Functional programming is all the rage. It can undoubtedly produce cleaner and more expressive code, but why switch to Scala or wait for Java 8? In fact, there are many ways to dramatically improve your Java code quality by using a more functional style today. This presentation discusses how techniques such as functional programming, using fluent APIs, and other clean coding practices can make your code more concise, more readable, and much easier to maintain. But more importantly, you will learn about libraries such as LambdaJ and Google Guava that make this possible today with your existing Java code base.

1. Improve Your Java Code, Functional-Style - Now!
John
Ferguson
Smart
john.smart@wakaleo.com
h2p://www.wakaleo.com
Twi2er:
wakaleo

2. So who is this guy, anyway?
Consulta
nt
Trainer
Mentor
Author
Speaker
Coder
John Fer
guson S
mar t

3. f(x,y) = x + y
Func:onal
Programming
is
the
new
black

4. Immutability
is
your
friend

5. Avoid
nasty
side-­‐effects

6. Benefit
from
concurrency

7. No
more
messy
loops

8. How
What

9. Func:onal
Programming
in
Java
Coming
to
a
JVM
near
you
the
summer
of
2013

10. Java
8
supports
Lambda
Expressions
List<String> names
= Arrays.asList("Aristotle", "Plato", "Socrates", "Pythagoras");
List<String> filteredNames = names
.filter(name -> name.startsWith("P"))
.into(new ArrayList<String>());
assertThat(filteredNames, contains("Plato", "Pythagoras"));

11. Java
8
supports
Lambda
Expressions
["Aristotle", "Plato", "Socrates", "Pythagoras"]
names.filter(name -> name.startsWith("P"))
.into(new ArrayList<String>());
["Plato", "Pythagoras"]

12. Java
8
supports
Lambda
Expressions
public class Pet {
private String name;
private Species species;
Pet(String name, Species species) {...}
public String getName() {...}
public Species getSpecies() {...}
@Override
public boolean equals(Object o) {...}
@Override
public int hashCode() {...}
enum Species {
Cat,
Dog,
GuineaPig;
Pet called(String name) { return new Pet(name, this); }
}
}

13. Java
8
supports
Lambda
Expressions
public class Pet {
List<Pet> private String name;
pets = Arrays.asList(Cat.called("Ginger"),
private Species species;
Dog.called("Spot"),
GuineaPig.called("Fluffy"),
Pet(String name, Species species) {...}
Dog.called("Rover"));
public String getName() {...}
List<Pet> public Species getSpecies()-> a.getSpecies() == Dog)
sortedDogs = pets.filter(a {...}
.sorted((a, b) -> a.getName().compareTo(b.getName()))
.into(new ArrayList<Pet>());
@Override
public boolean equals(Object o) {...}
assertThat(sortedDogs, contains(Dog.called("Rover"),
@Override Dog.called("Spot")));
public int hashCode() {...}
enum Species {
Cat,
Dog,
GuineaPig;
Pet called(String name) { return new Pet(name, this); }
}
}

14. Java
8
supports
Lambda
Expressions
[Ginger, Spot, Fluffy, Rover]
pets.filter(a -> a.getSpecies() == Dog)
.sorted((a, b) -> a.getName().compareTo(b.getName()))
.into(new ArrayList<Pet>());
[Rover, Spot]

15. Java
8
supports
Lambda
Expressions
[Ginger, Spot, Fluffy, Rover]
pets.filter(a -> a.getSpecies() == Dog)
.map(e -> { return e.getName(); })
.sorted((a, b) -> a.compareTo(b))
.into(new ArrayList<Pet>());
["Rover", "Spot"]

16. Java
8
supports
Lambda
Expressions
[Ginger, Spot, Fluffy, Rover]
Predicate<Pet> carnivores = (pet) -> (pet.getSpecies() == Dog || pet.getSpecies() == Cat);
List<Pet> carnivorousPets = pets.filter(carnivores).into(new ArrayList<Pet>());
["Ginger", "Spot", "Rover"]

