Kotlin for Java Developers (GDG Leeds June 2017)

Transcript

1. KOTLIN FOR JAVA DEVELOPERS DARREN ATHERTON
2. KOTLIN FOR JAVA DEVELOPERS WHAT IS KOTLIN? ▸ Statically-typed JVM language from JetBrains. ▸ OOP language with functional aspects. ▸ Focuses on safe, concise code while maintaining (and increasing) readability. ▸ First-class tooling support. ▸ Now officially supported by Google. KOTLIN
3. KOTLIN OPERABILITY WITH JAVA TECHNOLOGIES ▸ Android ▸ Android Studio 2.3 - requires plugin installation. ▸ Android Studio 3.0 (alpha) - no setup required.
 ▸ Spring/Spring Boot ▸ Spring Framework 5.0 - dedicated Kotlin support using Kotlin Extension functions. ▸ Spring Boot Initializr - can choose Kotlin as a language: https://start.spring.io
 ▸ Eclipse Vert.x KOTLIN FOR JAVA DEVELOPERS KOTLIN
4. PROPERTIES KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Properties are fields with auto-generated getters and setters. val name: String = "Darren" ▸ Immutable, equivalent to final var name: String = "Darren" name = "Still Darren" ▸ Mutable val name = "Darren" ▸ Types are auto-inferred ▸ Prefer val over var - code with immutable fields is less likely to contain bugs!
5. VISIBILITY MODIFIERS - CLASS LEVEL KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ For members declared inside a class: ▸ Private - member is visible inside the class only. ▸ Protected - member has the same rules as private but is also available in subclasses. ▸ Internal - any member inside the same module can see internal members ▸ Public - any member who can see the class can see it’s public members. ▸ Default visibility is public.
6. VISIBILITY MODIFIERS - TOP LEVEL KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ For declarations declared inside at the top level: ▸ Private - declaration only visible in the same file. ▸ Protected - not available as the top level is independent of any class hierarchy. ▸ Internal - declaration visible only in the same module. ▸ Public - declaration is visible everywhere. ▸ Default visibility is public.
7. internal fun sum(first: Int, second: Int): Int { return first + second } FUNCTION SYNTAX KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Access modifier ▸ Name ▸ Param name/type ▸ Return type ▸ Keyword
8. FUNCTION SYNTAX - VARIATIONS KOTLIN FOR JAVA DEVELOPERS internal fun sum(first: Int, second: Int): Int { return first + second } KOTLIN ▸ Full syntax
9. FUNCTION SYNTAX KOTLIN FOR JAVA DEVELOPERS internal fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int { return first + second } KOTLIN ▸ Full syntax ▸ Omit access modifier FUNCTION SYNTAX - VARIATIONS
10. FUNCTION SYNTAX KOTLIN FOR JAVA DEVELOPERS internal fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int = first + second KOTLIN ▸ Full syntax ▸ Omit access modifier ▸ Inline return FUNCTION SYNTAX - VARIATIONS
11. FUNCTION SYNTAX KOTLIN FOR JAVA DEVELOPERS internal fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int = first + second fun sum(first: Int, second: Int) = first + second KOTLIN ▸ Full syntax ▸ Omit access modifier ▸ Inline return ▸ Omit return type FUNCTION SYNTAX - VARIATIONS
12. FUNCTION SYNTAX KOTLIN FOR JAVA DEVELOPERS internal fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int { return first + second } fun sum(first: Int, second: Int): Int = first + second fun sum(first: Int, second: Int) = first + second val sum = { first: Int, second: Int -> first + second } val sum: (Int, Int) -> Int = { first, second -> first + second } KOTLIN ▸ Full syntax ▸ Omit access modifier ▸ Inline return ▸ Omit return type ▸ As a type FUNCTION SYNTAX - VARIATIONS
