This document provides examples of Kotlin code demonstrating various Kotlin language features including: - Defining immutable vals and mutable vars - Type inference for variables - Default function parameters - When expressions for conditional logic - Classes, interfaces, and inheritance - Null safety features like safe calls and elvis operator - Higher-order functions like map and filter

11. // val = immutable object
val aString : String = "Lorem ipsum" // no semicolons
aString = "Lorem Ipsum Lorem ipsum" // compiler error
// var = mutable obect
var mutableString :String = "Lorem ipsum"// underlined by IDE

12. val anInt = 42 // compiler infers type to Int
val someString = "Lorem Ipsum"

13. fun add(left: Int, right: Int): Int {
return left + right
}

14. fun sayHi(hello: String = "Hello",
world: String = "World") {
println("$hello, $world!")
}
sayHi() // Hello World!
sayHi(hello = "Stathis") // Hello Stathis!
sayHi(hello = "Stathis", // Stathis lorem ipsum!
world = "lorem ipsum")

15. fun getResult(condition: Boolean): String {
val result = null
if (condition) {
result = "Service OK"
} else {
result = "Service Failure"
}
return result
}

16. fun getResult(condition: Boolean): String {
return if (condition) {
"Service OK"
} else {
"Service Failure"
}
}

17. fun getResult(condition: Boolean) =
if (condition) "Service OK" else "Service Failure"

18. val customer = Customer("Stathis", "Souris")
fun main (args: List<String>) {
println("Name is ${customer.firstName}")
// or if i want the toString I just omit the properties
println("Name is $customer")
}

19. fun Date.isTuesday() : Boolean {
return day == 2
}
val epoch: Date = Date(1970, 0, 0)
if (epoch.isTuesday()) {
println("The epoch was a Tuesday.")
} else {
println("The epoch was not a Tuesday.")
}

20. // decomposing parameters
val (name, email, id) = Tuple("Stathis", "foo@bar.com", 1);
val (firstName, lastName) = Customer("lorem", "ipsum")
val countryAndCity = mapOf(
Pair("Madrid", "Spain"),
"Paris" to "France"))
for ((city, country) in countryAndCity) { ... }
// ranges
for (numer in 1..100) { .. }
1..100.filter { e -> e % 2 == 0 }.map { ... }

21. // classes by default final, add `open` to inherit from
open class Person { }
class Employee(val vacationDays: Int): Person() {}
class Contractor: Person()
fun validateVacations(person: Person) {
if (person is Employee) {
// casting is inferred by the compiler (smart cast)
if (person.vacationDays != 20) {
println("You need to take some more time off!")
}
}
}

22. val customer: Customer = ...
println(customer.firstName)
val customer: Customer = ...
println(customer?.firstName) // if customer null then prints null
// elvis operator
val l: Int = if (b != null) b.length else -1
val l = b?.length ?: -1

23. // Spring 5 FunctionRouter registration in kotlin
"/posts".nest {
GET("/{id}", blogHandler::getBlogPost)
GET("/", blogHandler::getBlogPosts)
}
accept(TEXT_EVENT_STREAM).nest {
GET("/match", blogHandler::matchClock)
}

24. typealias MapOfStringsToObjects = HashMap<String, Object>

25. val validNumbers = arrayOf(100, 42, 5566, 234)
when(x) {
in 1..10 -> print("x == 1")
in validNumbers -> print("x is valid range")
!in 10..20 -> print("x is ouside range")
is ClassName -> ... (smart cast also works here)
else -> print("None of the above")
}

26. public String foo(String name, int number, boolean toUpperCase) {
return (toUpperCase ? name.toUpperCase() : name) + number;
}
public String foo(String name, int number) {
return foo(name, number, false);
}
public String foo(String name, boolean toUpperCase) {
return foo(name, 42, toUpperCase);
}
public String foo(String name) {
return foo(name, 42);
}

27. fun foo(name: String,number: Int = 42, toUpperCase: Boolean = false) =
(if (toUpperCase) name.toUpperCase() else name) + number
fun useFoo() = listOf(
foo("a"),
foo("b", number = 1),
foo("c", toUpperCase = true),
foo(name = "d", number = 2, toUpperCase = true))

28. public void sendMessageToClient(
@Nullable Client client,
@Nullable String message) {
if (client == null || message == null) return;
PersonalInfo personalInfo = client.getPersonalInfo();
if (personalInfo == null) return;
String email = personalInfo.getEmail();
if (email == null) return;
sendMessage(email, message);
}

29. fun sendMessageToClient(
client: Client?, message: String?) {
sendMessage(
client?.personalInfo?.email ?: return,
message ?: return );
}
class Client (val personalInfo: PersonalInfo?)
class PersonalInfo (val email: String?)

30. class CopyPrinter(copier: Copy, printer: Print) : Copy by copier, Print by printer
interface Copy {
fun copy(page: Page): Page
}
interface Print {
fun print(page: Page)
}

31. public final class CopyPrinter implements Copy, Print {
private final Copy $$delegate_0;// $FF: synthetic field
private final Print $$delegate_1;// $FF: synthetic field
public CopyPrinter(@NotNull Copy copier, @NotNull Print printer) {
Intrinsics.checkParameterIsNotNull(copier, "copier");
Intrinsics.checkParameterIsNotNull(printer, "printer");
super();
this.$$delegate_0 = copier;
this.$$delegate_1 = printer;
}
@NotNull
public Page copy(@NotNull Page page) {
Intrinsics.checkParameterIsNotNull(page, "page");
return this.$$delegate_0.copy(page);
}
public void print(@NotNull Page page) {

32. sealed class Expr {
class Num(val value: Int): Expr()
class Sum(val left: Expr, val right: Expr): Expr()
}
fun eval(e: Expr): Int =
when (e) {
is Expr.Num -> e.value
is Expr.Sum -> eval(e.right) + eval(e.left)
}