Introduction to kotlin + spring boot demo

Introduction to Kotlin Riyadh Java Meetup Nov 2019 Mohammad Hewedy mhewedy @mohewedy 1 2

Agenda ● What is Kotlin? ● Language Features ● Demos 2

If I were writing a Spring app today, I would strongly consider Kotlin. It brings the same kind of simplification to Java code as Spring did to J2EE. Rod Johnson. 3

What is Kotlin? ● Statically with type inference and strongly typed(1). ● Multi paradigm (object-oriented and functional) ● Fully Interoperate with Java ● Mainly targets the JVM (targets other runtimes as well) ● Semicolons are optional ● Type comes after the variable (val age: Int = 30) (1) see appendix 1 4

What is Kotlin? ● val keyword used to declare immutable variables ● var keyword used to declare mutable variables ● Classes and class members are public by default ● Classes and methods are final by default ● open keyword used to mark class as extensible and method as overridable 5

What is Kotlin? ● Interface methods are abstract and open by default ● Has package-level functions as well as class-level functions ● Supports extension functions to patch any library classes with new functionality (a.k.a Monkey patching) ● Support limited set of operator overloading using operator keyword 6 operator fun String.not() = this.reversed() println(!"hello") // prints olleh

What is Kotlin? ● Functions in kotlin supports: ○ default arguments ○ named arguments ○ variable-length argument lists ● data classes provides: ○ equals()/hashCode() ○ toString() ○ componentN() functions ○ copy() ○ Destructuring declarations: val (name, age) = jane ● Multiline strings with string interpolation ● Delegated Properties that allows lazy variable evaluation 7

What is Kotlin? ● object used to declare objects, which might used as: ○ singletons ○ companion object ○ Anonymous classes ● Access modifiers: ○ public ○ private ○ protected ○ internal ● Can use to write Domain-Specific Languages (DSL) ● Has Tail call optimization (tail recursion) 8

Code or Data? 9 config { datasource { url = "jdbc:h2:mem" username = "root" } webservice { wsdl = "http://example.org/ws?wsdl" action = "urn:weather-ws" } } source: https://gist.github.com/mhewedy/88d0f63486101501c3d87ff9d2e0231b

Code as Data 10 fun main() { val config = Config() config { datasource { url = "jdbc:h2:mem" username = "root" } webservice { wsdl = "http://example.org/ws?wsdl" action = "urn:weather-ws" } } println(config) } source: https://gist.github.com/mhewedy/88d0f63486101501c3d87ff9d2e0231b

If as an expression ● Traditional usage: 12 var max = a if (a < b) max = b

If as an expression ● Traditional usage with else: 13 var max: Int if (a > b) { max = a } else { max = b }

If as an expression ● As an expression 14 val max = if (a > b) a else b

If as an expression ● As an expression 15 val max = if (a > b) { print("choose a") a } else { print("choose b") b }

When expression 16 when (x) { 1 -> print("x == 1") 2 -> print("x == 2") else -> { // Note the block print("x is neither 1 or 2") } }

When expression 17 when (x) { in 1..10 -> print("x is in range") in validNumbers -> print("x is valid") !in 10..20 -> print("x is outside the range3") else -> print("none of the above") }

When expression 18 fun hasPrefix(x: Any) = when (x) { is String -> x.startsWith("prefix") // note the smart cast else -> false }

When expression 19 when { x.isOdd() -> print("x is in the range") x.isEven() -> print("x is even") else -> print("x is funny") }

When expression ● when scoped variables: 20 fun Request.getBody() = when (val response = executeRequest()) { is Success -> response.body is HttpError -> throw HttpException(response.status) }

For Loops ● For loops used to for iterates through anything that provides an iterator, which means: ○ has a member- or extension-function iterator(), whose return type: ■ has a member- or extension-function next(), and ■ has a member- or extension-function hasNext() that returns Boolean. 21

For Loops 22 for (item: Int in ints) { }

For Loops 23 for (i in 1..3) { println(i) }

For Loops 24 for (i in 6 downTo 0 step 2) { println(i) }

For Loops 25 for (i in 6.downTo(0).step(2)) { println(i) }

Loops and labels ● Kotlin has while loops as well ● Both for and while loops support break and continue (w/o labels) 26 loop@ for (i in 1..10) { for (j in 1..10) { if (...) break@loop } }

For Loops 27 var counter = 0 operator fun Int.iterator() = this operator fun Int.next() = this operator fun Int.hasNext() = counter++ < this // usage: for (n in 20) { println(n) }

