This document provides an overview of functional programming concepts in Java including lambda expressions, functional interfaces like Function and Consumer, streams, and the Optional API. It discusses how these features allow expressing operations like mapping, filtering, reducing in a more declarative way. The document also mentions topics for further exploration like currying, monads, dependency injection and the future of design patterns with functional programming.

1. Yakhya DABO
Software craftsman && DevOps Engineer
@yakhyadabo
(Birmingham, United Kingdom)

2. Work & Interests

3. Continuous Delivery
Work and Interests

4. Software craftsmanship
Work and Interests

5. Don't just code Monkey (M. Fowler)
→ Knowledge
→ Responsibility
→ Impact
Work and Interests

6. Devoxx France
Paris Jug
Socrates Germany
London Software craftsmanship
Communities
Work and Interests

7. Functional Programming in Java 8

8. Avoid technical debt
Java 7 is no longer supported
Java 9 coming soon

9. A programming paradigm that ...
● Treats function as primitive
● Avoids state and mutable data
● Is declarative
Definition of Functional Programming

10. To be a function
function succ(int y){
return y++;
}
Definition

11. int x = 1;
function dummy(int y){
x++;
return x+y;
}
Not to be a function
Definition

12. int x = 1;
function dummy(int y){
x++;
return x+y;
}
Not to be a function
Definition

13. Functions as First-Class Citizens
- High-Order Functions
- Lambda
- Top-Level Functions
Immutable data
(Hadi Hariri Devoxx France 2015)
Functional Language

14. Write less and expressive code
Why do we need it ?

15. The Context

16. “Software product is embedded in a cultural matrix of
applications, users, laws, and machines vehicles.
These all change continually, and their changes inexorably
force change upon the software product.”
Frederic brooks
The context

17. Every 18 months a CPU's transistors will
shrink in size by a factor of two.
Moor Law
The context

18. - In 2002, limitations in a CPU's circuitry.
- The multi-core processor was born.
Moor Law (New Context)
The context

19. Lambda project

20. “Reduce the gap between the serial and parallel
versions of the same computation”
Brian Goetz (Lambda project leader)
The context

21. public List<Output> processInputs(List<Input> inputs)
throws InterruptedException, ExecutionException {
int threads = Runtime.getRuntime().availableProcessors();
ExecutorService service = Executors.newFixedThreadPool(threads);
List<Future<Output>> futures = new ArrayList<Future<Output>>();
for (final Input input : inputs) {
Callable<Output> callable = new Callable<Output>() {
public Output call() throws Exception {
Output output = new Output();
// process your input here and compute the output
return output;
}
};
futures.add(service.submit(callable));
}
service.shutdown();
List<Output> outputs = new ArrayList<Output>();
for (Future<Output> future : futures) {
outputs.add(future.get());
}
return outputs;
}
Pallelizing with OOP
The context

22. Output out = input.process(treatment);
Output out = Input.processInParallel(treatment);
Pallelizing with FP
The context

23. => Lambda expression
=> Functional Interface
=> Optional API
=> Stream API
….
Java 8 main new features

24. Lambda expressions

25. (argument_list) -> function_body
Lambda expression

26. x -> x + 1 ;
(int x, int y) -> x + y ;
() -> System.out.println("I am a lambda");
Lambda expressions
Lambda expressions

27. X= (int x) -> x + 1 ;
(int x, int y) -> {
z = x + y ;
z++;
}
Lambda expression
Lambda expressions

28. Functional Interface

29. Provide target types for lambda expressions ...
Has a single abstract method, ..., to which the
lambda expression's parameter and return types are
matched or adapted.
Functional Interface

30. int x -> x * 2;
public interface IntOperation {
int operate(int i);
}
Functional Interface

31. - Function
- Consumer
- Supplier
Functional Interface
Common Functional Interfaces

32. Functions accept one argument and produce a result.
@FunctionalInterface
Interface Function<T,R>{
R apply(T t);
}
Functional Interface
Function

33. // String to an integer
Function<String, Integer> stringToInt = x -> Integer.valueOf(x);
int toto = stringToInt(“10”);
Functional Interface
Function

34. Suppliers produce a result of a given generic type.
Unlike Functions, Suppliers don't accept arguments.
@FunctionalInterface
Interface Supplier<T>{
T get()
}
Functional Interface
Supplier

35. Supplier<Person> personSupplier = () -> new Person();
Person persion = personSupplier.get();
Functional Interface
Supplier

36. Consumers represents operations to be performed on a single
input argument.
@FunctionalInterface
public interface Consumer<T>{
void accept(T t)
}
Functional Interface
Consumer

37. Consumer<Person> personPrinter =
(p) -> System.out.println("Hello, " + p.firstName);
personPrinter.accept(new Person("Luke", "Skywalker"));
Functional Interface
Consumer

38. Predicates are boolean-valued functions of one
argument.
@FunctionalInterface
Interface Predicate<T>{
boolean test(T t)
}
Functional Interface
Predicate

39. Predicate<String> isNotEmpty = s -> s.isEmpty();
Predicate<String> isNotEmpty = isEmpty.negate();
Functional Interface
Predicate

40. Type Inference

41. What is the type of [x -> 2 * x] ?
public interface IntOperation {
int operate(int i);
}
public interface DoubleOperation {
double operate(double i);
}
Type Inference
Type Inference

42. Java does have type inferencing when using generics.
public <T> T foo(T t) {
return t;
}
Type Inference
Type Inference

