Fun with Kotlin

Fun With
Kotlin
by Egor Andreevici

INTRO
PRAGMATISM
Clarity Safety Interoperability Tooling
WHAT’S KOTLIN ABOUT?

INTRO
FOCUS ON USE CASES
INTEROPERATE
WHY PRAGMATIC?

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

SYNTAX
fun sum(a: Int, b: Int): Int = a + b
KOTLIN SYNTAX

SYNTAX
fun sum(a: Int, b: Int) = a + b
KOTLIN SYNTAX

CLASSES
Kotlin Java
class Person public class Person {}
CREATING A CLASS

CLASSES
class Person
class Student : Person()
INHERITANCE

CLASSES
class Person
class Student : Person() // won’t compile!
INHERITANCE

CLASSES
open class Person
class Student : Person() // voila!
INHERITANCE

CLASSES
open class Person(val name: String, var age: Int)
// Java
public class Person {
private final String name;
private Integer age;
public Person(String name, Integer age) { // }
public String getName() { // }
public Integer getAge() { // }
public void setAge(Integer age) { // }
}
CONSTRUCTORS

CLASSES
open class Person(val name: String, var age: Int)
Kotlin Java
val name: String final String name
var age: Int Integer age
VAL & VAR

CLASSES
data class Person(val name: String, var age: Int)
‣ equals() & hashCode() $0
‣ toString() $0
‣ componentN() functions $0
‣ copy() function $0
‣ TOTAL FREE!
DATA CLASSES

CLASSES
val name = person.component1()
val age = person.component2()
COMPONENTN() FUNCTIONS

CLASSES
val name = person.component1()
val age = person.component2()
for ((name, age) in people) {
println("$name is $age years old")
}
DESTRUCTURING DECLARATIONS

CLASSES
class Person {
companion object {
val FIELD_NAME = "name"
fun isOlderThan(person: Person, age: Int) =
person.age > age
}
}
println(Person.isOlderThan(person, 30))
STATIC -> OBJECT

CLASSES
public final class StringUtils {
public static String capitalize(String str) {
// TODO: return capitalized string
return str;
}
}
StringUtils.capitalize(“a”);
UTILS IN JAVA

CLASSES
fun String.capitalize(): String {
return this
}
"a".capitalize()
EXTENSIONS IN KOTLIN

CLASSES
fun String.capitalize(): String {
return this
}
"a".capitalize()
import me.egorand.kotlin.extensions.capitalize
EXTENSIONS IN KOTLIN

NULL-SAFETY
THE BILLION DOLLAR MISTAKE

NULL SAFETY
var a: String = "abc"
a = null // compilation error
var b: String? = "abc"
b = null // OK
val l = a.length
val l = b.length // error: variable 'b' can be null
NULLABLE VS NON-NULL TYPES

NULL SAFETY
val l = if (b != null) b.length else -1
CHECKING FOR NULL: SIMPLE APPROACH

NULL SAFETY
b?.length
bob?.department?.head?.name
CHECKING FOR NULL: SAFE CALLS

NULL SAFETY
CHECKING FOR NULL: ELVIS OPERATOR

NULL SAFETY
val l: Int = if (b != null) b.length else -1
val l = b?.length ?: -1
CHECKING FOR NULL: ELVIS OPERATOR

NULL SAFETY
val l = b!!.length()
CHECKING FOR NULL: LIVING DANGEROUSLY

FUNCTIONS
fun double(x: Int): Int {
return x * 2
}
val result = double(2)
FUNCTIONS IN KOTLIN

FUNCTIONS
fun Int.times(n: Int) = this * n
println(2.times(3))
INFIX NOTATION

FUNCTIONS
infix fun Int.times(n: Int) = this * n
println(2.times(3))
INFIX NOTATION

FUNCTIONS
infix fun Int.times(n: Int) = this * n
println(2 times 3)
INFIX NOTATION

FUNCTIONS
fun printString(str: String,
prefix: String = "",
postfix: String = "") {
println(prefix + str + postfix)
}
printString("hello")
printString("hello", "<")
printString("hello", "<", ">")
DEFAULT ARGUMENTS

FUNCTIONS
fun printString(str: String,
prefix: String = "",
postfix: String = "") {
println(prefix + str + postfix)
}
printString(“hello”,
prefix = “<“,
postfix = “>”)
NAMED ARGUMENTS

FUNCTIONS
fun <T> forEach(list: List<T>, f: (T) -> Unit) {
for (elem in list) {
f(elem)
}
}
val list = listOf("Alice", "Bob", "Claire")
forEach(list, ::println)
forEach(list, { str -> println(str) })
HIGHER-ORDER FUNCTIONS

FUNCTIONS
f(elem)
}
}
forEach(list, { println(it) })

FUNCTIONS
f(elem)
}
}
forEach(list) { println(it) }

SMART CASTS
// Java
private static void demo(Object x) {
if (x instanceof String) {
String str = (String) x;
System.out.println(str.length());
}
}
CASTING IN JAVA

SMART CASTS
// Java
private static void demo(Object x) {
if (x instanceof String) {
String str = (String) x;
System.out.println(str.length());
}
}
// Kotlin
fun demo(x: Any) {
if (x is String) {
println(x.length)
}
}
SMART CASTS IN KOTLIN

SMART CASTS
fun demo(x: Any) {
if (x is String) {
println(x.length)
}
}
IF EXPRESSION

SMART CASTS
fun demo(x: Any) {
if (x !is String) {
println("Not a String")
} else {
println(x.length)
}
}
IF-ELSE EXPRESSION

SMART CASTS
fun demo(x: Any) {
if (x !is String) return
println(x.length)
}
RETURN STATEMENT

SMART CASTS
if (x !is String || x.length == 0) return
if (x is String && x.length > 0)
print(x.length)
BOOLEAN EXPRESSIONS

WHAT’S NEXT?
POST-1.0 ROADMAP
▸ New language features
▸ Java 8/9 support
▸ JavaScript support
▸ Android tooling improvements
▸ IDE features

WHAT’S NEXT?
RESOURCES
▸ Kotlin - http://kotlinlang.org/
▸ Koans - http://try.kotlinlang.org/koans
▸ Blog - https://blog.jetbrains.com/kotlin/
▸ Forum - https://devnet.jetbrains.com/community/kotlin
▸ Slack - http://kotlinlang.slack.com/

Thank you!
@EgorAnd
+EgorAndreevich
http://blog.egorand.me/

Fun with Kotlin

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