These are the slides of the talk we made with Stuart Marks at Devoxx Belgium 2018. This first part covers Lambda Expressions and API design with functional interfaces. What is the state of lambda expressions in Java 11? Lambda expressions are the major feature of Java 8, having an impact on most of the API, including the Streams and Collections API. We are now living the Java 11 days; new features have been added and new patterns have emerged. This highly technical Deep Dive session will visit all these patterns, the well-known ones and the new ones, in an interactive hybrid of lecture and laboratory. We present a technique and show how it helps solve a problem. We then present another problem, and give you some time to solve it yourself. Finally, we present a solution, and open for questions, comments, and discussion. Bring your laptop set up with JDK 11 and your favorite IDE, and be prepared to think!

1. #Devoxx
Java Lambda Stream
Master Class – Part 1
@StuartMarks @JosePaumard

2. #LambdaHOL#Devoxx
Master Class?
Did you bring your laptop?

3. #LambdaHOL#Devoxx
Yes you did!
What about:
- 10-15mn of explaination
- 3-5mn of time to solve the exercice
- ~5mn to show a solution

4. #LambdaHOL#Devoxx
No you didn’t!
What about:
- 10-15mn of explaination
- 3-5mn to show a first example
- 3-5mn to show a second example

5. #LambdaHOL#Devoxx
The LambdaHOL
8 test classes, with 80 exercises to complete
- Lambda, functionnal interfaces
- Comparators
- Streams, Collectors
- Some challenges at the end
- Updated regularly
With solutions!

6. #LambdaHOL#Devoxx
The LambdaHOL
You can find it here
https://github.com/stuart-marks/LambdaHOLv2

7. #LambdaHOL#Devoxx
Agenda
1st part: API Design with lambdas and
Functional interfaces
2nd part: Streams, reductions, collectors

8. #LambdaHOL#Devoxx
API Design with lambdas
Chaining lambdas with default methods
- Consumer interface
- Predicate interface
Composing lambdas
- Comparator interface
Partial Application
- Currency converter example

9. #LambdaHOL#Devoxx
Stuart Marks
JDK Core Libraries Developer
Java Plaform Group, Oracle
Twitter: @stuartmarks

10. #LambdaHOL#Devoxx
@JosePaumard
https://www.youtube.com/user/JPaumard
https://www.slideshare.net/jpaumard
https://github.com/JosePaumard

11. #Devoxx #LambdaHOL
Questions?
#LambdaHOL

12. #LambdaHOL#Devoxx
For today
The starting point is here:
https://github.com/JosePaumard/lambda-master-class-part1

13. #Devoxx #LambdaHOL
A Few Words on Functional
Interfaces

14. #LambdaHOL#Devoxx
Functional interfaces
A functional interface is:
▪ An interface
▪ With one abstract method
▪ Methods from Object do not count
▪ May be annotated with @FunctionalInterface

15. #LambdaHOL#Devoxx
Functional interfaces
A functional interface can be implemented
with a lambda!
Comparator<String> comparator = new Comparator<String>() {
public int compareTo(String s1, String s2) {
return Integer.compare(s1.length(), s2.length()) ;
}
}
Comparator<String> comparator =
(String s1, String s2) ->
Integer.compare(s1.length(), s2.length()) ;

16. #Devoxx #LambdaHOL
Consumer

17. #LambdaHOL#Devoxx
1st example
A consumer that clears a string builder
@Test
public void consumer_1() {
Consumer<List<String>> consumer = null; // TODO
List<String> list =
new ArrayList<>(Arrays.asList("a", "b", "c"));
consumer.accept(list);
assertThat(list).isEmpty();
}

18. #LambdaHOL#Devoxx
2nd example
A consumer that calls two Consumers
@Test
public void consumer_2() {
Consumer<List<String>> c1 = list -> list.add("first");
Consumer<List<String>> c2 = list -> list.add("second");
Consumer<List<String>> consumer; // TODO
List<String> list =
new ArrayList<>(Arrays.asList("a", "b", "c"));
consumer.accept(list);
assertThat(list).containsExactly("a", "b", "c", "first", "second");
}

19. #Devoxx #LambdaHOL
Predicate

20. #LambdaHOL#Devoxx
1st example
A predicate that negates another predicate
@Test
public void predicate_1() {
Predicate<String> predicate = s -> s.isEmpty();
Predicate<String> notPredicate; // TODO
assertThat(notPredicate.test("")).isFalse();
assertThat(notPredicate.test("Not empty!")).isTrue();
}

21. #LambdaHOL#Devoxx
2nd example
A predicate that is true if a string is non null
and non empty
@Test
public void predicate_2() {
Predicate<String> p1 = s -> s != null;
Predicate<String> p2 = s -> s.isEmpty();
Predicate<String> p3; // TODO
assertThat(p3.test("")).isFalse();
assertThat(p3.test(null)).isFalse();
assertThat(p3.test("Not empty!")).isTrue();
}

22. #LambdaHOL#Devoxx
3rd example
How to design a xOr() method on Predicate?
@Test
public void predicate_3() {
Predicate<String> p1 = s -> s.length() == 4;
Predicate<String> p2 = s -> s.startsWith("J");
Predicate<String> p3; // TODO
assertThat(p3.test("True")).isTrue();
assertThat(p3.test("Julia")).isTrue();
assertThat(p3.test("Java")).isFalse();
}

