~> t2 = t.append(withName(“FCK”));
F95 /t

30. #Devoxx #Javaslang @koenighotze
Immutable
List<~> t = List.of(“F95”);
List<~> t2 = t.prepend(“FCK”);
F95 /t
FCKt2

31. #Devoxx #Javaslang @koenighotze
List<~> t = List.of(“F95”);
List<~> t2 = t.prepend(“FCK”);
F95 /
FCK
t
t2
Persistent and Efficient

32. #Devoxx #Javaslang @koenighotze
Streams are glorified Iterators
Stream<String> jdk
= Stream.of("a", “B");
jdk.map(String::toUpperCase);
jdk.map(String::toLowerCase);

33. #Devoxx #Javaslang @koenighotze
java.lang.IllegalStateException:
stream has already been operated upon
or closed
at java.util.stream.AbstractPipeline.<init>(AbstractPipeline.java:203)
at java.util.stream.ReferencePipeline.<init>(ReferencePipeline.java:94)
at
java.util.stream.ReferencePipeline$StatelessOp.<init>(ReferencePipeline.java
:618)
at java.util.stream.ReferencePipeline$3.<init>(ReferencePipeline.java:187)
at java.util.stream.ReferencePipeline.map(ReferencePipeline.java:186)

34. #Devoxx #Javaslang @koenighotze
Javaslang Streams
Stream<String> slang =
Stream.of("a", "B");
slang.map(String::toUpperCase);
// “A”,“B”
slang.map(String::toLowerCase);
// “a”,“b”

35. #Devoxx #Javaslang @koenighotze
Javaslang Streams
Stream<String> slang =
Stream.of("a", "B");
slang.map(String::toUpperCase)
.take(2);
slang.map(String::toLowerCase)
.take(2);

36. #Devoxx #Javaslang @koenighotze
Javaslang Streams
Stream<String> slang =
Stream.of("a", "B");
slang.map(String::toUpperCase)
.take(2);// “A”,“B”
slang.map(String::toLowerCase)
.take(2);// “a”,“b”

37. #Devoxx #Javaslang @koenighotze
Functional data structures
improve performance and
reduce the chance of
unexpected behaviour

38. #Devoxx #Javaslang @koenighotze
Functional Sugar

39. #Devoxx #Javaslang @koenighotze
“Find a user and, if an
address is available, fetch
the user’s street”

40. #Devoxx #Javaslang @koenighotze
Cascading Pile of Shame
User user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

41. #Devoxx #Javaslang @koenighotze
We’ve got java.util.Optional
Optional<User> opt =
Optional.ofNullable(user);
if (optional.isPresent()) {
…
}

42. #Devoxx #Javaslang @koenighotze

43. #Devoxx #Javaslang @koenighotze
Please stop using
Optional#isPresent()

44. #Devoxx #Javaslang @koenighotze
An Option is like a Gift Box
NoneSome

45. #Devoxx #Javaslang @koenighotze
Map opens the Gift Box
Map
Some Some
( )->

46. #Devoxx #Javaslang @koenighotze
Nothing from Nothing
( )->
Map
None None

47. #Devoxx #Javaslang @koenighotze
Optional or Option?
Optional

48. #Devoxx #Javaslang @koenighotze
Optional or Option?
Option
Some None

49. #Devoxx #Javaslang @koenighotze
Optional or Option?
Option
Some None
Value
Iterable

50. #Devoxx #Javaslang @koenighotze
Optional or Option?
Option
Some None
Value
Iterable

51. #Devoxx #Javaslang @koenighotze
Optional or Option?
Option
Some None
Value
Iterable
Serializable

52. #Devoxx #Javaslang @koenighotze
Fixing the Pile of Shame
User user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

53. #Devoxx #Javaslang @koenighotze
Option all the Things
User user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

54. #Devoxx #Javaslang @koenighotze
Option all the Things
Option<User> user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

55. #Devoxx #Javaslang @koenighotze
Option all the Things
Option<User> user = repo.findOne("id");
if (user != null) {
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

56. #Devoxx #Javaslang @koenighotze
Option all the Things
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
} Option<Address> getAddress()

57. #Devoxx #Javaslang @koenighotze
Option all the Things
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
if (null != address) {
return address.getStreet();
}
}

58. #Devoxx #Javaslang @koenighotze
Option all the Things
Option<User> user = repo.findOne("id");
user.flatMap(User::getAddress)
Address address = user.getAddress();
.map(Address::getStreet)
return address.getStreet();
}
}

59. #Devoxx #Javaslang @koenighotze
repo.findOne("id")
.flatMap(User::getAddress)
.map(Address::getStreet)
.getOrElse("");
“Find a user and, if an address is
available, fetch the user’s street”

60. #Devoxx #Javaslang @koenighotze
Option and map transform
nested code into an
understandable story

61. #Devoxx #Javaslang @koenighotze
Working with legacy code
Painting by Eero Järnefelt

62. #Devoxx #Javaslang @koenighotze
Nice Validation Code
public static String check(String iban){
if (validationMagic(iban)) {
return iban;
}
throw new IllegalArgumentException(“Peng”);
}

