10 Kotlin Tricks in 10(ish) Minutes (Devoxx US 2017)

Kotlin is new language growing in popularity as a complement to Java. Its major advantages and features compared to Java are immediately appealing. While it's quick to learn, it also has a lot of small and thoughtful parts which can be harder to discover. This short talk will cover 10 of my favorites with real-world examples. Attendees should come in having seen some Kotlin but looking to learn even more.

Video: https://youtu.be/0sPzDwS55wM

Transcript

1. Jake Wharton 10 Kotlin Tricks In 10(ish) Minutes
2. Why Kotlin?
3. Why Kotlin? val firstName: String = "Jake"
 val lastName: String? = null
4. Why Kotlin? val firstName = "Jake"
 val lastName: String? = null
5. Why Kotlin? class User {
 public String getName() {
 // ...
 }
 public void setName(String name) {
 // ...
 }
 } // ^^^ Java
6. Why Kotlin? class User {
 public String getName() {
 // ...
 }X
 public void setName(String name) {
 // ...
 }Y
 }Z // ^^^ Java vvv Kotlin val user = User()
 println("Name is " + user.name)
7. Why Kotlin? class User {
 public String getName() {
 // ...
 }X
 public void setName(String name) {
 // ...
 }Y
 }Z // ^^^ Java vvv Kotlin val user = User()
 println("Name is ${user.name}")
8. Why Kotlin? class User {
 public String getName() {
 // ...
 }X
 public void setName(String name) {
 // ...
 }Y
 }Z // ^^^ Java vvv Kotlin val user = User()
 println("Name is $user")
9. Why Kotlin? val user = User()
10. Why Kotlin? val user = User()
 user = User()
 
 var currentUser = User()
 currentUser = User()
11. Why Kotlin? fun Date.isTuesday(): Boolean {
 return day == 2
 }
12. Why Kotlin? fun Date.isTuesday(): Boolean {
 return day == 2
 } val epoch = Date(1970, 0, 0)
 if (epoch.isTuesday()) {
 println("The epoch was a tuesday.")
 } else {
 println("The epoch was not a tuesday.")
 }
13. Why Kotlin? val executor = Executors.newSingleThreadExecutor();
 executor.execute { println("Background thread!") }
14. Why Kotlin? fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }
15. Why Kotlin? fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }X
16. Why Kotlin? inline fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }X
17. Why Kotlin? inline fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }
18. Why Kotlin? data class User(val firstName: String, val lastName: String)
19. Why Kotlin? ...and much, much more!
20. #01 Explosive Placeholders
21. #01 Explosive Placeholders class Test : Runnable {
 override fun run() {
 // TODO doSomethingElse()?
 }B
 }A
22. #01 Explosive Placeholders class Test : Runnable {Y
 override fun run() {Z
 if (something()) {
 doSomething()
 } else {
 // TODO doSomethingElse()?
 }C
 }B
 }A
23. #01 Explosive Placeholders class Test : Runnable {Y
 override fun run() {Z
 if (something()) {
 doSomething()
 } else {
 // TODO doSomethingElse()?
 }C
 }B
 }A
24. #01 Explosive Placeholders class Test : Callable<String> {Y
 override fun call(): String {Z
 if (something()) {
 return doSomething()
 } else {
 // TODO return doSomethingElse()?
 }C
 }B
 }A Runnable
 run
25. #01 Explosive Placeholders class Test : Callable<String> {Y
 override fun call(): String {Z
 if (something()) {
 return doSomething()
 } else {
 // TODO return doSomethingElse()?
 }C
 }B
 }A Runnable
 run
26. #01 Explosive Placeholders class Test : Callable<String> {
 override fun call(): String {
 if (something()) {
 return doSomething()
 } else {
 throw UnsupportedOperationException("TODO doSomethingElse()?")
 }C
 }B
 }A 
 
 
 
 
 // return
27. #01 Explosive Placeholders class Test : Callable<String> {
 override fun call(): String {
 if (something()) {
 return doSomething()
 } else {
 TODO("doSomethingElse()?")
 }C
 }B
 }A
28. #01 Explosive Placeholders class Test : Callable<String> {
 override fun call(): String {
 if (something()) {
 return doSomething()
 } else {
 TODO("doSomethingElse()?")
 }C
 }B
 }A
29. #02 Semantic Validation
30. #02 Semantic Validation static String join(String sep, List<String> strings) {
 // ...
 }Z
31. #02 Semantic Validation static String join(String sep, List<String> strings) {
 if (sep == null) throw new NullPointerException("sep == null");
 if (strings == null) throw new NullPointerException("strings == null"); 
 // ...
 }Z
32. #02 Semantic Validation static String join(String sep, List<String> strings) {
 if (sep == null) throw new NullPointerException("sep == null");
 if (sep.length() < 2) { throw new IllegalArgumentException("sep.length() < 2: " + sep); }Y
 if (strings == null) throw new NullPointerException("strings == null"); 
 // ...
 }Z
33. #02 Semantic Validation static String join(String sep, List<String> strings) {
 Preconditions.checkNotNull(sep, "sep == null");
 if (sep.length() < 2) {
 throw new IllegalArgumentException("sep.length() < 2: " + sep);
 }Y
 Preconditions.checkNotNull(strings, "strings == null");
 