13. internal fun sum(first: Int, second: Int): Int { return first + second } FUNCTIONS - NAMED PARAMETERS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Parameter name ▸ Parameter type val total = sum(first = 2, second = 2) val total = sum(second = 2, first = 2)
14. FUNCTIONS - DEFAULT AND OPTIONAL PARAMETERS KOTLIN FOR JAVA DEVELOPERS KOTLIN internal fun sum(first: Int = 2, second: Int): Int { return first + second } val total = sum(second = 2)
15. FUNCTIONS - DEFAULT AND OPTIONAL PARAMETERS KOTLIN FOR JAVA DEVELOPERS JAVA void showText(String text, float fontSize, String typeface) { }
16. FUNCTIONS - DEFAULT AND OPTIONAL PARAMETERS KOTLIN FOR JAVA DEVELOPERS JAVA void showText(String text, float fontSize, String typeface) { } void showText(String text, String typeface) { showText(text, DEFAULT_FONT_SIZE, typeface); }
17. FUNCTIONS - DEFAULT AND OPTIONAL PARAMETERS KOTLIN FOR JAVA DEVELOPERS JAVA void showText(String text, float fontSize, String typeface) { } void showText(String text, String typeface) { showText(text, DEFAULT_FONT_SIZE, typeface); } void showText(String text, float fontSize) { showText(text, fontSize, DEFAULT_TYPEFACE); }
18. FUNCTIONS - DEFAULT AND OPTIONAL PARAMETERS KOTLIN FOR JAVA DEVELOPERS JAVA void showText(String text, float fontSize, String typeface) { } void showText(String text, String typeface) { showText(text, DEFAULT_FONT_SIZE, typeface); } void showText(String text, float fontSize) { showText(text, fontSize, DEFAULT_TYPEFACE); } void showText(String text) { showText(text, DEFAULT_FONT_SIZE, DEFAULT_TYPEFACE); }
19. FUNCTIONS - DEFAULT AND OPTIONAL PARAMETERS KOTLIN FOR JAVA DEVELOPERS KOTLIN fun showText(text: String, fontSize: Float = DEFAULT_FONT_SIZE, typeface: String = DEFAULT_TYPEFACE) {} ▸ Defaults showText("hello world") showText("hello world", 16f) showText("hello world", 16f, "Arial") showText("hello world", typeface = "Arial") showText("hello world", fontSize = 16f) showText(text = "hello world", fontSize = 16f, typeface = "Arial") ▸ Using order ▸ Using names
20. HIGHER-ORDER FUNCTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Functions which return a function or take a function as a parameter ▸ Predominantly used via lambdas ▸ Available at the Java 6 level (great for Android!) fun <T> Iterable<T>.filter(predicate: (T) -> Boolean): List<T> { return filterTo(ArrayList<T>(), predicate) } val list: List<String> = listOf("Darren") val filtered = list.filter { it.startsWith("D") } val filtered = list.filter { singleString -> singleString.startsWith("D") }
21. NULLABILITY KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN ▸ In Java, the NullPointerException is one of the biggest sources of errors. ▸ In Kotlin, ‘null’ is part of the type system. ▸ For each property, we can declare whether it can contain a nullable value. ▸ For each function, we can declare whether it returns a nullable value. ▸ This eliminates the NullPointerException*.
22. NULLABILITY KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN String myString = generateSomeString(); // method can return null String myString2 = myString.replace("old", "new"); ▸ Throws NPE!
23. NULLABILITY KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN String myString = generateSomeString(); // method can return null String myString2 = myString.replace("old", "new"); ▸ Throws NPE! val myString: String? = generateSomeString() val myString2 = myString.replace("old", "new") ▸ Does not compile
24. NULLABILITY KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN String myString = generateSomeString(); // method can return null String myString2 = myString.replace("old", "new"); ▸ Throws NPE! val myString: String? = generateSomeString() val myString2 = myString.replace("old", "new") ▸ Does not compile if (myString != null) { val myString2 = myString.replace("old", "new") } ▸ Compiles!
