How to migrate from java to Kotlin

1. @PaulienVanAlst#Devoxx #Kotlin
From Java to Kotlin
The adventures of a smooth migration
Paulien van Alst
Open Value

2. @PaulienVanAlst#Devoxx #Kotlin
Who am I?

3. @PaulienVanAlst#Devoxx #Kotlin
So… what is Kotlin
Developed by Jetbrains
Adopted by Google for Android
Development

4. @PaulienVanAlst#Devoxx #Kotlin
So… what is Kotlin
J V M
J S
M u l t i p l a t f o r m

5. @PaulienVanAlst#Devoxx #Kotlin
So… what is Kotlin
J V M
J S
M u l t i p l a t f o r m

6. @PaulienVanAlst#Devoxx #Kotlin
So… what is Kotlin
Expressiveness
Reuse
Interoperability
Safety

7. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin

8. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin
boardGameRepository.findAllByCategory(ADVENTURE).stream()
.filter(game -> game.getMinimalNumberOfPlayers() > 2)
.filter(game -> game.getMinimalAge() > 10)
.map(BoardGameEntity::getPrice)
.reduce(BigDecimal.ZERO, BigDecimal::add);

9. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin
boardGameRepository.findAllByCategory(ADVENTURE).stream()
.filter(game -> game.getMinimalNumberOfPlayers() > 2)
.filter(game -> game.getMinimalAge() > 10)
.map(BoardGameEntity::getPrice)
.reduce(BigDecimal.ZERO, BigDecimal::add);
boardGameRepository.findAllByCategory(ADVENTURE)
.moreThanTwoPlayers()
.olderThanTenToPlay()
.meanPrice()

10. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin
boardGameRepository.findAllByCategory(ADVENTURE).stream()
.filter(game -> game.getMinimalNumberOfPlayers() > 2)
.filter(game -> game.getMinimalAge() > 10)
.map(BoardGameEntity::getPrice)
.reduce(BigDecimal.ZERO, BigDecimal::add);
boardGameRepository.findAllByCategory(ADVENTURE)
.moreThanTwoPlayers()
.olderThanTenToPlay()
.meanPrice()

11. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin
boardGameRepository.findAllByCategory(ADVENTURE).stream()
.filter(game -> game.getMinimalNumberOfPlayers() > 2)
.filter(game -> game.getMinimalAge() > 10)
.map(BoardGameEntity::getPrice)
.reduce(BigDecimal.ZERO, BigDecimal::add);
boardGameRepository.findAllByCategory(ADVENTURE)
.moreThanTwoPlayers()
.olderThanTenToPlay()
.meanPrice()

12. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin
boardGameRepository.findAllByCategory(ADVENTURE).stream()
.filter(game -> game.getMinimalNumberOfPlayers() > 2)
.filter(game -> game.getMinimalAge() > 10)
.map(BoardGameEntity::getPrice)
.reduce(BigDecimal.ZERO, BigDecimal::add);
boardGameRepository.findAllByCategory(ADVENTURE)
.moreThanTwoPlayers()
.olderThanTenToPlay()
.meanPrice()

13. @PaulienVanAlst#Devoxx #Kotlin
My story with Kotlin
boardGameRepository.findAllByCategory(ADVENTURE).stream()
.filter(game -> game.getMinimalNumberOfPlayers() > 2)
.filter(game -> game.getMinimalAge() > 10)
.map(BoardGameEntity::getPrice)
.reduce(BigDecimal.ZERO, BigDecimal::add);
boardGameRepository.findAllByCategory(ADVENTURE)
.moreThanTwoPlayers()
.olderThanTenToPlay()
.meanPrice()

14. @PaulienVanAlst#Devoxx #Kotlin
Pro’s to switch to Kotlin
Regular updates
Interoperability
Multiplatform

15. @PaulienVanAlst#Devoxx #Kotlin
Pro’s to switch to Kotlin
Null safety Immutability

16. @PaulienVanAlst#Devoxx #Kotlin
Pro’s to switch to Kotlin
Null safety Immutability
Operator overloading