17. Java
8
supports
Lambda
Expressions
[Ginger, Spot, Fluffy, Rover]
pets.filter((pet) -> (pet.getSpecies() == Dog || pet.getSpecies() == Cat))
.into(new ArrayList<Pet>());
["Ginger", "Spot", "Rover"]

18. Java
8
supports
Lambda
Expressions
public class VetStay {
private Pet pet;
private Date startOfStay;
private String diagnosis;
public VetStay(Pet pet, Date startOfStay, String diagnosis) {
this.pet = pet;
this.startOfStay = startOfStay;
this.diagnosis = diagnosis;
}
}
List<VetStay> vetStays = pets.map(pet -> {
return new VetStay(pet, new Date(), "sore paw");})
.into(new ArrayList<VetStay>());

19. Java
8
supports
Lambda
Expressions
List<Pet> pets = Arrays.asList(Cat.called("Ginger"),
Dog.called("Spot"),
GuineaPig.called("Fluffy"),
Dog.called("Rover")
);
assert pets.anyMatch(pet -> pet.getSpecies() == Dog)

20. But
Func:onal
Programming
in
Java
today?
Surely
this
is
madness!

21. Google Guava

22. Immutable
Collec:ons
Defensive
Thread-safe
Efficient

23. Immutable
Collec:ons
Creating an immutable list
List<String> colors = ImmutableList.of("red", "green", "blue");
Set<String> colors = ImmutableSet. of("red", "green", "blue");
Set<String> myColors = ImmutableSet.copyOf(colors);
Creating an immutable copy

24. No
more
returning
Null
“Null sucks.”
- Doug Lea
“I call it my billion-dollar mistake.”
- Sir Tony Hoare

25. No
more
returning
Null
What does a null return value mean?
interface ClientService {
Client findByName(String name);
}
No matching client found (but we were expecting one)?
No matching client found (but we’re cool with that)?
Something went wrong?

26. No
more
returning
Null
Sometimes we might not return a client
interface ClientService {
Optional<Client> findByName(String name);
}
This forces us to cater for this case in our code
Optional<Client> clientOptional = clientService.findByName("Smith");
if (clientOptional.isPresent()) {
Client client = clientOptional.get();
} else {
// No client was found
}

27. No
more
returning
Null
A person may not have a favorite color
class Person {
Optional<Color> getFavoriteColor();
}
Color colorToUse = person.getFavoriteColor().or(Blue)
If not, use Blue

28. And
lots
of
other
stuff...

29. No
more
returning
Null
What does a null return value mean?
interface ClientService {
Client findByName(String name);
}
No matching client found (but we were expecting one)?
No matching client found (but we’re cool with that)?
Something went wrong?

30. No
more
returning
Null
Sometimes we might not return a client
interface ClientService {
Optional<Client> findByName(String name);
}
This forces us to cater for this case in our code
Optional<Client> clientOptional = clientService.findByName("Smith");
if (clientOptional.isPresent()) {
Client client = clientOptional.get();
} else {
// No client was found
}

31. No
more
returning
Null
A person may not have a favorite color
class Person {
Optional<Color> getFavoriteColor();
}
Color colorToUse = person.getFavoriteColor().or(Blue)
If not, use Blue

32. lambdaj
– DSL for manipulating collections in a functional style
– Replace loops with more concise and readable code

33. LambdaJ support many high level
collection-related functions
filter aggregate
sort
convert
extract group

34. Filtering
in
LambdaJ
import static ch.lambdaj.Lambda.filter;
import static org.hamcrest.Matchers.startsWith;
.
.
.
List<String> names
= Arrays.asList("Aristotle", "Plato", "Socrates", "Pythagoras");
List<String> filteredNames = filter(startsWith("P"), names);
assertThat(filteredNames, contains("Plato", "Pythagoras"));
LambdaJ
Hamcrest

35. Filtering
in
LambdaJ
["Aristotle", "Plato", "Socrates", "Pythagoras"]
filter(startsWith("P"), names)
["Plato", "Pythagoras"]