25. HANDLING NULL VALUES KOTLIN FOR JAVA DEVELOPERS KOTLIN if (myString != null) { val myString2 = myString.replace("old", "new") } ▸ Classic null-check
26. HANDLING NULL VALUES KOTLIN FOR JAVA DEVELOPERS KOTLIN if (myString != null) { val myString2 = myString.replace("old", "new") } val myString2 = myString?.replace("old", "new") ▸ Classic null-check ▸ Safe-call operator
27. HANDLING NULL VALUES KOTLIN FOR JAVA DEVELOPERS KOTLIN if (myString != null) { val myString2 = myString.replace("old", "new") } val myString2 = myString?.replace("old", "new") val myString2 = myString?.replace("old", "new") ?: "default" ▸ Classic null-check ▸ Safe-call operator ▸ Elvis operator
28. NULLABILITY KOTLIN FOR JAVA DEVELOPERS @Nullable public Account getAccountForUsername(@Nullable String username) { if (username != null) { return accountManager.getAccount(username); } else { return null; } } JAVA/KOTLIN
29. NULLABILITY KOTLIN FOR JAVA DEVELOPERS @Nullable public Account getAccountForUsername(@Nullable String username) { if (username != null) { return accountManager.getAccount(username); } else { return null; } } fun getAccountForUsername(username: String?): Account? { return if (username != null) { accountManager.getAccount(username) } else { null } } JAVA/KOTLIN
30. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS public class VideoGame { private String name; private String publisher; private int reviewScore; public VideoGame(String name, String publisher, int reviewScore) { this.name = name; this.publisher = publisher; this.reviewScore = reviewScore; } } JAVA
31. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS public String getName() {} public void setName(String name) {} public String getPublisher() {} public void setPublisher(String publisher) {} public int getReviewScore() {} public void setReviewScore(int reviewScore) {} JAVA
32. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; VideoGame that = (VideoGame) o; if (reviewScore != that.reviewScore) return false; if (name != null ? !name.equals(that.name) : that.name != null) { return false; } return publisher != null ? publisher.equals(that.publisher) : that.publisher == null; } JAVA
33. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS @Override public int hashCode() { int result = name != null ? name.hashCode() : 0; result = 31 * result + (publisher != null ? publisher.hashCode() : 0); result = 31 * result + reviewScore; return result; } @Override public String toString() { return "VideoGame{" +"name='" + name + '\'' + ", publisher='" + publisher + '\'' + ", reviewScore=" + reviewScore +'}'; } JAVA
34. ▸ By specifying the ‘data’ keyword, we get all of this for free and some extras. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS KOTLIN data class VideoGame(val name: String, val publisher: String, var reviewScore: Int)
35. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS KOTLIN val game = VideoGame("Gears of War", "Epic Games", 8) ▸ Data classes are instantiated in the same way as standard classes:
36. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS KOTLIN val game = VideoGame("Gears of War", "Epic Games", 8) ▸ Data classes are instantiated in the same way as standard classes: ▸ We can now access the members of game: print(game.name) // "Gears of War" print(game.publisher) // "Epic Games" print(game.reviewScore) // 8 game.reviewScore = 7
37. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ We can pretty-print the contents of the class via toString(): /* Prints: * Game(name=Gears Of War, publisher=Epic Games, reviewScore=7) */ print(game.toString())
38. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ We can pretty-print the contents of the class via toString(): ▸ We can copy an object and specify new values for specific parameters: /* Prints: * Game(name=Gears Of War, publisher=Epic Games, reviewScore=7) */ print(game.toString()) val game = VideoGame("Gears of War", "Epic Games", 8) val betterGame = game.copy(reviewScore = 10)
