Workshop Kotlin @ BKK

1. Introduction to KotIin

2. What’s Kotlin
● New language support by Jetbrain
● Run on JVM
● Static type
● 100% interoperable with Java
● https://try.kotlinlang.org/

3. ● Declaration type (val/ var)
● Property
● Function
● Lambda
● Higher order function
● Data class
Agenda

4. 2 Types of Variable
(Mutable & Immutable)

5. Java Kotlin
Person person = new Person();
int number = 10;
var (variable: mutable)
String name = “Kotlin”;
String surname = “Pro”
surname = “melon”
var name: String = “Kotlin”
var surname = “Pro”
surname = “melon”
var number = 10
var person = Person()

6. val (value: immutable)
Java Kotlin
final String name = “Kotlin”;
final int number = 10;
final Person person = new Person();
val name: String = “Kotlin”
val surname = “Pro”
surname = “melon”
val number = 10
val person = Person()

7. Properties
(Field + Accessor)

8. Class & Properties
Java Kotlin
Java Kotlin Output
person.setName(“Kotlin”)
System.out.print(person.getName())
person.name = “Kotlin”
print(person.name)
Kotlin
public class Person {
private String name = “melon” ;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
class Person {
var name = “melon”
}

9. Properties Initialization
● Default Getter/Setter
● Custom Getter/Setter
● Late init
● Lazy

10. Custom Getter/Setter
Java Kotlin
Java Kotlin Output
person.setName(“Kotlin”)
System.out.print(person.getName())
person.name = “Kotlin”
print(person.name)
MY NAME IS KOTLIN
public class Person {
private String name = “”;
public String getName() {
return name.toUpperCase();
}
public void setName(String name) {
this.name = “My name is ” + name;
}
}
class Person {
var name: String = “”
get() = field.toUpperCase()
set(value) {
field = “My Name is $value”
}
}

11. Kotlin
class Test {
var subject: TestSubject
}
Compile error: Property must initialized

12. LATE INIT(1)
Kotlin
class Test {
lateinit var subject: TestSubject
@Setup fun setUp( ) {
subject = TestSubject()
}
@Test fun test ( ) {
subject.method()
}
}
Initialize later

13. Kotlin
class Person {
lateinit var name: String
fun getPersonInfo(): String {
return name
}
}
Input Output
print(person.name)
kotlin.UninitializedPropertyAccessException

14. LAZY : (Save memory and skip the initialisation until the property is required. )
Kotlin
class EditProfilePersenter {
val database by lazy {
Database()
}
fun saveToDatabase(name: Person) {
database.save(name)
…...
}
}
DATABASE OBJECT IS INITIATE AT THIS LINE.

15. Nullable type

16. TYPE? TYPE= NULLOR
var y: String = “Hi”
y = null
var x: String? = “Hi”
x = null
Compile error: y can’t be null

17. Java Kotlin
Assertion !!
Elvis operator ?:
Safety operator ?
String getUppercaseName (String name) {
if (name != null) {
return name.toUpperCase();
}
return null;
}
String getUppercaseName (String name) {
if (name != null) {
return name.toUpperCase();
}
return “”;
}
String getUppercaseName (String name) {
return name.toUpperCase();
}
fun getUppercaseName (name: String?) : String? {
return name?.toUpperCase()
}
fun getUppercaseName (name: String?) : String {
return name?.toUpperCase() ?: “”
}
fun getUppercaseName (name: String?) : String {
return name!!.toUpperCase()
}

18. Constructor
(constructor block + initialize block)

19. Java Kotlin
Constructor block
Initialize block
How to use : Person(“Kotlin”)
Constructor block
Initialize block
public class Person {
Person(String name) {
System.out.println(name);
}
}
class Person(name: String) {
init {
println(name)
}
}
public class Person {
private String name;
Person(String name) {
this.name = name;
System.out.println(name);
}
public String getNameUpperCase() {
return name.toUpperCase()
}
}
class Person(val name: String) {
init {
println(name)
}
fun getNameUpperCase() : String {
return name.toUpperCase()
}
}

20. Function

21. Java Kotlin
Usage:
plus(1,2) outcome: 3
plus(x=1, y=2) outcome: 3
plus(y=2, x=1) outcome: 3
plus(y=2) outcome: 2 <— x+y = 0+2 = 2
public int plus(int x, int y) {
return x+y;
}
fun plus(x: Int, y: Int) : Int {
return x+y
}
fun plus(x: Int, y: Int) = x+y
Single Expression
fun plus(x: Int = 0, y: Int) = x+y
Parameter can set default value

22. Lambda
(Anonymous function)

23. (Parameter Type) Return Type
val resultOfsum = { x: Int, y:Int -> x + y }
val resultOfsum = sum(2, 3)
fun sum(x: Int, y: Int) {
return x + y
}
Usage:
resultOfsum(2, 2) outcome: 4
resultOfsum(2, 3) outcome: 5
(Int, Int) Int
Implement something{ }

24. Higher order function
(A function that takes another function as an argument or returns one.)

25. fun twoAndThreeOperation(operation: (Int, Int) -> Int) {
val result = operation(2, 3)
println("The result is $result")
}
Higher order function
Usage:
twoAndThreeOperation ({ x, y —> x + y}) outcome : The result is 5 //x=2, y= 3 → 2+3
twoAndThreeOperation { x, y —> x * y} outcome : The result is 6 //x=2, y= 3 → 2*3
twoAndThreeOperation { x, y —> y - x + 10 } outcome : The result is 11 //x=2, y= 3 → 3-2+10

26. public inline fun <T, R> Iterable<T>.map(transform: (T) -> R): List<R> {
return mapTo(ArrayList<R>(collectionSizeOrDefault(10)), transform)
}
person1: Person(name = “A”, age = 20)
person2: Person(name = “B”, age = 21)
person3: Person(name = “C”, age = 22)
val persons = listOf(person1, person2, person3)
List Of Person List Of person’s name
persons.map { person -> person.name }
map (transform)
List Of Person List Of person’s age
persons.map { person -> person.age }
map (transform)
Java :
List<String> personName = new ArrayList<> ();
for (Person person: persons) {
personName.add(person.name)
}
Java :
List<Integer> personAge = new ArrayList<>();
for (Person person: persons) {
personAge.add(person.age)
}

27. Data class

28. Java Kotlin
class Person {
private String name;
private int age;
@Override
String toString() {}
@Override
int hashCode() {}
@Override
boolean equals(Object o) {}
Person copy() {}
….getter()/setter() of name, age….
}
data class Person (
val name: String,
var age: Int
)

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