Hello, I'm Juan Medina

I've been coding since I was 10

Three years ago I was a manager

I was disconnected from technology

Transformation was happening

Research time

Conversations

Opportunity

Building

Upgrading coding skills

Learning by Example

Blog and Articles

Kotlin mainstream language

• Google's support for Kotlin in Android
• Spring Framework 5 Kotlin extensions
• Reactor Kotlin extensions
• Spark Kotlin support

Learning Kotlin

Packt Publishing want a book

Writing a book

There is never enough feedback

Finally the book is done done

What I have learned

If you really want it, you can do it.

Why a book about Kotlin?

What is Kotlin

• A programming language
• Created by JetBrains
• Strong Typed
• C++/Java looks-like
• Concise
• Not a paradigm shift
• Modern constructs
• LLVM compiler

Can produce code for

• JVM
• JavaScript
• Native
  • applications
  • imports/export C libraries
  • WebAssembly
• Cross platform modules

What it looks like

fun main(args : Array<String>) {
  println("Hello, world!")
}

Kotlin provides several features

Null Safety

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

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

val l = b?.length ?: -1

Object Oriented

class Greeter(val name: String) {
   fun greet() {
      println("Hello, $name")
   }
}

fun main(args: Array<String>) {
   Greeter(args[0]).greet()
}

Type inference

val a = "abc"
val b = 1

fun getData() : String {
	return "hello world"
}

val c = getData()
print(c) // will print "hello world"

fun getData() : Customer {
	return Customer(1, "super customer") //a class that have id, name
}
print(c) // will print "Customer(id=1, name=super customer)"

Single expression functions

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

fun sum2(a : Int, b : Int) = a + b

fun bigger(a : Int, b : Int) = if (a>b) a else b

val value = if (bigger(x, y)==x) z else 1-z

val other = when(getValue(x, y)){
	0, 1 -> 0
	-1 -> y
	else -> x
}

Default and named arguments

fun displayBorder(character: Char = '=', length: Int = 15) {
  for (i in 1..length) {
    print(character)
  }
  println("")
}

fun main(args : Array<String>) {
  displayBorder()           //print: ===============
  displayBorder('*')        //print: ***************
  displayBorder('*', 5)     //print: *****
  displayBorder(length = 2) //print: ==
}

Data classes

//  - equals() / hashCode()
//  - toString() of the form "User(name=John, age=42)"
//  - copy()
//  - componentN() : component1() = name, component2() = age
data class User(val name: String = "", val age: Int = 0)

val user = User("John", "42")
val olderUser = user.copy(age=50);

val name, age = user;

fun getUser() = User("John", "42")

val userName, userAge = getUser()

Extensions functions

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

val l = mutableListOf(1, 2, 3)
l.swap(0, 2) // the list will be (3, 2, 1)

Lambdas

val strings = listOf("hello", "I'm", "a list", "of", "strings")

strings.filter {
	value : String -> value.length > 3
}.forEach{ value : String -> println(value) }

strings.filter{
	value -> value.length > 3
}.forEach{ value -> println(value) }

strings.filter { it.length > 3 }.forEach(::println)

Type-Safe builders

fun result(args: Array<String>) =
	house {
		rooms {
			room {
				person {+"Lonley"}
			}
			room  {
				person {
					for (arg in args)
						+arg
				}
		}
	}

They are dozen more features

Kotlin vs Java

  Mono<SunriseSunset> createResult(final Mono<GeoTimesResponse> geoTimesResponseMono) {
      return geoTimesResponseMono.flatMap(geoTimesResponse -> {
          if ((geoTimesResponse.getStatus() != null) && (geoTimesResponse.getStatus()
              .equals(STATUS_OK))) {
              return Mono.just(new SunriseSunset(geoTimesResponse.getResults().getSunrise(),
                      geoTimesResponse.getResults().getSunset()));
          } else {
              return Mono.error(new GetSunriseSunsetException(SUNRISE_RESULT_NOT_OK));
          }
      });
  }

  open internal fun createResult(geoTimesResponseMono: Mono<GeoTimesResponse>) =
    geoTimesResponseMono.flatMap {
        with(it){
            if (status == STATUS_OK) with(results) { SunriseSunset(sunrise, sunset).toMono() }
            else GetSunriseSunsetException(SUNRISE_RESULT_NOT_OK).toMono()
        }
    }

Kotlin is not only about shortening our code, is about reducing the boilerplate, making our code more comprehensible and easier to maintain

And as a bonus we can take our code to other platforms

Reactive Revolution

• ReactiveX
• Lightbend Akka
• Eclipse Verte.x
• Project Reactor
• Reactive Streams
• Spring WebFlux

Reactive Manifesto

We believe that a coherent approach to systems architecture is needed, and we believe that all necessary aspects are already recognised individually: we want systems that are Responsive, Resilient, Elastic and Message Driven. We call these Reactive Systems.

What is a reactive system

non-reactive system

reactive system

If software is eating the world

Creating a Reactive Microservice

Prerequisites

• JDK 1.8+
• MongoDB 3.4+ (running on localhost:27017)
• Any IDE such: Intellij IDEA Community Edition

Initialize some data

{ "_id": 1, "name": "Kotlin"}
{ "_id": 2, "name": "Reactive"}
{ "_id": 3, "name": "Microservices" }

$ mongoimport --collection customers --drop --file customers.json

$ mvnw spring-boot:run
INFO 1792 --- [localhost:27017] org.mongodb.driver.connection            : Opened connection [connectionId{localValue:1, serverValue:1}] to localhost:27017
INFO 1792 --- [           main] o.s.b.web.embedded.netty.NettyWebServer  : Netty started on port(s): 8080
INFO 1792 --- [           main] c.j.m.R.ReactiveKotlinApplicationKt      : Started ReactiveKotlinApplicationKt in 3.351 seconds (JVM running for 3.721)

package com.juan.medina.ReactiveKotlin

data class Customer(var id : Int = 0, var name : String = "")

@Repository
class CustomerRepository(private val template: ReactiveMongoTemplate) {
  fun findById(id: Int) = template.findById<Customer>(id)
  fun getAll() = template.findAll<Customer>()
}

fun findById(id: Int) = template.findById<Customer>(id)
fun getAll() = template.findAll<Customer>()

@Component
class CustomerHandler(val customerRepository: CustomerRepository) {
  fun getCustomer(serverRequest: ServerRequest) = ServerResponse.ok()
      .body<Customer>(customerRepository.findById(serverRequest.pathVariable("id").toInt()))

  fun getAllCustomers(serverRequest: ServerRequest) = ServerResponse.ok()
      .body<Customer>(customerRepository.getAll())
}

ServerResponse.ok().body<Customer>(Mono<Customer>)
ServerResponse.ok().body<Customer>(Flux<Customer>)

@Component
class CustomerRouter(val customerHandler: CustomerHandler) {
  @Bean
  fun customerRoutes() = router {
    "/customer".nest {
      GET("/{id}", customerHandler::getCustomer)
      GET("/", customerHandler::getAllCustomers)
    }
  }
}

{
	"id" : 1,
	"name" : "Kotlin"
}

[
	{"id": 1, "name" : "Kotlin"},
	{"id": 2, "name" : "Reactive"},
	{"id": 3, "name" : "Microservice"}
]

Now we have a reactive REST API