This document discusses various aspects of control flow and objects in Kotlin including: - Constructors can be declared as part of a class and initialize properties. Initialization blocks and secondary constructors are also supported. - Objects can be used to create singletons and anonymous classes. Companion objects allow associating static-like functionality with a class. - Control flow avoids goto and supports return, break, continue with labels. Conditional expressions like if and when can be used anywhere expressions are expected. - Basic control structures like for loops, while loops, and try/catch work similarly to Java but offer additional functionality like expression bodies.

1. Kotlin for Android
May 24, 2018
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2. Objects, Constructors and Control Flow
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3. 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)

4. ● 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
)

5. ● One primary constructor as part of class declaration
○ Declare property visibility and default values in the
parameter list
● Initialization blocks - init
constructors
OBJECTS AND FLOW
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class Person(
val name: String = "John",
private var age: Int
){
init {
// Initialize things here...
}
}

6. ● One primary constructor as part of class declaration
○ Declare property visibility and default values in the
parameter list
● Initialization blocks - init
● Optional secondary constructors
● Property initialization
constructors
OBJECTS AND FLOW
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class Person(
val name: String = "John",
private var age: Int
){
var hairColor: Color = Color.BLACK
init {
// Initialize things here...
}
constructor(age: Int, hairColor: Color): this(age = age){
this.hairColor = hairColor
}
}

7. constructors
OBJECTS AND FLOW
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DEMO

8. 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

9. object
OBJECTS AND FLOW
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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();

10. object
OBJECTS AND FLOW
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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();

11. companion object
advanced
OBJECTS AND FLOW
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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
UserRepo.intance() // returns the real repo
BizRepo.intance() // returns the real repo

12. companion object
advanced
OBJECTS AND FLOW
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interface Factory<T: Any> {
var mock: T?
fun instance(): T
}
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
UserRepo.intance() // returns the real repo
BizRepo.intance() // returns the real repo

13. companion object
advanced
OBJECTS AND FLOW
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interface Factory<T: Any> {
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
}
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
UserRepo.intance() // returns the real repo
BizRepo.intance() // returns the real repo

14. companion object
advanced
OBJECTS AND FLOW
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interface Factory<T: Any> {
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() })
}
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
UserRepo.intance() // returns the real repo
BizRepo.intance() // returns the real repo

15. ● No goto
Control Flow
OBJECTS AND FLOW
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16. 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

17. 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")
}
// 12
● No goto
● return, break and continue

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

19. Control Flow
OBJECTS AND FLOW
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fun foo() {
listOf(1, 2, 3, 4, 5).forEach {
if (it == 3) return@forEach
print(it)
}
println(" this is reachable")
}
// 1245 this is reachable
● No goto
● return, break and continue can use labels

20. 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)
}
println(" this is reachable")
}
// 1245 this is reachable
● No goto
● return, break and continue can use labels

21. Control Flow
OBJECTS AND FLOW
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fun foo() {
listOf(1, 2, 3, 4, 5).run { forEach {
if (it == 3) return@run
print(it)
}}
println(" this is reachable")
}
// 12 this is reachable
● No goto
● return, break and continue can use labels

22. 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)
}
println(" this is reachable")
}
● No goto
● return, break and continue can use labels
● for loop - no parameterized for(;;)

23. Control Flow
OBJECTS AND FLOW
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● No goto
● return, break and continue can use labels
● for loop - no parametrized for(;;)
● while - same as java

24. Control Flow
OBJECTS AND FLOW
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return if (isJvmRuntime) {
RuntimeEnvironment.application
} else {
InstrumentationRegistry.getContext()
}
● No goto
● return, break and continue can use labels
● for loop - no parametrized for(;;)
● while - same as java
● if - same as java, can be used as an expression

25. Control Flow
OBJECTS AND FLOW
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return when (isJvmRuntime) {
true -> RuntimeEnvironment.application
else -> InstrumentationRegistry.getContext()
}
● No goto
● return, break and continue can use labels
● for loop - no parametrized for(;;)
● while - same as java
● if - same as java, can be used as an expression
● when - replaces switch from java, can be used as expr.

26. 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
● return, break and continue can use labels
● for loop - no parametrized for(;;)
● while - same as java
● if - same as java, can be used as an expression
● when - replaces switch from java, can be used as expr.

27. Control Flow
OBJECTS AND FLOW
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val isJvmRuntime = try {
Class.forName("org.robolectric.RuntimeEnvironment")
true
} catch (notFound: ClassNotFoundException) {
false
}
● No goto
● return, break and continue can use labels
● for loop - no parametrized for(;;)
● while - same as java
● if - same as java, can be used as an expression
● when - replaces switch from java, can be used as expr.
● try catch - same as java, can be used as an expression

28. Questions?
Next Up: Functions