39. DATA CLASSES KOTLIN FOR JAVA DEVELOPERS package com.kotlinreference.samples.dataclasses; public class VideoGamePojo { private String name; private String publisher; private int reviewScore; public VideoGamePojo(String name, String publisher, int reviewScore) { this.name = name; this.publisher = publisher; this.reviewScore = reviewScore; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getPublisher() { return publisher; } public void setPublisher(String publisher) { this.publisher = publisher; } public int getReviewScore() { return reviewScore; } public void setReviewScore(int reviewScore) { this.reviewScore = reviewScore; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; VideoGamePojo that = (VideoGamePojo) o; if (reviewScore != that.reviewScore) return false; if (name != null ? !name.equals(that.name) : that.name != null) { return false; } return publisher != null ? publisher.equals(that.publisher) : that.publisher == null; } @Override public int hashCode() { int result = name != null ? name.hashCode() : 0; result = 31 * result + (publisher != null ? publisher.hashCode() : 0); result = 31 * result + reviewScore; return result; } @Override public String toString() { return "VideoGamePojo{" + "name='" + name + '\'' + ", publisher='" + publisher + '\'' + ", reviewScore=" + reviewScore + '}'; } } package com.kotlinreference.samples.dataclasses data class VideoGame(val name: String, val publisher: String, var reviewScore: Int) JAVA/KOTLIN
40. WHEN KOTLIN FOR JAVA DEVELOPERS fun evaluateNumber(num: Int) { when (num) { 26 -> print("num is exactly 26") 41 -> print("num is exactly 41") in 0..10 -> print("num is in the range 0 - 10") else -> print("none of the above") } } KOTLIN ▸ Branching by value:
41. WHEN KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Branching by type: fun evaluateObject(obj: Any): String { return when (obj) { is String -> "obj is a String" is Int -> "obj is an Int" is Float -> "obj is a Float" else -> "obj is something else" } }
42. WHEN KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Branching with expressions: fun evaluateNumber(num: Int) { when { num.isEven() -> print("num is even") num.isOdd() -> print("num is odd") else -> { print("the universe is broken") } } }
43. SMART CASTING KOTLIN FOR JAVA DEVELOPERS open class FeedItem class Tweet : FeedItem() class RedditPost : FeedItem() class DribbblePost : FeedItem() open class FeedScreen class TweetScreen(val tweet: Tweet) : FeedScreen() class RedditScreen(val redditPost: RedditPost) : FeedScreen() class DribbbleScreen(val dribbblePost: DribbblePost) : FeedScreen() KOTLIN
44. SMART CASTING KOTLIN FOR JAVA DEVELOPERS public FeedScreen getScreenOnClick(FeedItem feedItem) { if (feedItem instanceof Tweet) { Tweet tweet = (Tweet) feedItem; return new TweetScreen(tweet); } else if (feedItem instanceof RedditPost) { RedditPost redditPost = (RedditPost) feedItem; return new RedditScreen(redditPost); } else if (feedItem instanceof DribbblePost) { DribbblePost dribbblePost = (DribbblePost) feedItem; return new DribbbleScreen(dribbblePost); } else { return null; } } JAVA
45. SMART CASTING KOTLIN FOR JAVA DEVELOPERS fun getScreenOnClick(feedItem: FeedItem): FeedScreen? { return if (feedItem is Tweet) { TweetScreen(feedItem) } else if (feedItem is RedditPost) { RedditScreen(feedItem) } else if (feedItem is DribbblePost) { DribbbleScreen(feedItem) } else { null } } KOTLIN
46. fun getScreenOnClick(feedItem: FeedItem): FeedScreen? { return if (feedItem is Tweet) { TweetScreen(feedItem) } else if (feedItem is RedditPost) { RedditScreen(feedItem) } else if (feedItem is DribbblePost) { RedditScreen(feedItem) } else { null } } SMART CASTING KOTLIN FOR JAVA DEVELOPERS KOTLIN
47. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS public FeedScreen getScreenOnClick(FeedItem feedItem) { if (feedItem instanceof Tweet) { Tweet tweet = (Tweet) feedItem; return new TweetScreen(tweet); } else if (feedItem instanceof RedditPost) { RedditPost redditPost = (RedditPost) feedItem; return new RedditScreen(redditPost); } else if (feedItem instanceof DribbblePost) { DribbblePost dribbblePost = (DribbblePost) feedItem; return new DribbbleScreen(dribbblePost); } else { return null; // not meaningful! } } JAVA
48. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS public FeedScreen getScreenOnClick(FeedItem feedItem) { if (feedItem instanceof Tweet) { Tweet tweet = (Tweet) feedItem; return new TweetScreen(tweet); } else if (feedItem instanceof RedditPost) { RedditPost redditPost = (RedditPost) feedItem; return new RedditScreen(redditPost); } else { // still not meaningful! DribbblePost dribbblePost = (DribbblePost) feedItem; return new DribbbleScreen(dribbblePost); } } JAVA
49. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS open class FeedItem class Tweet : FeedItem() class RedditPost : FeedItem() class DribbblePost : FeedItem() open class FeedScreen class TweetScreen(val tweet: Tweet) : FeedScreen() class RedditScreen(val redditPost: RedditPost) : FeedScreen() class DribbbleScreen(val dribbblePost: DribbblePost) : FeedScreen() KOTLIN
50. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS sealed class FeedItem { class Tweet : FeedItem() class RedditPost : FeedItem() class DribbblePost : FeedItem() } sealed class FeedScreen { class TweetScreen(val tweet: Tweet) : FeedScreen() class RedditScreen(val redditPost: RedditPost) : FeedScreen() class DribbbleScreen(val dribbblePost: DribbblePost) : FeedScreen() } KOTLIN
51. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS fun getScreenOnClick(feedItem: FeedItem): FeedScreen { when (feedItem) { is Tweet -> return TweetScreen(feedItem) is RedditPost -> return RedditScreen(feedItem) is DribbblePost -> return DribbbleScreen(feedItem) } } KOTLIN
52. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS fun getScreenOnClick(feedItem: FeedItem): FeedScreen { return when (feedItem) { is Tweet -> TweetScreen(feedItem) is RedditPost -> RedditScreen(feedItem) is DribbblePost -> DribbbleScreen(feedItem) } } KOTLIN
53. SEALED CLASSES KOTLIN FOR JAVA DEVELOPERS fun getScreenOnClick(feedItem: FeedItem) = when (feedItem) { is Tweet -> TweetScreen(feedItem) is RedditPost -> RedditScreen(feedItem) is DribbblePost -> DribbbleScreen(feedItem) } KOTLIN
54. RETURN/ASSIGN EVERYTHING KOTLIN FOR JAVA DEVELOPERS fun getScreenOnClick(feedItem: FeedItem) = when (feedItem) { is Tweet -> TweetScreen(feedItem) is RedditPost -> RedditScreen(feedItem) is DribbblePost -> DribbbleScreen(feedItem) } KOTLIN fun sum(first: Int = 2, second: Int) = first + second val fullName = if (surname != null) { "$firstName $surname" } else { firstName }
55. WITH() KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN ▸ ‘With’ is used when we are calling multiple methods on the same object. ▸ Acts as if we are ‘inside’ the objects’ scope. void printGame(VideoGame game) { print(game.getName()); print(game.getPublisher()); print(game.getReviewScore()); } fun printGame(game: VideoGame) = with(game) { print(name) print(publisher) print(reviewScore) }
56. LET() KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ A scoping function which gives us access to an object inside the defined scope but not beyond. storageProvider.provideDatabase().let { database -> // database only available inside this scope } fun getGameByName(name: String?): VideoGame? { return name?.let { database.getGame(name) } } ▸ When combined with ‘?’, ‘let’ smart-casts your nullable field to a non-null field: ▸ Now non-nullable
57. RUN() KOTLIN FOR JAVA DEVELOPERS KOTLIN fun printGame(game: VideoGame?) = game?.run { print(name) print(publisher) print(reviewScore) } ▸ ‘Run’ is a combination of ‘with’ and ‘let’. ▸ The null-check power of ‘let’ and the scoping of ‘with’.
58. APPLY() KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN public File makeFile() { File file = new File("path"); file.mkdirs(); file.setReadable(true); file.setWritable(false); return file; } fun makeFile() = File("path").apply { mkdirs() setReadable(true) setWritable(false) } ▸ ‘Apply’ defines an extension on all types. ▸ The object you call apply on is passed into the closure, where you can work on it, and then returns the same object.
59. USE() KOTLIN FOR JAVA DEVELOPERS JAVA/KOTLIN Properties properties = new Properties(); try (FileInputStream stream = new FileInputStream("config.properties")) { properties.load(stream); } // 'stream' automatically closed val properties = Properties() FileInputStream("config.properties").use { properties.load(it) } // stream automatically closed ▸ Kotlin version of Java ‘try-with-resources’ (Java 1.7+)
60. LAZILY-INITIALISED SINGLETONS KOTLIN FOR JAVA DEVELOPERS class MySingleton { private static MySingleton instance; @Nullable String myString = null; private MySingleton() {} static MySingleton getInstance() { if (instance == null) { instance = new MySingleton(); } return instance; } } MySingleton.getInstance().myString = "hello"; JAVA/KOTLIN
61. LAZILY-INITIALISED SINGLETONS KOTLIN FOR JAVA DEVELOPERS object MySingleton { var myString: String? = null } MySingleton.myString = "hello" class MySingleton { private static MySingleton instance; @Nullable String myString = null; private MySingleton() {} static MySingleton getInstance() { if (instance == null) { instance = new MySingleton(); } return instance; } } MySingleton.getInstance().myString = "hello"; ▸ MySingleton is initialised at this point JAVA/KOTLIN
62. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ The Collections API - a library of functions which manipulate collections of data, e.g: ▸ Mapping ▸ Grouping ▸ Sorting ▸ Filtering ▸ Equivalent to Java 8 streams ▸ Much more fluent syntax ▸ Available on Java 6 (again, great for Android!)
63. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN val games = listOf( VideoGame("Gears of War", "Epic Games", 8), VideoGame("Bioshock", "2K Games", 8), VideoGame("Snake", "Nokia", 10)) val mutableGames = mutableListOf( VideoGame("Gears of War", "Epic Games", 8), VideoGame("Bioshock", "2K Games", 8), VideoGame("Snake", "Nokia", 10)) games.forEach { print(it.name) } games.forEachIndexed { index, videoGame -> print("$index - $videoGame") }
64. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return a list of items which match the predicate: val bestGames = games.filter { it.reviewScore == 10 }
65. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return a list of items which match the predicate: val bestGames = games.filter { it.reviewScore == 10 } ▸ Return a list of items which do not match the predicate: val worstGames = games.filterNot { it.reviewScore >= 5 }
66. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return a list of items which match the predicate: val bestGames = games.filter { it.reviewScore == 10 } ▸ Return a list of items which do not match the predicate: val worstGames = games.filterNot { it.reviewScore >= 5 } ▸ Return a list of items equal to the first or last n items: val firstTwoGames = games.take(2) val lastTwoGames = games.takeLast(2)
67. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return the number of items which match the predicate: val numOfTens: Int = games.count { it.reviewScore == 10 }
68. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return the number of items which match the predicate: ▸ Return ‘true’ if all items match the predicate: val numOfTens: Int = games.count { it.reviewScore == 10 } val allTens: Boolean = games.all { it.reviewScore == 10 }
69. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return the number of items which match the predicate: ▸ Return ‘true’ if all items match the predicate: ▸ Return ‘true’ if any items match the predicate: val numOfTens: Int = games.count { it.reviewScore == 10 } val allTens: Boolean = games.all { it.reviewScore == 10 } val anyTens: Boolean = games.any { it.reviewScore == 10 }
70. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return the number of items which match the predicate: ▸ Return ‘true’ if all items match the predicate: ▸ Return ‘true’ if any items match the predicate: val numOfTens: Int = games.count { it.reviewScore == 10 } val allTens: Boolean = games.all { it.reviewScore == 10 } val anyTens: Boolean = games.any { it.reviewScore == 10 } ▸ Return ‘true’ if no items match the predicate: val noTens: Boolean = games.none { it.reviewScore == 10 }
71. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN val combined = listOf(1, 2) + listOf(3, 4) ▸ Combine two lists:
72. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return an item at the given index or the result of our defaultValue function if the index given is out of bounds: val gameOrElse = games.elementAtOrElse(0, { VideoGame("default", "default", 1) }) val combined = listOf(1, 2) + listOf(3, 4) ▸ Combine two lists:
73. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Return the first item that matches the predicate or ‘null’ if no item was found: val firstOrNull: VideoGame? = games.find { it.name == "Bioshock" }
74. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Returns the single element that matches the predicate or throws an exception if there is zero or more than one matching item: ▸ Return the first item that matches the predicate or ‘null’ if no item was found: val firstOrNull: VideoGame? = games.find { it.name == "Bioshock" } val singleGame = games.single { it.name == "Bioshock" }
75. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Returns the single element that matches the predicate or throws an exception if there is zero or more than one matching item: ▸ Return the first item that matches the predicate or ‘null’ if no item was found: val firstOrNull: VideoGame? = games.find { it.name == "Bioshock" } val singleGame = games.single { it.name == "Bioshock" } ▸ Same as above but returns ‘null’ instead of throwing an exception: val singleOrNull = games.singleOrNull { it.name == "Bioshock" }
76. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Apply the transform function to each item in the collection and return a list of the transformed items: val allScoresDeducted = games.map { it.copy(reviewScore = it.reviewScore - 2) }
77. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Apply the transform function to each item in the collection and return a list of the transformed items: val allScoresDeducted = games.map { it.copy(reviewScore = it.reviewScore - 2) } ▸ Create a new collection for each item in the collection and then flatten them into a unique list containing all of the elements: val playerIds = games.flatMap { getListOfPlayersForGame(it) }
78. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Group items according to the ‘keySelector’ function we pass as the parameter: val grouped: Map<String, List<VideoGame>> = games.groupBy { if (it.reviewScore > 6) "good" else "bad" }
79. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Group items according to the ‘keySelector’ function we pass as the parameter: val alphabeticalAZ = games.sortedBy { it.name } val alphabeticalZA = games.sortedByDescending { it.name } ▸ Sort items according to the ‘selector’ function we pass as the parameter: val grouped: Map<String, List<VideoGame>> = games.groupBy { if (it.reviewScore > 6) "good" else "bad" }
80. COLLECTIONS KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ We often combine these transformations ▸ Example: “Take the first 100 games, find which ones start with 'a' or 'b', give them a random review score, sort them by highest review score and show me the top 3.” games.take(100) .filter { it.name.startsWith("a") || it.name.startsWith("b") } .map { it.copy(reviewScore = random.nextInt()) } .sortedByDescending { it.reviewScore } .take(3)
81. A PARADIGM SHIFT IN THINKING KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ Kotlin is an object-oriented language with functional constructs. ▸ Lets the developer choose how to use it: ▸ Object-Oriented Programming via classes and instances. ▸ Functional Programming via higher-order functions. ▸ A mix of both, Kotlin is a flexible, interoperable language. ▸ Learn to maintain readability while increasing conciseness.
82. A PARADIGM SHIFT IN THINKING KOTLIN FOR JAVA DEVELOPERS KOTLIN ▸ My personal preference: ▸ Prefer to think in terms of data and functions, rather than objects - steer away from complex class hierarchies and explore the top- level. ▸ Compose functionality from functions - e.g. eliminate your mapper class. ▸ Prefer to write stateless code with clear inputs and outputs via functions over OOP which promotes mutability via objects. ▸ Still adhere to the more sensible OOP principles when possible. Kotlin will be part of a bigger Java eco-system for a while so look at the bigger picture.
83. KOTLIN FOR JAVA DEVELOPERS BRINGING KOTLIN TO A BUSINESS ▸ Barriers ▸ Initial learning curve - syntax/best practices/paradigm shift ▸ Change in perspective to functional programming ▸ Dex Limit on Android ▸ Benefits ▸ Increase code safety - NPE/ClassCastException ▸ Less code overall ▸ Interoperable with existing app code and Java libraries ▸ Increase developer productivity/happiness KOTLIN
84. KOTLIN FOR JAVA DEVELOPERS BRINGING KOTLIN TO A JAVA TEAM ▸ My situation: ▸ Current team: ~6 Java developers ▸ Start with 1 or 2 developers using Kotlin ▸ How to introduce Kotlin: ▸ Use Kotlin in an isolated component ▸ Involve other developers in code reviews ▸ Workshops/Codelabs ▸ Listen to IntelliJ suggestions! KOTLIN
85. KOTLIN FOR JAVA DEVELOPERS USEFUL LINKS ▸ Kotlin Koans: http://try.kotlinlang.org ▸ Install Kotlin with Gradle: https://medium.com/@CodingDoug/ kotlin-android-a-brass-tacks-experiment-part-1-3e5028491bcc ▸ Compilation Speeds: https://medium.com/keepsafe-engineering/ kotlin-vs-java-compilation-speed-e6c174b39b5d ▸ Collection operations: https://antonioleiva.com/collection- operations-kotlin ▸ Standard library: http://beust.com/weblog/2015/10/30/ exploring-the-kotlin-standard-library/ KOTLIN
86. KOTLIN FOR JAVA DEVELOPERS MORE KOTLIN TALKS ▸ Introduction to Kotlin (Google I/O ’17): https:// www.youtube.com/watch?v=X1RVYt2QKQE ▸ Android Development with Kotlin (Jake Wharton): https:// www.youtube.com/watch?v=A2LukgT2mKc ▸ Kotlin in Production (Droidcon NYC ’16): https:// www.youtube.com/watch?v=mDpnc45WwlI ▸ Life is Great and Everything Will Be Ok, Kotlin is Here (Google I/O ’17): https://www.youtube.com/watch?v=fPzxfeDJDzY KOTLIN
87. KOTLIN FOR JAVA DEVELOPERS DARREN ATHERTON twitter.com/darrenatherton darrenatherton.co.uk