Kotlin

Kotlin - why kotlin and lesson learned
Nodirbek Usmanov
Developer @ DKB Code Factory GmbH
Berlin, 06.09.2019

Table of Contents
1 Table of Contents
2 Introduction
3 Philosophy
4 Java vs Kotlin
5 Variables
6 New instance
8 Collection api
9 Nullsafe
10 Code - a max function
11 Code - ﬁnding the oldest person
12 Smartcast
13 Summary

What is Kotlin
Introduction
• since 2010 (release 1.0 on 15.02.2016)
• JVM language*
• developed by Jetbrains (IntelliJ)
• open source

Why Kotlin
Introduction
• 100% interoperable with Java
• statically typed
• concise and less boilerplate code
• target platforms: server-side, android, native, everywhere
where java runs

Why Kotlin
Introduction
• object oriented and functional programming
• solves nullability problem
• higher order functions
• smart casts

Kotlin
Philosophy
pragmatic, concise and safe language

Java
Class with getter/setter/equals/hashcode/toString
1public class Person {
2
3private String name;
4private int age;
5
6public Person(String name, int age) {
7this.name = name;
8this.age = age;
9}
10
11public Person(String name) {
12this.name = name;
13}
14
15public String getName() {
16return name;
17}
18
19public int getAge() {
20return age;
21}
22
23public void setAge(int age) {
24this.age = age;
25}
26
27public boolean equals(Object object) {
28if (this == object) return true;
29if (object == null || getClass() != object.getClass()) return false;
30if (!super.equals(object)) return false;
31Person personJava = (Person) object;
32return age == personJava.age &&
33java.util.Objects.equals(name, personJava.name);
34}
35
36public int hashCode() {
37return Objects.hash(super.hashCode(), name, age);
38}
39

Kotlin
Class with getter/setter/equals/hashcode/toString
1data class Person(val name: String,
2var age: Int? = null)

Kotlin
Variables
1// immutable, not nullable, read-only
2val name: String
3
4// immutable, not mullable, read-only
5val age: Int

Kotlin
Variables
2val name: String
3
4// mutable (can be initialized multiple times), not nullable
5var age: Int

Kotlin
Variables
2val name: String
3
4// mutable, nullable
5var age: Int?

Kotlin
Variables
2val name: String
3
4// mutable, nullable, default value
5var age: Int? = null

Kotlin
new instance
1val person: Person = Person("Bob")

Kotlin
new instance
1val person = Person("Bob")

Kotlin
functions
In mathematics:
f : Z × Z → Z, f (a, b) = a + b (1)

Kotlin
functions
In mathematics:
f : Z × Z → Z, f (a, b) = a + b (2)
In Kotlin:
fun f(a: Int, b: Int) : Int = a + b

Kotlin
functions
1fun f(a:Int, b:Int) : Int {
2return a + b
3}

Kotlin
functions with a return value + type (inference)
1fun f(a:Int, b:Int) = a + b

Kotlin
collection api - creating a read-only list of persons
1val people = listOf(Person("Bob"),
2Person("Tim", age = 40))

Kotlin
collection api - creating a read-only set of persons
1val people = setOf(Person("Bob"),
2Person("Tim", age = 40))

Java
stream - collection processing
List<String> list = people.stream()
.map(Person::getName)
.collect(Collectors.toList());

Java
List<String> list = people.stream()
.map(Person::getName)
.collect(Collectors.toList());
Kotlin
real functional approach
val list = people.map{it.name}

Java
int total = employees.stream()
collect(Collectors.summingInt(Employee::getSalary)));

Java
int total = employees.stream()
collect(Collectors.summingInt(
Employee::getSalary)));
Kotlin
real functional approach
val total = employees.sumBy{it.salary}

Kotlin
Nullsafe - when to return a null
When do you return a null?
(no matter which language)

Kotlin
• Eﬀective Java suggestion:
• an empty collection instead of null

Kotlin
• Eﬀective Java suggestion:
• an empty collection instead of null
• What about a function which returns an object and not a
collection?
• Optional with extra overhead for avoiding null

