Kotlin for Java developers.

Kotlin ve Java’nin yeni
versiyonları
Hadi Tok
@hadi_tok
Technical Lead @CitizenMe

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About Kotlin
Kotlin is a cross-platform, statically typed, general-purpose
programming language with type inference. Kotlin is designed to
interoperate fully with Java. Kotlin mainly targets the JVM, but also
compiles to JavaScript or native code.
Source: wikipedia.org / License: WP:CC BY-SA

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Why Kotlin?
Concise
Drastically reduce the amount of boilerplate code
Source: kotlinlang.org /License: Apache 2.0

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Why Kotlin?
Safe
Avoid entire classes of errors such as null pointer exceptions

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Why Kotlin?
Interoperable
Leverage existing libraries for the JVM, Android, and the browser

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Kotlin for Java
Programmers

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Functions

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fun main() {
println("Hello world!")
}

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fun sum(a: Int, b: Int): Int {
return a + b
}

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fun sum(a: Int, b: Int): Int {
return a + b
}
fun sum(a: Int, b: Int) = a + b

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Default argumnets

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fun namePrinter(firstName: String, lastName: String) {
print("$firstName $lastName")
}
namePrinter("Hadi", "Tok")

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fun namePrinter(firstName: String, lastName: String="Tok") {
}
namePrinter("Hadi")

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Named argumnets

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fun namePrinter(firstName: String="Hadi", lastName:
String="Tok") {
}
namePrinter(lastName = "Hariri")

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infix notation

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class MyStringCollection {
infix fun add(s: String) { /*...*/ }
fun build() {
this add "abc" // Correct
add("abc") // Correct
//add "abc" Incorrect: the receiver must be specified
}
}

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Extension Functions

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fun MutableList.swap(index1: Int, index2: Int) {
val tmp = this[index1] // 'this' corresponds to the list
this[index1] = this[index2]
this[index2] = tmp
}

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Higher Order Functions

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class Request {
void longRunningOperation(Callback callback) {
String result = "result ok"; //assume long running
callback.getResult(result);
}
interface Callback {
void getResult(String result);
}
}

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Request request =new Request();
request.longRunningOperation(new Callback() {
@Override
public void getResult(String result) {
System.out.println(result);
}
});

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class Request{
fun longRunningOperation(callback:(String)->Unit){
val result = "result ok" //assume long running
callback(result)
}
}

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val request= Request()
request.longRunningOperation({result->
print(result)
})

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val request= Request()
request.longRunningOperation { result->
print(result)
}

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val request = Request()
request.longRunningOperation(this::printResult)
fun printResult(result:String){
print(result)
}

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Lambda expressions

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val sum: (Int, Int) -> Int = { x: Int, y: Int -> x + y }

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val sum: (Int, Int) -> Int = { x: Int, y: Int -> x + y }
sum(3,4)

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Variables

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val a: Int = 1 // immediate assignment
val b = 2 // `Int` type is inferred

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var x = 5 // `Int` type is inferred
x += 1

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Null Safety

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var a: String = "abc"
a = null // compilation error

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var b: String? = "abc"
b = null // ok

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val l: Int = a.length

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val l: Int = a.length
val m = b.length // error: variable 'b' can be null

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val c: String? = null
val l = if (c != null) c.length else -1

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val a = "Kotlin"
val b: String? = null
println(b?.length)
println(a?.length) // Unnecessary safe call

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val listWithNulls: List = listOf("Kotlin", null)
for (item in listWithNulls) {
item?.let { println(it) } //prints Kotlin and ignores null
}

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val l = c?.length ?: -1

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var b: String? = "abc"
val l = b!!.length

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String Templates

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var a = 1
// simple name in template:
val s1 = "a is $a"
a = 2
// arbitrary expression in template:
val s2 = "${s1.replace("is", "was")}, now is $a"

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String Literals

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val s = "Hello, world!\n"

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val s = "Hello, world!\n"
val text = """
for (c in "foo")
print(c)
"""

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val s = "Hello\nworld!"
val text = """
for (c in "foo")
print(c)
"""
val text = """
|Tell me and I forget.
|Teach me and I remember.
|Involve me and I learn.
|(Benjamin Franklin)
""".trimMargin()

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Type checks and automatic
casts

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if (obj is String) {
// `obj` is automatically cast to `String` in
// this branch
return obj.length
}

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if (obj !is String) return null
// `obj` is automatically cast to `String` in
// this branch
return obj.length

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// `obj` is automatically cast to `String` on the
// right-hand side of `&&`
if (obj is String && obj.length > 0) {
return obj.length
}

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Loops

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val items = listOf("apple", "banana")
for (item in items) {
println(item)
}

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for (x in 1..5) {
print(x)
}

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for (x in 1..5) {
print(x)
}
for (x in 1..10 step 2) {
print(x)
}

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for (x in 1..5) {
print(x)
}
for (x in 1..10 step 2) {
print(x)
}
for (x in 9 downTo 0 step 3) {
print(x)
}

