Kotlin For Android - Constructors and Control Flow (part 2 of 7)

Kotlin for Android
May 24, 2018
y/kotlin-workbook

Objects, Constructors and Control Flow
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classes
BASIC SYNTAX
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interface Runnable {
fun run()
}
interface RepeatingRunnable : Runnable {
fun run(times: Int) {
(1..times).forEach { run() }
}
}
open class Greeting : Runnable {
override fun run() = println("Hi!")
}
class RepeatingGreeting : Greeting(), RepeatingRunnable
RepeatingGreeting().run(3)

● One primary constructor as part of class declaration
○ Declare property visibility and default values in the
parameter list
constructors
OBJECTS AND FLOW
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class Person(
val name: String = "John",
private var age: Int
)

parameter list
● Initialization blocks - init
constructors
OBJECTS AND FLOW
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class Person(
){
init {
// Initialize things here...
}
}

parameter list
● Initialization blocks - init
● Optional secondary constructors
● Property initialization
constructors
OBJECTS AND FLOW
y/kotlin-workbook
class Person(
){
var hairColor: Color = Color.BLACK
init {
// Initialize things here...
}
constructor(age: Int, hairColor: Color): this(age = age){
this.hairColor = hairColor
}
}

constructors
OBJECTS AND FLOW
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DEMO

object
OBJECTS AND FLOW
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● Used to create anonymous inner classes and singletons
● Can have static methods in the JVM
● Can have static fields in the JVM
● Every class can have a companion object
● Cannot inherit from object
○ object can inherit and extend though

object
OBJECTS AND FLOW
y/kotlin-workbook
object Foo {
val BAR = "BAR"
fun hi() = "hi"
}
// kotlin
val bar = Foo.BAR
val foo = Foo.hi()
// java
String bar = Foo.INSTANCE.BAR;
String foo = Foo.INSTANCE.hi();

object
OBJECTS AND FLOW
y/kotlin-workbook
object Foo {
const val BAR = "BAR"
@JvmStatic fun hi() = "hi"
}
// kotlin
val bar = Foo.BAR
val foo = Foo.hi()
// java
String bar = Foo.INSTANCE.BAR;
String foo = Foo.INSTANCE.hi();
// java improved interop
String bar = Foo.BAR;
String foo = Foo.hi();

companion object
advanced
OBJECTS AND FLOW
y/kotlin-workbook
UserRepo.intance() // returns the real repo
BizRepo.intance() // returns the real repo
UserRepo.mock = mock(UserRepo)
BizRepo.mock = mock(BizRepo)
UserRepo.intance() // returns the mock repo
BizRepo.intance() // returns the mock repo
UserRepo.mock = null
BizRepo.mock = null

companion object
advanced
OBJECTS AND FLOW
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interface Factory<T: Any> {
var mock: T?
fun instance(): T
}
BizRepo.mock = null

companion object
advanced
OBJECTS AND FLOW
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var mock: T?
fun instance(): T
}
open class Provider<T: Any>(init: () -> T): Factory<T> {
override var mock: T? = null
private val instance: T by lazy(init)
override fun instance() = mock ?: instance
}
BizRepo.mock = null

companion object
advanced
OBJECTS AND FLOW
y/kotlin-workbook
var mock: T?
fun instance(): T
}
open class Provider<T: Any>(init: () -> T): Factory<T> {
override var mock: T? = null
private val instance: T by lazy(init)
override fun instance() = mock ?: instance
}
class UserRepo {
companion object : Provider<UserRepo>({ UserRepo() })
}
class BizRepo {
companion object : Provider<BizRepo>({ BizRepo() })
}
BizRepo.mock = null

● No goto
Control Flow
OBJECTS AND FLOW
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Control Flow
OBJECTS AND FLOW
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fun foo() {
listOf(1, 2, 3, 4, 5).forEach {
if (it == 3) return
print(it)
}
println(" this is unreachable")
}
● No goto
● return, break and continue

Control Flow
OBJECTS AND FLOW
y/kotlin-workbook
fun foo() {
if (it == 3) return
print(it)
}
println(" this is unreachable")
}
// 12
● No goto
● return, break and continue

Control Flow
OBJECTS AND FLOW
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fun foo() {
if (it == 3) return@forEach
print(it)
}
println(" this is reachable")
}
● No goto
● return, break and continue can use labels

Control Flow
OBJECTS AND FLOW
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fun foo() {
if (it == 3) return@forEach
print(it)
}
}
// 1245 this is reachable
● No goto

Control Flow
OBJECTS AND FLOW
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fun foo() {
listOf(1, 2, 3, 4, 5).forEach loop@{
if (it == 3) return@loop
print(it)
}
}
● No goto

Control Flow
OBJECTS AND FLOW
y/kotlin-workbook
fun foo() {
listOf(1, 2, 3, 4, 5).run { forEach {
if (it == 3) return@run
print(it)
}}
}
● No goto

Control Flow
OBJECTS AND FLOW
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fun foo() {
for (int in listOf(1, 2, 3, 4, 5)) {
if (int == 3) break
print(int)
}
}
● No goto
● for loop - no parameterized for(;;)

Control Flow
OBJECTS AND FLOW
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● No goto
● for loop - no parametrized for(;;)
● while - same as java

Control Flow
OBJECTS AND FLOW
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return if (isJvmRuntime) {
RuntimeEnvironment.application
} else {
InstrumentationRegistry.getContext()
}
● No goto
● if - same as java, can be used as an expression

Control Flow
OBJECTS AND FLOW
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return when (isJvmRuntime) {
true -> RuntimeEnvironment.application
else -> InstrumentationRegistry.getContext()
}
● No goto
● when - replaces switch from java, can be used as expr.

Control Flow
OBJECTS AND FLOW
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val z = when {
x.isOdd() -> "x is odd"
y.isEven() -> "y is even"
else -> "x is funny, y is cool"
}
● No goto

Control Flow
OBJECTS AND FLOW
y/kotlin-workbook
val isJvmRuntime = try {
Class.forName("org.robolectric.RuntimeEnvironment")
true
} catch (notFound: ClassNotFoundException) {
false
}
● No goto
● try catch - same as java, can be used as an expression

Kotlin For Android - Constructors and Control Flow (part 2 of 7)

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Editor's Notes