Kotlin
ﬁnd the problem
fun foo() = length(null)
fun length(s: String): Int {
return s.length
}

Kotlin
No more NPE - if else
// <<Compiler>>: Null can not be a value of a non-null type
String
fun length(s: String): Int {
return s.length
}

Kotlin
No more NPE - if else
fun length(s: String?): Int {
return if (s != null) s.length else 0
}

Kotlin
No more NPE - elvis operator
fun length(s: String?): Int {
return s?.length ?: 0
}

Kotlin
Let’s code - expression over statement
// find the max of two given integer numbers

Kotlin
Let’s code
1fun max(a:Int, b:Int):Int{
2return if (a > b) a else b
3}

Kotlin
Let’s code - concise
1fun max(a:Int, b:Int):Int = if (a > b) a else b

Kotlin
Let’s code - concise
1fun max(a:Int, b:Int) = if (a > b) a else b

Kotlin
Let’s code
1// find the oldest person in a list of persons
2
3data class Person(val name: String,
4val age: Int? = null)
5
6
7fun main(args: Array<String>) {
8
9// 1 - create a list with two persons
10
11// 2 - find the oldest person
12
13// 3 - print the result
14

Kotlin
Let’s code
// find the oldest person
data class Person(val name: String,
val age: Int? = null)
fun main() {
val persons = listOf( ... )
val oldest =
}

// find the oldest person SOLUTION
data class Person(val name: String,
val age: Int? = null)
fun main() {
val persons = listOf(Person("Bob"),
Person("Tim", age = 30))
val oldest = persons.maxBy{ it.age ?: 0 }
println("The oldest is: $oldest")
}

Kotlin
smart cast
1
2open class Vehicle
3
4
5class Boat : Vehicle() {
6fun swim() {
7println("swim")
8}
9}
10
11class Car : Vehicle() {
12fun drive() {
13println("drive")
14}
15}
16
17fun move(veh: Vehicle) {
18if (veh is Car) {
19veh.drive()
20} else if (veh is Boat) {
21veh.swim()
22} else {
23throw IllegalArgumentException()
24}
25}

Kotlin
smart cast
2if (veh is Car) {
3veh.drive()
4} else if (veh is Boat) {
5veh.swim()
6} else {
7throw IllegalArgumentException()
8}
9}

Kotlin
smart cast with when-constructor
2when (veh) {
3is Car -> veh.drive()
4is Boat -> veh.swim()
5else -> throw IllegalArgumentException()
6}
7}

Lesson learned
• you can implement a spaghetti code even with kotlin

Lesson learned
• review in short period and motivate developers to read kotlin
spec./doc

Lesson learned
spec./doc
• don’t use type? everywhere

Lesson learned
spec./doc
• avoid .?let

Lesson learned
spec./doc
• avoid .?let
• no arrow code

Lesson learned
spec./doc
• avoid .?let
• no arrow code
• ﬂuent testing lib: kotlin-test (especially for param. tests)

Summary I
• Kotlin is a compiled, statically typed language

Summary I
• Interoperable with the ecosystem of Java libraries
• Seamless bi-directional interoperability with Java

Summary I
• Interoperable with the ecosystem of Java libraries
• Seamless bi-directional interoperability with Java
• Code will fail fast — during compilation rather than runtime
• Kotlin oﬀers more compile time safety when compared to a lot of
other statically typed languages

Summary II
• Easy to learn
• Fast and pleasing learning curve for Java developers
• Moving from Java to Kotlin is not only fast, it’s also enjoyable

Summary II
• Easy to learn
• Freedom to mix the imperative and functional styles of
programming

Summary II
• Easy to learn
• Freedom to mix the imperative and functional styles of
programming
• Jetbrains claims that Kotlin code is 40% more concise than
Java

References I
Dmitry Jemerov, Svetlana Isakova.
Kotlin in Action.
Manning Publications Co., 02.2017.
Dr. Venkat Subramaniam.
Programming Kotlin.
O’Reilly UK Ltd., 07.2019.
Pierre-Yves Saumont.
The Joy of Kotlin.
Manning Publications Co., 05.2019.

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