[PPT] - Kotlin 1.4 Language Features Svetlana Isakova October 12, 2020 PowerPoint Presentation

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Kotlin 1.4 Online Event October 12, 2020 @sveta_isakova

Kotlin 1.4 Language Features Svetlana Isakova

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Kotlin 1.4 Language Features

SAM conversions for Kotlin classes
Explicit API mode
Trailing comma
Break and continue inside when expressions
Mixing named and positional arguments
New type inference
Unified exception type for null checks

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SAM conversions for Kotlin interfaces

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SAM = Single abstract method

interface Action { fun run() }

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SAM conversion

Ability to pass a lambda or callable reference when a SAM interface is expected

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runAction { println("I'm Kotlin 1.3") }

SAM conversion for a Java interface

public interface Action { void run(); } public static void runAction(Action action) { action.run(); }

Java: Kotlin:

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runAction { println("I'm Kotlin 1.3") }

SAM conversion for a Kotlin interface

interface Action { fun run() } fun runAction(a: Action) = a.run()

Java: Kotlin: Kotlin: Doesn’t work. Please use function types!

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KT-7770 SAM for Kotlin classes

SAM conversion only works for Java interfaces That’s by design: everyone should use functional types But that is inconvenient!

211 50+comments

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fun interface Action { fun run() }

Functional interfaces

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fun interface Action { fun run() } fun runAction(a: Action) = a.run() runAction { println("Hello, Kotlin 1.4!") }

Functional interfaces

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fun interface Action { fun run() fun runWithDelay() }

Why functional interfaces?

No longer a SAM interface!

Fun interfaces must have exactly one abstract method

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Explicit API mode

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Kotlin style guide

kotlinlang.org Ability to pass a lambda or callable reference when a SAM interface is expected

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Specify member visibility

fun privateFun() { … }

It’s PUBLIC! to avoid accidentally exposing declarations as public APIs

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Specify member visibility

private fun privateFun() { … } public fun publicFun() { … }

to avoid accidentally exposing declarations as public APIs

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Explicitly specify function return types and property types

fun getAnswer(finished: Boolean) = if (finished) "42" else "unknown" : String

to avoid accidentally changing the return type when the implementation changes

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Explicitly specify function return types and property types

to avoid accidentally changing the return type when the implementation changes

: Any fun getAnswer(finished: Boolean) = if (finished) 42 else "unknown"

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fun getAnswer(finished: Boolean) = if (finished) 42 else "unknown"

Explicit API mode

The visibility must be specified in explicit API mode The return type must be specified in explicit API mode

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public fun getAnswer(finished: Boolean): Any = if (finished) 42 else "unknown"

Explicit API mode

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Strict mode: reports errors

kotlin { explicitApi() } kotlin { explicitApi = 'strict' }

build.gradle: build.gradle.kts:

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Warning mode: reports warnings

kotlin { explicitApiWarning() } kotlin { explicitApi = 'warning' }

build.gradle: build.gradle.kts:

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Trailing comma

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Argument list

val colors = listOf( "red", "green", "blue" )

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Argument list

val colors = listOf( "red", "blue" "green", )

Compiler error: Expecting ','

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Argument list

val colors = listOf( "red", "green", "blue", )

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Argument list

val colors = listOf( "red", "green", "blue", )

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Argument list

val colors = listOf( "red", "blue", "green", )

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Function declaration

fun displayRectangle( color: Color, width: Int, height: Int, ) { ../ }

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Class declaration

data class Contact( val address: String, val phoneNumber: String, val email: String, )

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Break & continue in when expressions

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Kotlin 1.3 forbids break and continue in when expressions

fun foo(list: List<Int>) { for (i in list) { when (i) { 42 -? continue else -? println(i) } } }

'break' and 'continue' are not allowed in 'when' statements. Consider using labels to continue/break from the outer loop.

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Workaround: using labels

fun foo(list: List<Int>) { l@ for (i in list) { when (i) { 42 -? continue@l else -? println(i) } } }

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Kotlin 1.4 allows break and continue in when expressions

fun foo(list: List<Int>) { for (i in list) { when (i) { 42 -? continue else -? println(i) } } }

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Kotlin 1.4 allows break and continue in when expressions

fun foo(list: List<Int>) { for (i in list) { when (i) { 42 -? break else -? println(i) } } }

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Mixing named and positional arguments

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Kotlin 1.3: using named args

drawRectangle( width = 10, height = 20, color = Color.BLUE )

redundant helpful helpful

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Kotlin 1.3: mixing not allowed

drawRectangle( width = 10, height = 20, Color.BLUE )

Mixing named and positioned arguments is not allowed.

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Kotlin 1.4

drawRectangle( width = 10, height = 20, color = Color.BLUE )

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New Type Inference

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