23. #Devoxx #LambdaHOL
Comparator

24. #LambdaHOL#Devoxx
A first example
What is this code doing?
Comparator<Person> cmp = new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
return p1.getLastName().compareTo(p2.getLastName());
}
};

25. #LambdaHOL#Devoxx
A first example
What is this code doing?
Comparator<Person> cmp = new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
int cmp = p1.getLastName().compareTo(p2.getLastName());
if (cmp == 0) {
return p1.getFirstName().compareTo(p2.getFirstName());
} else {
return cmp;
}
}
};

26. #LambdaHOL#Devoxx
A first example
What is this code doing?Comparator<Person> cmp = new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
int cmp = p1.getLastName().compareTo(p2.getLastName());
if (cmp == 0) {
cmp = p1.getFirstName().compareTo(p2.getLastName());
if (cmp == 0) {
return Integer.compare(p1.getAge(), p2.getAge());
} else {
return cmp;
}
} else {
return cmp;
}
}
};

27. #LambdaHOL#Devoxx
A first example
What is this code doing?Comparator<Person> cmp = new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
int cmp = p1.getLastName().compareTo(p2.getLastName());
if (cmp == 0) {
cmp = p1.getFirstName().compareTo(p2.getFirstName());
if (cmp == 0) {
return Integer.compare(p1.getAge(), p2.getAge());
} else {
return cmp;
}
} else {
return cmp;
}
}
};

28. #LambdaHOL#Devoxx
A first example
What is this code doing?
Comparator<Person> cmp = Comparator.comparing(Person::getLastName)
.thenComparing(Person::getFirstName)
.thenComparing(Person::getAge);

29. #LambdaHOL#Devoxx
1st example
A comparator that puts null values at the end
Person michael = new Person("Michael", "Jackson", 51);
Person rod = new Person("Rod", "Stewart", 71);
Person paul = new Person("Paul", "McCartney", 74);
Person mick = new Person("Mick", "Jagger", 73);
Person jermaine = new Person("Jermaine", "Jackson", 61);
@Test
public void comparator_1() {
Comparator<Person> cmp = null; // TODO
assertThat(cmp.compare(michael, rod)).isLessThan(0);
assertThat(cmp.compare(paul, paul)).isEqualTo(0);
assertThat(cmp.compare(michael, jermaine)).isGreaterThan(0);
assertThat(cmp.compare(mick, null)).isLessThan(0);
assertThat(cmp.compare(null, mick)).isGreaterThan(0);
}

30. #Devoxx #LambdaHOL
API Design

31. #LambdaHOL#Devoxx
A Currency Converter
Given the following data…
Date=2018/11/05
EUR=1
AUD=1.58125
BRL=4.20735
NOK=9.51682
GBP=0.87749
PLN=4.30044
CAD=1.49181
RUB=75.25596

32. #LambdaHOL#Devoxx
A Currency Converter
… write the following currency converter:
LocalDate date = ...;
CurrencyConverter plnToAud =
CurrencyConverter.of(date)
.from("PLN")
.to("AUD");
float pln = ...;
float aud = plnToAud.convert(pln);

33. #LambdaHOL#Devoxx
A Validator
Given the following bean
public class Person {
private String lastName ;
private int age ;
// getters, setters, constructeurs
}

34. #LambdaHOL#Devoxx
A Validator
Implement the following pattern
If the user is not valid: get() throws an exception, with all
the exceptions of each validator in the suppressed
exceptions
Validator<Person> validator = Validator.
.firstValidate(p -> p.getName() == null, "name is null")
.thenValidate(p -> p.getAge() < 0, "age is negative")
.thenValidate(p -> p.getAge() > 150, "age is greater than
150");
person = validator.validate(person).get();

35. #LambdaHOL#Devoxx
A Validator
If the user is not valid: get() throws an exception, with all
the exceptions of each validator in the suppressed
exceptions
Validator<Person> validator =
Validator
.firstValidate(p -> p.getName() == null, "name is null")
.thenValidate(p -> p.getAge() < 0, "age is negative")
.thenValidate(p -> p.getAge() > 150, "age is greater than 150");
person = validator.validate(person).get();

36. #Devoxx
Don’t miss the 2nd day:
1) More functional
programming!
2) Stream, reduction
3) Collectors
Day break!

37. #Devoxx
Java Lambda Stream
Master Class – Part 2
@StuartMarks @JosePaumard

38. #Devoxx #LambdaHOL
Map Filter Reduce

39. #Devoxx #LambdaHOL
Advanced Reduction

40. #Devoxx
Come back for the last part!
1) Collectors
2) Challenges
Coffee break!

41. #Devoxx #LambdaHOL
Collectors

42. #Devoxx #LambdaHOL
Challenges

43. #Devoxx
Questions?
@StuartMarks @JosePaumard
#LambdaHOL
https://github.com/JosePaumard/lambda-master-class-part1
https://github.com/JosePaumard/lambda-master-class-part2