63. #Devoxx #Javaslang @koenighotze
Awesome WTF Code
String iban;
try {
iban = check("AL47");
}
catch (IllegalArgumentException ex) {
iban = "";
}

64. #Devoxx #Javaslang @koenighotze
From Exceptions to Options
String iban = lift(Iban::check)
.apply("AL47...")
.getOrElse("");

65. #Devoxx #Javaslang @koenighotze
From Exceptions to Options
String iban = lift(Iban::check)
.apply("AL47...")
.getOrElse("");

66. #Devoxx #Javaslang @koenighotze
Wrapping Exceptions with Try
Try.of(() -> stuffToDo())

67. #Devoxx #Javaslang @koenighotze
Wrapping Exceptions with Try
Failure
Exception
Success
Result
Try.of(() -> stuffToDo())

68. #Devoxx #Javaslang @koenighotze
Exceptions to Options with Try
Try.of(() -> check("AL.."))
.getOrElse("")

69. #Devoxx #Javaslang @koenighotze
Exceptions to Options with Try
Try.of(() -> check("AL.."))
.getOrElse("")

70. #Devoxx #Javaslang @koenighotze
Lifting and Try-ing reduce
exception handling clutter
and side-effects

71. #Devoxx #Javaslang @koenighotze
Structural Decomposition
Image by Arend, https://flic.kr/p/pkBe4g

72. #Devoxx #Javaslang @koenighotze
Classic HTTP Handling
if (OK.equals(res.getStatusCode()))
{
return res.getBody();
}
return emptyList();

73. #Devoxx #Javaslang @koenighotze
(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?
^_`{|}~-]+)*|"(?:[x01-x08x0bx0cx0e-x1fx21x23-
x5bx5d-x7f]|[x01-x09x0bx0cx0e-x7f])*")@(?:(?:
[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9](?:[a-
z0-9-]*[a-z0-9])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9]
[0-9]?).){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-
z0-9-]*[a-z0-9]:(?:[x01-x08x0bx0cx0e-x1fx21-
x5ax53-x7f]|[x01-x09x0bx0cx0e-x7f])+)])
Pattern Matching Basics

74. #Devoxx #Javaslang @koenighotze
Pattern Matching Basics
Match(expression)
.of(cases)

75. #Devoxx #Javaslang @koenighotze
Cases map functions to patterns
Case(pattern, function)

76. #Devoxx #Javaslang @koenighotze
Example Patterns
$() wildcard pattern
$(“foo”) equals pattern
isIn(“a”, “b”) conditional pattern

77. #Devoxx #Javaslang @koenighotze
HTTP Handling Fixed
*Note: some type details missing
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);

78. #Devoxx #Javaslang @koenighotze
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);
HTTP Handling Fixed
*Note: some type details missing
OK or anything else

79. #Devoxx #Javaslang @koenighotze
HTTP Handling Fixed
*Note: some type details missing
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);

80. #Devoxx #Javaslang @koenighotze
HTTP Handling Fixed
*Note: some type details missing
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);

81. #Devoxx #Javaslang @koenighotze
HTTP Handling Fixed
Match(res.getStatusCode())
.of(
Case($(OK), res.getBody()),
Case($(), emptyList())
);
*Note: some type details missing

82. #Devoxx #Javaslang @koenighotze
Matching a Try
public Try<…> fetchFromUrl(…) {
…
}

83. #Devoxx #Javaslang @koenighotze
Matching a Try
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

84. #Devoxx #Javaslang @koenighotze
Matching a Try
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

85. #Devoxx #Javaslang @koenighotze
Matching a Try
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

86. #Devoxx #Javaslang @koenighotze
Presto!
Match(fetchFromUrl(“…”))
.of(
Case(Success($()), identity()),
Case(Failure($()), emptyList())
);

87. #Devoxx #Javaslang @koenighotze
Pattern matching
replaces complex if-
then-else sequences
with clear expressions

88. #Devoxx #Javaslang @koenighotze
HashMap.of("Foo", "Bar",
"Qux", "Baz")
.bimap(String::toLowerCase,
String::toUpperCase)
.get("qux"); // BAZ
Javaslang offers much more

89. #Devoxx #Javaslang @koenighotze
Javaslang offers much more
Tuple.of("Foo", 1)
.map((s, i) ->
Tuple.of(s + "Bar",
i + 5))
// “FooBar”, 6

90. #Devoxx #Javaslang @koenighotze
Javaslang offers much more

91. #Devoxx #Javaslang @koenighotze
So, is this is the mother
of all free lunches?

92. #Devoxx #Javaslang @koenighotze
What are the drawbacks?

93. #Devoxx #Javaslang @koenighotze
What are the drawbacks?
count(Collection-libs)
>
count(Logging-libs)

94. #Devoxx #Javaslang @koenighotze
What are the drawbacks?
Option.of(foo)
.map
.filter
.flatMap

95. #Devoxx #Javaslang @koenighotze
Wrapping up
Slick, stable, consistent API
Object-functional
Complex(?)
Version 3 in 2017

96. #Devoxx #Javaslang @koenighotze
.iohttp://
Do you want to know more?

97. #Devoxx #Javaslang @koenighotze
Thank You!
<david.schmitz@senacor.com>