36. Filtering
in
LambdaJ
List<String> filteredNames = new ArrayList<>();
for(String name : names) {
if (name.startsWith("P")) {
filteredNames.add(name);
}
}
Old-style Java
names.filter(name -> name.startsWith("P"))
.into(new ArrayList<String>());
Java 8
filter(startsWith("P"), names)
Lambda J

37. Filtering
in
LambdaJ
List<Pet> pets = Arrays.asList(Cat.called("Ginger"),
Dog.called("Spot"),
GuineaPig.called("Fluffy"),
Dog.called("Rover"));
List<Pet> sortedDogs = filter(having(on(Pet.class).getSpecies(),
is(Dog)), pets);
assertThat(sortedDogs, contains(Dog.called("Rover"), Dog.called("Spot")));

38. Filtering
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
filter(having(on(Pet.class).getSpecies(), is(Dog)), pets);
[Rover, Spot]

39. Sor:ng
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
sort(sortedDogs, on(Pet.class).getName());
[Fluffy, Ginger, Rover, Spot]

40. Filtering
and
Sor:ng
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
List<Pet> filteredDogs = filter(having(on(Pet.class).getSpecies(), is(Dog)), pets);
List<Pet> sortedDogs = sort(filteredDogs, on(Pet.class).getName());
[Rover, Spot]

41. Extrac:ng
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
pets.filter(a -> a.getSpecies() == Dog)
.map(e -> { return e.getName(); })
.sorted((a, b) -> a.compareTo(b))
.into(new ArrayList<Pet>());
Extracting with Java 8
["Rover", "Spot"]

42. Extrac:ng
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
List<Pet> dogs = filter(having(on(Pet.class).getSpecies(), is(Dog)), pets);
List<String> dogNames = extract(dogs, on(Pet.class).getName());
List<String> sortedDogNames = sort(dogNames,on(String.class));
Extracting with
LambdaJ
["Rover", "Spot"]

43. Extrac:ng
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
List<Pet> filteredDogs = new ArrayList();
for(Pet pet : pets) {
if (pet.getSpecies() == Dog) {
filteredDogs.add(pet);
}
}
Collections.sort(filteredDogs, new Comparator<Pet>() {
@Override
public int compare(Pet pet, Pet pet1) {
return pet.getName().compareTo(pet1.getName());
}
});
Extracting with old-style
["Rover", "Spot"] Java

44. Extrac:ng
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
List<Pet> dogs = filter(having(on(Pet.class).getSpecies(), is(Dog)), pets);
List<String> dogNames = extract(dogs, on(Pet.class).getName());
List<String> sortedDogNames = sort(dogNames,on(String.class));
Extracting with
LambdaJ
["Rover", "Spot"]

45. Conver:ng
in
LambdaJ
List<VetStay> vetStays = pets.map(pet -> {
return new VetStay(pet, new Date(), "sore paw");})
.into(new ArrayList<VetStay>());
Converting with Java 8
convert(pets, toVetStay());
private Converter<Pet, VetStay> toVetStay() {
return new Converter<Pet, VetStay>() {
@Override
public VetStay convert(Pet pet) {
return new VetStay(pet, new Date(), "sore paw");
}
};
}
Converting with LambdaJ

46. Grouping
in
LambdaJ
[Ginger, Spot, Fluffy, Rover]
Group<Pet> petsBySpecies = group(pets,
by(on(Pet.class).getSpecies()));
List<Pet> dogs = petsBySpecies.find(Dog);
[Spot,Rover]

47. FlaOening
in
LambdaJ
[[Spot, Rover], [Ginger], [Fluffy]]
List<List<Pet>> petsBySpecies = ...
List<Pet> allPets = flatten(petsBySpecies);
[Spot, Rover, Ginger, Fluffy]

48. Checking
existence
in
LambdaJ
[Spot, Rover, Ginger, Fluffy]
exists(pets, having(on(Pet.class).getSpecies(), is(Cat)))
true

49. Using
Predicates
in
LambdaJ
Predicate<Pet> carnivores = (pet) -> (pet.getSpecies() == Dog || pet.getSpecies() == Cat);
List<Pet> carnivorousPets = pets.filter(carnivores).into(new ArrayList<Pet>());
Java 8
Matcher<Pet> carnivore = new Predicate<Pet>() {
public boolean apply(Pet pet) {
return (pet.getSpecies() == Dog || pet.getSpecies() == Cat);
}
};
List<Pet> carnivores = filter(carnivore, pets);
LambdaJ