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val items = listOf("apple", "banana")
var index = 0
while (index < items.size) {
println("item at $index is ${items[index]}")
index++
}

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when expression

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fun describe(obj: Any): String =
when (obj) {
1 -> "One"
"Hello" -> "Greeting"
is Long -> "Long"
!is String -> "Not a string"
else -> "Unknown"
}

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Collections

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val fruits = listOf("banana", "avocado")
fruits
.filter { it.startsWith("a") }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }

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Classes

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class Person {
var firstName: String
var lastName: String
constructor(firstName: String, lastName: String) {
this.firstName = firstName
this.lastName = lastName
}
}

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class Person(var firstName: String, var lastName: String)

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val person = Person("Hadi", "Tok")

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Getter setter

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class Person(var firstName: String, var lastName: String){
var name = "$firstName $lastName"
}

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class Person(var firstName: String, var lastName: String) {
var name: String
get() = "$firstName $lastName"
}

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class Person(var firstName: String, var lastName: String) {
var name: String
get() = "$firstName $lastName"
set(value) {
val array = value.split(" ")
firstName = array[0]
lastName = array[1]
}
}

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Data Classes

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class Person {
String firstName;
String lastName;
}

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class Person{
private String firstName;
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}

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class Person{
return firstName;
}
}
return lastName;
}
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
return Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
}

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class Person{
return firstName;
}
}
return lastName;
}
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
return Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
@Override
public String toString() {
return "Person{" +
"firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
'}';
}
}

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class Person(var firstName: String, var lastName: String)

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data class Person(var firstName: String, var lastName: String)

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Sealed classes

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sealed class Result()
data class Success(val value: String) : Result()
data class Failure(val e: Exception) : Result()

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when(expr) {
is Success -> println(result.value)
is Failure -> println(result.e.message)
// the else is not required because covered all the cases
}

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Objects

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object PersonPrinter {
fun printPerson(person: Person) {
print(person.toString())
}
}

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object PersonPrinter {
fun printPerson(person: Person) {
print(person.toString())
}
}
PersonPrinter.printPerson(Person("Hadi", "Tok"))

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Inline classes

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inline class Address(val value: String)

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inline class Address(val value: String)
object AddressPrinter{
fun printAddress(address: Address){
print(address.value)
}
}

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Delegated Properties

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val lazyValue: String by lazy {
println("computed!")
"Hello"
}
fun main() {
println(lazyValue)
println(lazyValue)
}

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Coroutines

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fun printer(){
GlobalScope.launch {
val result = longRunningOperation()
println("result is $result")
}
}
suspend fun longRunningOperation(): String {
delay(5000L)
return "200"
}

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New Vesions of Java

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What is new with Java?
Java 9
● Private methods in interfaces

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Java 10
● var keyword (type inference)

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var a = "asd";
System.out.println(a);

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Java 11
● Local Variable Syntax for Lambda Parameters

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interface LambdaInterface {
abstract void doSomething(String x);
}
LambdaInterface lambdaInterface = ((String x) ->
System.out.println(x)
);

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interface LambdaInterface {
abstract void doSomething(String x);
}
LambdaInterface lambdaInterface = ((var x) ->
System.out.println(x)
);

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Java 12

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Java 13

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Java 14
● Switch Expressions

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String result = "";
int number = 0;
switch (0) {
case 1:
case 2:
case 3:
result = "low";
break;
case 4:
result = "ok";
break;
case 5:
case 6:
case 7:
result = "high";
break;
default:
result ="unknown";
}
System.out.println("result");

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String result =""
switch (0) {
case 1,2,3:
result = "low";
break;
case 4:
result = "ok";
break;
case 5,6,7:
result = "high";
break;
default:
result ="unknown";
}

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String result = switch (0) {
case 1, 2, 3 -> "low";
case 4 -> "ok";
case 5, 6, 7 -> "high";
default -> "unknown";
};

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Java 15 and later
● Text Blocks

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String html = """

Hello, world

""";
Source: http://openjdk.java.net/jeps/368

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Java 15 and later
● Text Blocks
● Pattern Matching for instanceof

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if(obj instanceof String){
String s = (String) obj;
System.out.println(s);
}

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if(obj instanceof String s){
System.out.println(s);
}

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Java 15 and later
● Text Blocks
● Pattern Matching for instanceof
● Records

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record Person(String firstName, String lastName) {
}

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}
Person p = new Person("Hadi", "Tok");

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}
Person p = new Person("Hadi", "Tok");
System.out.println(p.firstName() + " " + p.lastName());

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Questions?
Usefull Links
● Kotlin web site:
○ https://kotlinlang.org/
● Try Kotlin
○ https://play.kotlinlang.org/
● KotlinConf 2019: What's New in Java 19: The end of Kotlin? by Jake
Wharton :
○ https://www.youtube.com/watch?v=te3OU9fxC8U

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Thank you!
Hadi Tok
@hadi_tok
Technical Lead @CitizenMe

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Kotlin for Java developers.

Kotlin for Java developers.

Hadi Tok

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