Off to a new island - Kotlin for backend development Tobias Schneck, Simon Hofmann Get a first insight to new JVM language Kotlin and learn how you can use Kotlin also for backend development. We will compare traditional Java code to the new fancy Kotlin code and show why we fell in love with it. We will also show how you can spin up your Kotlin Spring Boot backend server in just a few minutes and how you use Java and Kotlin code together in one project.

1. Off to a new Island!
Kotlin for backend development
Tobias Schneck / Simon Hofmann
26.05.2018

2. Kotlin
Agenda
• Introduction
• Kotlin Basics
• Variables, Constants
• Functions
• Classes
• Optionals
• Control Flow
• Extension Functions
• Lambdas
• Build Setup with Maven
• Spring Boot Webservice

3. Kotlin
Links
• Slides:
• GitHub Examples:
https://github.com/ConSol/KotlinSpringBoot
https://github.com/ConSol/KotlinBasics
tiny.cc/consol-hackaburg2018

4. Introduction

5. Kotlin
Why Kotlin?
Concise SafeInter-
operable
Tool-
friendly

6. Kotlin
Where to use?
JVM Android Browser Native

7. Variables, Constants

8. Kotlin
String myVariable = "Change me";
final String myConstant = "Can't change me";
var myVariable: String = "Change me“
val myConstant: String = "Can't change me"

9. Kotlin
String myVariable = "Change me";
final String myConstant = "Can't change me";
var myVariable: String = "Change me“
val myConstant: String = "Can't change me"

10. Kotlin
String myVariable = "Change me";
final String myConstant = "Can't change me";
var myVariable = "Change me“
val myConstant = "Can't change me"

11. Functions

12. Kotlin
Functions
fun bark(times: Int): String {
return "Wuff".repeat(times)
}

13. Kotlin
Functions
fun bark(times: Int): String {
return "Wuff".repeat(times)
}
Function keyword Return type

14. Kotlin
Functions
fun bark(times: Int = 1): String {
return "Wuff".repeat(times)
}
Default value

15. Kotlin
Functions
fun bark(times: Int): String {
return "Wuff".repeat(times)
}

16. Kotlin
Functions
fun bark(times: Int = 1) = "Wuff".repeat(times)

17. Kotlin
Functions
dog.bark()
dog.bark(3)
dog.bark(times = 3)

18. Classes and Inheritance

19. Kotlin
Classes
public class JavaDTO{
private int id;
private String name;
public JavaDTO(int id, String name) {
this.id = id;
this.name = name;
}
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}
class KotlinDTO(var id: Int, var name: String)

20. Kotlin
Classes
class KotlinDTO(var id: Int, var name: String)

21. Kotlin
Classes
class KotlinDTO(var id: Int, var name: String)
Primary Constructor

22. Kotlin
class KotlinDTO(var id: Int, var name: String) {
constructor(json: JSONObject) :
this(json.getInt(”id"), json.getString(”name")) {
}
}
Classes
Secondary
Constructor
Has to call
Primary Constructor

23. Kotlin
Classes
class KotlinDTO(var id: Int, var name: String) {
override fun toString() {
return ”ID: $id, Name: $name"
}
}
String template

24. Kotlin
Classes
val dto = KotlinDTO(1, ”First DTO")
No more “new”

25. Kotlin
Classes
val secondDTO = KotlinDTO(name = ”FooBar“, id = 10)

26. Kotlin
Getters and Setters
class Person(var name: String, var age: Int)
Default getters and setters are generated

27. Kotlin
Getters and Setters
private class Person {
var name = "defaultName"
set(value) {
// Special logic
}
get() = "Person $field"
}
Overwrite default getters
and setters

28. Kotlin
Getters and Setters
private class Person {
var name = "defaultName"
set(value) {
// Special logic
}
get() = "Person $field"
}
@Test
fun testPerson() {
val p = Person()
p.name = "unusedValue"
assertEquals("Person defaultName", p.name)
}
no value set!

29. Kotlin
Property Access
val human = Person()
human.name = ”Simon“
val personName = human.name

30. Time to hack!
https://github.com/ConSol/KotlinBasics

31. Time to hack!
On 1_SimpleDogClass.kt

32. Kotlin
Solution
private class Dog(val name: String, val type: String) {
fun bark(times: Int = 1): String {
return "Wuff".repeat(n = times)
}
}
@Test
fun testMyNewDogClass() {
val bruno = Dog("Bruno", "Labrador")
assertEquals("WuffWuffWuff", bruno.bark(3))
}

33. Optionals

34. Kotlin
Java: Nullpointer exceptions
public void sayHello(Dog otherDog){
System.out.println(“Wuff, "+ otherDog.getName());
}

35. Kotlin
Java: Nullpointer exceptions
public void sayHello(Dog otherDog){
System.out.println(“Wuff, "+ otherDog.getName());
}
sayHello(null);

36. Kotlin
Java: Nullpointer exceptions
public void sayHello(Dog otherDog){
System.out.println(“Wuff, "+ otherDog.getName());
}
sayHello(null);