17. @PaulienVanAlst#Devoxx #Kotlin
Pro’s to switch to Kotlin
Null safety Immutability
Operator overloading
Extension functions

18. @PaulienVanAlst#Devoxx #Kotlin
Pro’s to switch to Kotlin
Null safety Immutability
Operator overloading
Extension functions
Coroutines

19. @PaulienVanAlst#Devoxx #Kotlin
What to keep in mind
Interoperability
Idioms
Magic problem solver

20. @PaulienVanAlst#Devoxx #Kotlin
What to keep in mind
Interoperability
Idioms
Magic problem solver

21. @PaulienVanAlst#Devoxx #Kotlin
Warning!
DO this at home
or
In a POC

22. @PaulienVanAlst#Devoxx #Kotlin
Let’s start!
Rates 6 8
Games above … 8 9 9,5

23. @PaulienVanAlst#Devoxx #Kotlin
Let’s start!
Spring boot 2
In memory database
UT’s and IT’s

24. @PaulienVanAlst#Devoxx #Kotlin
Let’s start!
Code!

25. @PaulienVanAlst#Devoxx #Kotlin
Migration Plan
1. Set-up dependencies
2. Migrate PoJo’s
3. Rewrite unit tests
4. Business logic / integration tests
5. Configuration

26. @PaulienVanAlst#Devoxx #Kotlin
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib-jdk8</artifactId>
<version>${kotlin.version</version>
</dependency>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-test-junit</artifactId>
<version>${kotlin.version</version>
<scope>test</scope>
</dependency>
Set-up dependencies

27. @PaulienVanAlst#Devoxx #Kotlin
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib-jdk8</artifactId>
<version>${kotlin.version</version>
</dependency>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-test-junit</artifactId>
<version>${kotlin.version</version>
<scope>test</scope>
</dependency>
Set-up dependencies

28. @PaulienVanAlst#Devoxx #Kotlin
<plugin>
<groupId/>
<artifactId>kotlin-maven-plugin</artifactId>
<configuration><jvmTarget>1.8</jvmTarget></configuration>
<executions>
<execution><id> compile </id> </execution>
<execution><id> test-compile </id> </execution>
</executions>
</plugin>
Set-up dependencies

29. @PaulienVanAlst#Devoxx #Kotlin
<plugin>
<groupId/>
<artifactId>kotlin-maven-plugin</artifactId>
<configuration><jvmTarget>1.8</jvmTarget></configuration>
<executions>
<execution><id> compile </id> </execution>
<execution><id> test-compile </id> </execution>
</executions>
</plugin>
Set-up dependencies

30. @PaulienVanAlst#Devoxx #Kotlin
<plugin>
<groupId/>
<artifactId>kotlin-maven-plugin</artifactId>
<configuration><jvmTarget>1.8</jvmTarget></configuration>
<executions>
<execution><id> compile </id> </execution>
<execution><id> test-compile </id> </execution>
</executions>
</plugin>
Set-up dependencies

31. @PaulienVanAlst#Devoxx #Kotlin
<plugin>
<groupId/>
<artifactId>kotlin-maven-plugin</artifactId>
<configuration><jvmTarget>1.8</jvmTarget></configuration>
<executions>
<execution><id> compile </id> </execution>
<execution><id> test-compile </id> </execution>
</executions>
</plugin>
Set-up dependencies

32. @PaulienVanAlst#Devoxx #Kotlin
Set-up dependencies
Code!

33. @PaulienVanAlst#Devoxx #Kotlin
Migration Plan
1. Set-up dependencies
2. Migrate PoJo’s
3. Rewrite unit tests
4. Business logic / integration tests
5. Configuration

34. @PaulienVanAlst#Devoxx #Kotlin
Migrate PoJo’s
@Value
public class BoardGame {
private final String name;
private final Category category;
private final AgeRange ageRange;
private final NumberOfPlayers numberOfPlayers;
private final Rating rating;
}

35. @PaulienVanAlst#Devoxx #Kotlin
Migrate PoJo’s
@Value
public class BoardGame {
private final String name;
private final Category category;
private final AgeRange ageRange;
private final NumberOfPlayers numberOfPlayers;
private final Rating rating;
}
data class BoardGame ( val name: String,
val category: Category,
val ageRange: AgeRange,
val numberOfPlayers: NumberOfPlayers,
val rating: Rating
)

36. @PaulienVanAlst#Devoxx #Kotlin
Migrate PoJo’s
Code!