Classes 28 class Person constructor(firstName: String)

Classes 29 class Person(val name: String) { var children: MutableList<Person> = mutableListOf(); constructor(name: String, parent: Person) : this(name) { parent.children.add(this) } }

Classes 30 class DontCreateMe private constructor() {/* */ }

Classes 31 val invoice = Invoice() val customer = Customer("Joe Smith")

Inheritance 33 class Example // implicitly inherits from Any

Inheritance 34 open class Base(p: Int) class Derived(p: Int) : Base(p)

Inheritance 35 class MyView : View { constructor(ctx: Context) : super(ctx) constructor(ctx: Context, attrs: AttributeSet) : super(ctx, attrs) }

Inheritance 36 open class Polygon { open fun draw() {} } abstract class Rectangle : Polygon() { abstract override fun draw() }

Inheritance 37 open class Shape { open fun draw() {/* ... */} fun fill() {/* ... */ } } class Circle : Shape() { override fun draw() {/* ... */} }

Inheritance 38 typealias NodeList = ArrayList<Node> sealed class Node object EmptyNode : Node() data class ValueNode(val value: Any?) : Node() data class ObjectNode(val props: ArrayList<Pair<String, Any?>>) : Node() data class ArrayNode(val elements: NodeList) : Node() source: https://github.com/mhewedy/eureka-klient/blob/de7a1bf2ebec72a3bc1c9ca32868f5d93da32765/src/main/kotlin/helpers/json/Deserializer.kt#L30

Inheritance 39 fun eval(expr: Node): Unit = when (expr) { is ValueNode -> println(expr.value) // smart case is ObjectNode -> println(expr.props) is ArrayNode -> expr.elements.forEach { eval(it) } is EmptyNode -> print("empty") // the `else` clause is not required }

Properties and Fields 40 class State(val name: String) class Address { var name: String = "Holmes, Sherlock" var state: State? = null } // used as val address = Address() println("${address.name} at ${address.state?.name}") // note the safe navigation operator with nullable types

Properties and Fields 41 class MyClass { lateinit var subject: TestSubject @Setup fun setup() { subject = TestSubject() } @Test fun test() { subject.method() // dereference directly } }

Interfaces 42 interface MyInterface { fun foo() fun bar() { // optional body } }

Interfaces 43 interface A{ fun foo() { print("A") } fun bar() } interface B{ fun foo() { print("B") } fun bar() { print("bar") } } class C : A { override fun bar() { print("bar") } } class D: A, B { override fun foo() { super<A>.foo() super<B>.foo() } override fun bar() { super.bar() } }

Functions 44 fun double(x: Int): Int { return 2 * x }

Functions 45 fun double(x: Int): Int = x * 2

Functions 46 fun double(x: Int) = x * 2

Functions 47 fun foo(vararg strings: String) { /* ... */} // calling as foo(strings = *arrayOf("a", "b", "c"))

Functions 48 fun read(b: Array<Byte>, off: Int = 0, len: Int = b.size) { /* ... */ }

Functions 49 fun reformat( str: String, normalizeCase: Boolean = true, upperCaseFirstLetter: Boolean = true, divideByCamelHumps: Boolean = false, wordSeparator: Char = ' ' ) { /* ... */ } // calling as reformat(str, normalizeCase = true, upperCaseFirstLetter = true, divideByCamelHumps = false, wordSeparator = '_') // can shuffle args as well

Functions 50 infix fun Int.shl(x: Int): Int { /* ... */ } // calling the function using the infix notation 1 shl 2 // is the same as 1.shl(2)

Functions 51 fun dfs(graph: Graph) { fun dfs(current: Vertex, visited: MutableSet<Vertex>) { if (!visited.add(current)) return for (v in current.neighbors) dfs(v, visited) } dfs(graph.vertices[0], HashSet()) }

Functions 52 class Sample { fun foo() { print("Foo") } }

Functions ● Classes and objects can extend functions 53 object Identity : (Int) -> Int { override fun invoke(p1: Int) = p1 } // called as Identity(10)

Functions ● Lambda with receiver 54 operator fun Config.invoke(block: Config.() -> Unit) { block.invoke(this) }

Functions 55 fun add(x: Int): (Int) -> Int { return fun(y: Int): Int { return x + y } } val result = add(10)(30)

Functions 56 fun add(x: Int): (Int) -> Int { return { y: Int -> x + y } } val addFunc = add(10) val result = addFunc(30)