37. Kotlin
Kotlin: Built-in null-safety
fun sayHello(dog: Dog) {
println("Hi, " + dog.name);
}

38. Kotlin
Kotlin: Built-in null-safety
fun sayHello(otherDog: Dog) {
println("Hi, " + dog.name);
}
sayHello(null);

39. Kotlin
Kotlin: Built-in null-safety
fun sayHello(otherDog: Dog) {
println("Hi, " + dog.name);
}
sayHello(null);

40. Kotlin
Kotlin: Built-in null-safety
var dog: Dog = null

41. Kotlin
Explicit nullable types
var dog: Dog? = null

42. Kotlin
Calling Optionals
val dog: Dog? = findDog()
dog.bark()

43. Kotlin
Calling Optionals
val dog: Dog? = findDog()
if(dog != null){
dog.bark(3)
}

44. Kotlin
Safe Call Operator
val dog: Dog? = findDog()
dog?.bark()

45. Kotlin
For all NPE lovers (The not-null assertion operator)
val dog: Dog? = findDog()
dog!!.bark()

46. Kotlin
Elvis Operator
val foundDog = findDog()
val myDog = if (foundDog != null) {
foundDog
} else {
buyNewDog()
}

47. Kotlin
Elvis Operator
val myDog = findDog() ?: buyNewDog()

48. Kotlin
Elvis Operator
val myDog = findDog() ?: buyNewDog()

49. Kotlin
Automatic casting
if(dog != null){
dog.bark(3)
}
val anyObject: Any = getAnimal()
if(anyObject is Dog){
anyObject.bark()
}

50. Time to hack!

51. Time to hack!
On 2_Optionals.kt

52. Kotlin
Solution
private fun letDogBark(dog: Dog?) {
// TODO TASK 1
dog?.bark()
}
private fun getDogName(dog: Dog?): String {
// TODO TASK 2
return dog?.name ?: "No dog found"
}
private fun getNameOf(any: Any): String {
// TODO TASK 3
return (any as? Dog)?.name ?: "type unknown"
}

53. Control Flow

54. Kotlin
For Loop
val dogs = getDogs()
for (dog in dogs){
dog.bark()
}

55. Kotlin
For Loop
val dogs = getDogs()
for (index in 0..10){
dogs[index].bark()
}
Range Operator

56. Kotlin
For Loop
for (index in 0..10){
print(index)
}
>>> 012345678910
Ranges are
inclusive

57. Kotlin
For Loop
for (index in 10 downTo 1){
print(index)
}
>>> 10987654321

58. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int): String {
}

59. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int): String {
when(dogCount){
}
}

60. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int): String {
when(dogCount){
0 -> return ”No dogs"
1 -> return ”One dog"
else -> return "$dogCount dogs"
}
}

61. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int): String {
when(dogCount){
0 -> return ”No dogs"
1 -> return ”One dog"
else -> return "$dogCount dogs"
}
}

62. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int): String {
return when(dogCount){
0 -> ”No dogs"
1 -> ”One dog"
else -> "$dogCount dogs"
}
}

63. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int): String {
return when(dogCount){
0 -> ”No dogs"
1 -> ”One dog"
else -> "$dogCount dogs"
}
}

64. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int) = when (dogCount) {
0 -> ”No dogs"
1 -> ”One dog"
else -> "$dogCount dogs"
}

65. Kotlin
When switch becomes more powerful
fun getDogPluralString(dogCount: Int) = when (dogCount) {
0 -> ”No dogs"
1 -> ”One dog”
2..4 -> “Many dogs
else -> ”Too many dogs"
}

66. Kotlin
When without argument
fun evaluatePassword(password: String): String {
return when {
}
}

67. Kotlin
When without argument
fun evaluatePassword(password: String): String {
return when {
password.isEmpty() -> ”Please enter password"
password.length < 5 -> ”Password not long enough"
!password.containsNumber() -> ”Password must contain a number"
else -> ”Password valid"
}
}

68. Time to hack!

69. Time to hack!
On 3_ControlFlow.kt

70. Kotlin
Solution
// TODO TASK 1
fun findDogOwnerName(dog: Dog): String? {
return when(dog.name){
"Bruno" -> "Hans"
"Ignatz" -> "Peter"
else -> null
}
}
// TODO TASK 2
fun ageToString(dog: Dog): String {
return when(dog.age){
0,1 -> "Baby Dog"
in 2..8 -> "Normal Dog"
else -> "Old Dog"
}
}

71. Extensions

72. Kotlin
Extension Functions
fun Int.isEven(): Boolean {
return this % 2 == 0
}

73. Kotlin
Extension Functions
fun Int.isEven(): Boolean {
return this % 2 == 0
}
println(1.isEven()) // false
println(2.isEven()) // true
println(3.isEven()) // false

74. Time to hack!

75. Time to hack!
On 4_Extensions.kt

76. Kotlin
Solution
// TODO TAKS 1
fun String.scream(): String {
return this.toUpperCase()+"!!!"
}
// TODO TASK 2
private fun applyAllCapsExtension(text: String): String {
return text.scream()
}
@Test
fun testAllCapsDogLanguage() {
// TODO TASK 3
val allCapsDogLanguage = "Ich habe ganz viel Hunger".barkify().scream()
assertEquals("WUFF WUFF WUFF WUFF WUFF!!!", allCapsDogLanguage)
}