37. @PaulienVanAlst#Devoxx #Kotlin
Migration Plan
1. Set-up dependencies
2. Migrate PoJo’s
3. Rewrite unit tests
4. Business logic / integration tests
5. Configuration

38. @PaulienVanAlst#Devoxx #Kotlin
Rewrite unit tests
@Test
void when_game_is_rated_then_the_game_with_new_average_rate_is_returned()

39. @PaulienVanAlst#Devoxx #Kotlin
Rewrite unit tests
@Test
void when_game_is_rated_then_the_game_with_new_average_rate_is_returned()
@Test
fun `when game is rated then the game with new average rate is returned`()

40. @PaulienVanAlst#Devoxx #Kotlin
Rewrite unit tests
Code!

41. @PaulienVanAlst#Devoxx #Kotlin
Migration Plan
1. Set-up dependencies
2. Migrate PoJo’s
3. Rewrite unit tests
4. Business logic / integration tests
5. Configuration

42. @PaulienVanAlst#Devoxx #Kotlin
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-reflect</artifactId>
<version>${kotlin.version</version>
</dependency>
Business logic / integration tests

43. @PaulienVanAlst#Devoxx #Kotlin
<plugin>
<groupId/>
<artifactId>kotlin-maven-plugin</artifactId>
<configuration>
<compilerPlugins><plugin>jpa</plugin></compilerPlugins>
<jvmTarget>1.8</jvmTarget>
</configuration>
</plugin>
Business logic / integration tests

44. @PaulienVanAlst#Devoxx #Kotlin
<plugin>
<groupId/>
<artifactId>kotlin-maven-plugin</artifactId>
<configuration/>
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-noargs</artifactId>
<version>${kotlin.version</version>
<scope>test</scope>
</dependency>
</dependencies>
</plugin>
Business logic / integration tests

45. @PaulienVanAlst#Devoxx #Kotlin
Business logic / integration tests
Code!

46. @PaulienVanAlst#Devoxx #Kotlin
Migration Plan
1. Set-up dependencies
2. Migrate PoJo’s
3. Rewrite unit tests
4. Business logic / integration tests
5. Configuration

47. @PaulienVanAlst#Devoxx #Kotlin
Configuration
@SpringBootApplication
public class Application {
public static void main (String[] args) {
SpringApplication.run(Application.class, args);
}
}

48. @PaulienVanAlst#Devoxx #Kotlin
Configuration
@SpringBootApplication
public class Application {
public static void main (String[] args) {
SpringApplication.run(Application.class, args);
}
}
@SpringBootApplication
open class Application
fun main(args: Array<String>) {
SpringApplication.run(Application::class.java, *args)
}

49. @PaulienVanAlst#Devoxx #Kotlin
Configuration
Code!

50. @PaulienVanAlst#Devoxx #Kotlin
Migration Plan
1. Set-up dependencies
2. Migrate PoJo’s
3. Rewrite unit tests
4. Business logic / integration tests
5. Configuration

51. @PaulienVanAlst#Devoxx #Kotlin

52. @PaulienVanAlst#Devoxx #Kotlin

53. @PaulienVanAlst#Devoxx #Kotlin
Thank you!
Rate this talk!

54. @PaulienVanAlst#Devoxx #Kotlin

55. @PaulienVanAlst#Devoxx #Kotlin

56. @PaulienVanAlst#Devoxx #Kotlin
So… what is Kotlin
J S
J V M
M U L T I P L A T F O R M

57. @PaulienVanAlst#Devoxx #Kotlin
So… what is Kotlin