Functions 57 fun add(x: Int): (Int) -> Int { return { x + it } } val addFunc = add(10) val result = addFunc(30)

Functions 58 fun <T> map(x: String, op: (String) -> T): T { return op(x) } // used as map("John Smith", fun(n: String): Int { return n.length })

Functions 59 fun <T> map(x: String, op: (String) -> T): T { return op(x) } // used as map("John Smith", { it.length })

Functions 60 fun <T> map(x: String, op: (String) -> T): T { return op(x) } // used as map("John Smith") { it.length }

Functions 61 fun <E> List<E>.test(op: (e: E) -> Boolean): List<E> { val ret = arrayListOf<E>() for (element in this) { if (op(element)) ret += element } return ret } // used as employees.test { it.age > 25 }

Functions ● fold is one of most important functional operators 62 fun <T, R> Collection<T>.fold( initial: R, combine: (acc: R, nextElement: T) -> R ): R { var accumulator: R = initial for (element: T in this) { accumulator = combine(accumulator, element) } return accumulator }

fold (a.k.a reduce): how it works 63source: http://perugini.cps.udayton.edu/teaching/books/PL/www/lecture_notes/currying.html

Collections 65 val numbers = mutableListOf("one", "two", "three", "four") numbers += "five" val immutableNumbers = listOf("one", "two", "three", "four") immutableNumbers += "five" // compilation error

Collection Operations ● Transformations ○ Mapping (map) ○ Zipping (zip) ○ Flattening (flatmap) ● Filtering ○ Filtering (filter) ○ Partitioning (partition) ○ Testing predicates (any, none, all) ● Plus and Minus 66

Collection Operations ● Grouping ○ Group By (groupBy) ● Retrieving Collection Parts ○ Take and drop (takeWhile, dropWhile, takeLast, dropLast, take(n), drop(n)) ○ Partitioning (partition) ○ Testing predicates (any, none, all) ● Retrieve: ○ At index: (elementAt, []) ○ By condition: (first, last, find, findLast) 67

Collection Operations ● Order: ○ Sorting: (sortBy, sortedByDescending, reversed, shuffled) ● Aggregate: ○ Built-in: min, max, average, sum, count, maxBy, minBy ○ Custom: reduce, fold 68

Write your test cases in Kotlin 70 @Test fun `Test getContentType for image pdf byte stream`() { val pdfBytes = byteArrayOf(0x25, 0x50, 0x44, 0x46, 0x2d, 0x31, 0x2e, 0x34) // %PDF-1.4 val contentType = StreamUtil. getContentType(ByteArrayInputStream(pngPdfBytes), "abc.png") //give invalid name assertThat(contentType).isEqualTo("application/pdf") }

Test Spring Apps 71 @Test fun `test calling translateToCategories will calculate the agent category correctly`() { // setup val agentRepository = mock(AgentRepository::class.java) val agentCategoryRepository = mock(AgentCategoryRepository::class.java) whenever(agentRepository.findAllById(any())) .thenReturn(listOf(createAgentOfExperience(10), createAgentOfExperience(5))) whenever(agentCategoryRepository.findAll()).thenReturn(dbLookup()) val carrierService = CarrierService(agentRepository, agentCategoryRepository) // SUT // when val translatedList: List<Long> = carrierService.translateToCategories() // then assertThat(translatedList).asList().containsExactlyInAnyOrder(CAT_5, CAT_6) }

● https://github.com/mhewedy/eureka-klient (pure Kotlin app) ● https://gitlab.com/ticketz/ticketz-api (Kotlin + spring boot) ● https://github.com/mhewedy/condition-engine (Kotlin + java + spring boot) 73

1. Strong vs Weak types 75 // Kotlin (strong typed) fun main() { val x: Float = 10.5f val y: Int = x // compilation error println("y = $y") } // C (weak typed) #include <stdio.h> int main() { float x = 10.5; int y = x; printf("y = %dn", y); // prints 10 }

Resources ● https://www.wikiwand.com/en/Kotlin_(programming_language) ● https://kotlinlang.org/docs/reference/ (very good resource) ● http://shop.oreilly.com/product/0636920052982.do ● http://shop.oreilly.com/product/0636920052999.do ● https://www.manning.com/books/kotlin-in-action ● https://medium.com/sngular-devs/from-closures-to-curry-and-compose-197d2abcadd8 77

Introduction to kotlin + spring boot demo

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