77. Lambdas

78. Kotlin
Lambda Functions
val dog = Dog("Bruno")
val funRef = dog::bark
funRef(times = 3)
returns the function itself

79. Kotlin
Lambda Functions
val dog = Dog("Bruno")
val greetDog: (Dog) -> String = { dog -> "Hey! ${dog.name}" }
println(greetDog(dog))

80. Kotlin
Lambda Functions
val dog = Dog("Bruno")
val greetDog: (Dog) -> String = { dog -> "Hey! ${dog.name}" }
println(greetDog(dog))
Parameter type Return type

81. Kotlin
Lambda Functions
val dog = Dog("Bruno")
val greetDog: (Dog) -> String = { dog -> "Hey! ${dog.name}" }
println(greetDog(dog))
Parameter name Lambda Body

82. Kotlin
Filter Functions

83. Kotlin
Filter Functions

84. Kotlin
Filter Functions
var dogs: List<Dog> = animals.filter(
…
)

85. Kotlin
Filter Predicate
{ animal -> animal is Dog }

86. Kotlin
Filter Functions
var dogs = animals.filter({ animal ->
animal is Dog
})

87. Kotlin
Filter Functions
var dogs = animals.filter(){ animal ->
animal is Dog
}

88. Kotlin
Filter Functions
var dogs = animals.filter { animal ->
animal is Dog
}

89. Kotlin
Filter Functions
var dogs = animals.filter { it is Dog }

90. Kotlin
Mapping Functions
age = 8 age = 2 age = 3age = 6 age = 11

91. Kotlin
Mapping Functions
age = 8 age = 2 age = 3age = 6 age = 11
age = 8 age = 2 age = 3age = 6 age = 11

92. Kotlin
Mapping Functions
val dogs = listOf(Dog("Bello",age = 8),Dog("Rex",age = 2),
Dog("Lessi",age = 6),Dog("Bruno",age = 3),Dog("Bello",age = 11))

93. Kotlin
Mapping Functions
val dogs = listOf(Dog("Bello",age = 8),Dog("Rex",age = 2),
Dog("Lessi",age = 6),Dog("Bruno",age = 3),Dog("Bello",age = 11))
val dogAges = dogs.map { … }

94. Kotlin
Map Predicate
{ dog -> dog.age }
age = 8 age = 8

95. Kotlin
Mapping Functions
val dogs = listOf(Dog("Bello",age = 8),Dog("Rex",age = 2),
Dog("Lessi",age = 6),Dog("Bruno",age = 3),Dog("Bello",age = 11))
val dogAges = dogs.map { dog -> dog.age }

96. Kotlin
Mapping Functions
val dogs = listOf(Dog("Bello",age = 8),Dog("Rex",age = 2),
Dog("Lessi",age = 6),Dog("Bruno",age = 3),Dog("Bello",age = 11))
val dogAges = dogs.map { it.age }

97. Kotlin
Mapping Functions
val dogs = listOf(Dog("Bello",age = 8),Dog("Rex",age = 2),
Dog("Lessi",age = 6),Dog("Bruno",age = 3),Dog("Bello",age = 11))
val dogAges = dogs.map { it.age }
val average = dogAges.average() (= 6)

98. Kotlin
Map / Filter Combinations
val oldDogAgeAverage = dogs.map { it.age }.filter { it > 5 }.average()

99. Time to hack!
On 5_Lambdas.kt

100. Kotlin
Solution
// TODO TASK 1
private fun findDogNames(dogs: List<Dog>): List<String> {
return dogs.map { it.name }
}
// TODO TASK 2
private fun findOldDogs(dogs: List<Dog>): List<Dog> {
return dogs.filter { it.age > 5 }
}
// TODO TASK 3
private fun findNamesOfOldDogs(dogs: List<Dog>): List<String> {
return dogs.filter { it.age > 5 }.map { it.name }
}

101. Maven Setup

102. Kotlin
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib</artifactId>
<version>${kotlin.version}</version>
</dependency>
<!--TEST Dependencies-->
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-test-junit</artifactId>
<version>${kotlin.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>${junit.version}</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
<testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
<plugins>
<plugin>
<artifactId>kotlin-maven-plugin</artifactId>
<groupId>org.jetbrains.kotlin</groupId>
<version>${kotlin.version}</version>
<configuration/>
<executions>
<execution>
<id>compile</id>
<phase>compile</phase>
<goals>
<goal>compile</goal>
</goals>
</execution>
<!--TEST -->
<execution>
<id>test-compile</id>
<phase>test-compile</phase>
<goals>
<goal>test-compile</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>

103. Spring Boot Webservice

104. Kotlin
https://start.spring.io/
Basic Project Setup

105. Time to hack!
https://github.com/ConSol/KotlinSpringBoot

106. Any questions?

107. Thank you!

108. ConSol
Consulting & Solutions Software GmbH
Franziskanerstr. 38
D-81669 Munich
Germany
Tel.: +49-89-45841-100
info@consol.de
www.consol.com
Twitter: @consol_de