50. Using
Aggregates
in
LambdaJ
List<Pet> pets = Arrays.asList(Cat.called("Ginger"),
Dog.called("Spot"),
GuineaPig.called("Fluffy"),
Dog.called("Rover")
);
int ageOfOldestPet = maxFrom(pets).getAge();
int ageOfYoungestPet = minFrom(pets).getAge();
int totalPetAges = sumFrom(pets).getAge();

51. Some
real-­‐world
examples
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS TestOutcome
3
TestOutcome
4
FAILURE
PENDING
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS

52. Some
real-­‐world
examples
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS TestOutcome
3
TestOutcome
4
FAILURE
PENDING
filter(having(on(TestOutcome.class).getResult(), is(SUCCESS)), outcomes);
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS

53. Some
real-­‐world
examples
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS TestOutcome
3
TestOutcome
4
FAILURE
PENDING
filter(withResult(SUCCESS)), outcomes);
private Matcher<?> withResult(TestResult expectedResult) {
return having(on(TestOutcome.class).getResult(), is(expectedResult));
}
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS

54. Some
real-­‐world
examples
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS TestOutcome
3
TestOutcome
4
FAILURE
PENDING
filter(anyOf(withResult(SUCCESS), withResult(PENDING)), outcomes);
TestOutcome
1
SUCCESSTestOutcome
2
SUCCESSTestOutcome
4
PENDING

55. Some
real-­‐world
examples
TestOutcome
1
SUCCESS TestOutcome
2
SUCCESS TestOutcome
3
TestOutcome
4
FAILURE
PENDING
TestOutcome
2
Step
1
Step
2
Step
3
Step
4

56. Some
real-­‐world
examples
Tag
Tag 1
TagProvider1 Tag
TagProvider2 Tag
Tag
Tag
Tag
flatten(extract(TagProviders, on(TagProvider.class).getTags()));
Tag
Tag 1
Tag
Tag
Tag
Tag
Tag

57. And
performance
in
all
that?

58. Keeping
things
maintainable
“Excessive use of Guava's functional programming idioms can
lead to verbose, confusing, unreadable, and inefficient code.”
- the Guava team
Can also be true of LambdaJ

59. Keeping
things
maintainable
RULE 1
Use a functional style when it makes the intent more readable

60. Keeping
things
maintainable
sort(extract(filter(having(on(Pet.class).getSpecies(), is(Dog)), pets),
on(Pet.class).getName()), on(String.class));
List<Pet> dogs = filter(having(on(Pet.class).getSpecies(), is(Dog)), pets);
List<String> dogNames = extract(dogs, on(Pet.class).getName());
List<String> sortedDogNames = sort(dogNames, on(String.class));

61. Keeping
things
maintainable
RULE 2
One-liners are not always better

62. Keeping
things
maintainable
convert(pets,
new Converter<Pet, VetStay>() {
@Override
public VetStay convert(Pet pet) {
return new VetStay(pet, new Date(), "sore paw");
}
}
);
convert(pets, toVetStay());
private Converter<Pet, VetStay> toVetStay() {
return new Converter<Pet, VetStay>() {
@Override
public VetStay convert(Pet pet) {
return new VetStay(pet, new Date(), "sore paw");
}
};
}

63. Keeping
things
maintainable
RULE 3
Write your own domain-specific matchers

64. Keeping
things
maintainable
private Matcher<?> withResult(TestResult expectedResult) {
return having(on(TestOutcome.class).getResult(), is(expectedResult));
}
filter(withResult(SUCCESS)), outcomes);
select(not(withResult(SUCCESS))), outcomes);
select(anyOf(withResult(SKIPPED), withResult(IGNORED)), outcomes);

65. Thank You
John
Ferguson
Smart
john.smart@wakaleo.com
h2p://www.wakaleo.com
Twi2er:
wakaleo