43. ● DoubleOperation doubleOp = x -> x * 2;
● IntOperation intOp = x -> x * 2;
Inference by declaration
Type Inference

44. ● DoubleOperation doubleOp ;
doubleOp = x -> x * 2;
● IntOperation intOp ;
intOp = x -> x * 2;
Inference by affectation
Type InferenceType Inference

45. public Runnable toDoLater() {
return () -> System.out.println("later");
}
Inference by return type
Type Inference

46. Object runnable = (Runnable) () ->
{ System.out.println("Hello"); };
Object callable = (Callable) () →
{ System.out.println("Hello"); };
Inference by cast
Type Inference

47. Stream API

48. Collections support operations that work on a
single element :
add(), remove() and contains()
Stream API

49. Streams have bulk operations that access all
elements in a sequence.
forEach(), filter(), map(), and reduce()
Stream API

50. Three central concepts of functional
programming
Map/Filter/Reduce
Stream API

51. map(f, [d0, d1, d2...]) -> [f(d0), f(d1), f(d2)...]
Map
Stream API

52. Legacy
List<String> names = …
List<Person> people = ...
for (Person person : people){
names.add(person.getName());
}
Map
Stream API

53. List<Person> people = ...
List<String> names = people.streams()
.map(person -> person.getName())
.collect(Collectors.toList());
Functional Style
Map
Stream API

54. filter(is_even, [1, 2, 7, -4, 3, 12.0001]) -> [2, -4]
Filter
Stream API

55. List<Person> people = …
List<Person> peopleOver50 = ...
for (Person person : people){
if(person.getAge() >= 50){
peopleOver50.add(person.getName());
}
}
Legacy
Filter
Stream API

56. List<Person> people = …
List<String> peopleOver50 = people.streams()
.filter(person -> person.getAge()>50)
.collect(Collectors.toList());
Functional Style
Filter
Stream API

57. reduce(add, [1, 2, 4, 8]) -> 15
Reduce
Stream API

58. List<Integer> salaries = …
int averageSalaries = ...
for (Integer salary : salaries){
…
}
Legacy
Stream API
Reduce

59. List<Integer> salaries = …
Int averageAge = salaries.streams()
.average();
Functional Style
Stream API
Reduce

60. int sum = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.mapToInt(s -> s.getWeight())
.sum();
Stream API
Pipeline Structure

61. Optional API
Forgetting to check for null references is a
common source of bugs in application code. One
way to eliminate NullPointerExceptions is to
make sure that methods always return a non null
value.

62. Example
User user = userRepository.getUser(“id”);
If (user != null){
user.doSomeThing();
}
Optional API

63. With Optional
Optional<User> user = userRepository.getUser(“id”);
If (user.isPresent()){
user.doSomeThing();
}
Optional API

64. Optional API
Functional style
Optional<User> user = userRepository.getUser(“id”);
user.ifPresent(u - > u.doSomeThing());

65. Optional API
Functional style
Optional<User> user = userRepository.getUser(“id”);
user.ifPresent(u - > u.doSomeThing());
userRepository.getUser(“id”)
.ifPresent(user - > user.doSomeThing());

66. Optional<SomeObject> opt = Optional.empty();
Optional<SomeObject> opt = Optional.of(notNull);
Optional<SomeObject> opt = Optional.ofNullable(mayBeNull);
Declaration
Optional API

67. Common Optional patterns
Optional API

68. if (x != null && x.contains("ab")) {
print(x);
}
Optional API
If (condition) doSomething

69. if (x != null && x.contains("ab")) {
print(x);
}
Optional API
If (condition) doSomething
opt
.filter(x -> x.contains("ab"))
.ifPresent(x -> print(x));

70. Optional API
If (condition) return else return
if (x !=null){
return x.length();
} else {
return -1;
}

71. Optional API
If (condition) return else return
opt
.map(x -> x.length() )
.orElse(-1);
if (x !=null){
return x.length();
} else {
return -1;
}

72. if (s != null && !s.isEmpty()){
return s.charAt(0);
else
throw new IllegalArgumentException();
}
Optional API
Throwing Exception

73. if (s != null && !s.isEmpty()){
return s.charAt(0);
else
throw new IllegalArgumentException();
}
Optional API
Throwing Exception
opt
.filter(s -> ! s.isEmpty()).
.map(s -> s.charAt(0)).
.orElseThrow(IllegalArgumentException::new);

74. if (x !=null){
print(x.length());
} else {
Print(“-1”);
}
Optional API
If (condition) do else … do

75. if (x !=null){
print(x.length());
} else {
Print(“-1”);
}
Optional API
If (condition) do else … do
???

76. Avoid having states
Avoid using for, while
Think twice before using if
Summary

77. Curryfication, partial functions
Dependency injection
Monad ...
Next step ...

78. Function<Integer,Function<Integer,Integer>> sum =
x -> y -> x + y;
Function<Integer, Integer> plus10 = sum.apply(10);
Integer res = plus10.apply(5);
… Next step
Curryfication

79. ...
… Next step
Monad

80. ...
… Next step
Dependency injection

81. ...
… Next step
Future of GoF

82. Futher readings
Devoxx
France 2015 : Hadi Hariri (Refactoring to functional)
UK 2014 : Raoul Gabriel Urma && Richard Warburton Pragmatic
Functional Refactoring with Java 8
Books
● Functional programming for Java developers
● Java SE8 for the Really Impatient
● Java 8 in action