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

Functions
57
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
return op(x)
}
// used as
map("John Smith", { it.length })

Functions
60
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

● 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

● 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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