 // ...
 }Z 
 if == null) throw new NullPointerException(
 
 if == null) throw new NullPointerException(
34. #02 Semantic Validation static String join(String sep, List<String> strings) {
 Preconditions.checkNotNull(sep, "sep == null");
 Preconditions.checkArgument(sep.length() < 2, "sep.length() < 2: " + sep);
 Preconditions.checkNotNull(strings, "strings == null");
 
 // ...
 }Z 
 
 if ) {
 throw new IllegalArgumentException(
 }Y
35. #02 Semantic Validation static String join(String sep, List<String> strings) {
 Preconditions.checkNotNull(sep, "sep == null");
 Preconditions.checkArgument(sep.length() < 2, "sep.length() < 2: " + sep);
 Preconditions.checkNotNull(strings, "strings == null");
 
 // ...
 }Z 
 
 if ) {
 throw new IllegalArgumentException(
 }Y
36. #02 Semantic Validation fun join(sep: String, strings: List<String>): String {
 Preconditions.checkArgument(sep.length < 2, "sep.length < 2: " + sep)
 // ...
 }Z static 
 Preconditions.checkNotNull(sep, "sep == null");
 ;
 Preconditions.checkNotNull(strings, "strings == null");
37. #02 Semantic Validation fun join(sep: String, strings: List<String>): String {
 require(sep.length < 2) { "sep.length < 2: " + sep }
 // ...
 }Z 
 Preconditions.checkArgument , )
38. #02 Semantic Validation fun join(sep: String, strings: List<String>): String {
 require(sep.length < 2) { "sep.length < 2: " + sep }
 // ...
 }Z
39. #02 Semantic Validation fun join(sep: String, strings: List<String>): String {
 require(sep.length < 2) { "sep.length < 2: " + sep }
 // ...
 }Z 
 Preconditions.checkArgument , )
40. #02 Semantic Validation fun join(sep: String, strings: List<String>): String {
 if (sep.length < 2) { throw IllegalArgumentException("sep.length < 2: " + sep) }
 // ...
 }Z 
 require ) { }
41. #02 Semantic Validation // throws IllegalArgumentException require(value: Boolean) require(value: Boolean, lazyMessage: () -> Any) requireNotNull(value: T?): T requireNotNull(value: T?, lazyMessage: () -> Any): T // throws IllegalStateException check(value: Boolean) check(value: Boolean, lazyMessage: () -> Any) checkNotNull(value: T?): T checkNotNull(value: T?, lazyMessage: () -> Any): T // throws AssertionError assert(value: Boolean) assert(value: Boolean, lazyMessage: () -> Any)
42. #03 Anything and Nothing
43. #03 Anything and Nothing Runnable
44. #03 Anything and Nothing Runnable Any
45. #03 Anything and Nothing Runnable Int Any
46. #03 Anything and Nothing Runnable Int Any Nothing
47. #03 Anything and Nothing fun printName(user: User?) {
 val name = user?.name ?: throw IllegalArgumentException("User was null")
 println("Name is $name.")
 }
48. #03 Anything and Nothing fun printName(user: User?) {
 val name = user?.name ?: throw IllegalArgumentException("User was null")
 println("Name is $name.")
 }Z
49. #03 Anything and Nothing fun printName(user: User?) {
 val name = user?.name ?: throw IllegalArgumentException("User was null")
 println("Name is $name.")
 }Z
50. #03 Anything and Nothing fun printName(user: User?) {
 val name = user?.name ?: throw IllegalArgumentException("User was null")
 println("Name is $name.")
 }Z
51. #03 Anything and Nothing fun printName(user: User?) {
 val name = user?.name ?: throw IllegalArgumentException("User was null")
 println("Name is $name.")
 }Z
52. #03 Anything and Nothing fun printName(user: User?) {
 if (user != null) {
 println("Name is ${user.name}.")
 }Y
 throw IllegalArgumentException("User was null")
 }Z 
 val name = ?.name ?:
53. #03 Anything and Nothing fun printName(user: User?) {
 if (user != null) {
 println("Name is ${user.name}.")
 }Y
 throw IllegalArgumentException("User was null")
 }Z
54. #03 Anything and Nothing fun printName(user: User?) {
 if (user != null) {
 println("Name is ${user.name}.")
 }Y
 throw IllegalArgumentException("User was null")
 println("A user has no name.")
 }Z
55. #03 Anything and Nothing fun runServer(serverSocket: ServerSocket) {
 while (true) {
 handleSocket(serverSocket.accept())
 }
 }
56. #03 Anything and Nothing fun runServer(serverSocket: ServerSocket) {X
 while (true) {
 handleSocket(serverSocket.accept())
 }Y
 }Z runServer(ServerSocket(80))
 println("Running!")
57. #03 Anything and Nothing fun runServer(serverSocket: ServerSocket): Nothing {X
 while (true) {
 handleSocket(serverSocket.accept())
 }Y
 }Z runServer(ServerSocket(80))
 println("Running!")
58. #03 Anything and Nothing fun runServer(serverSocket: ServerSocket): Nothing {
 while (true) {
 handleSocket(serverSocket.accept())
 }
 } runServer(ServerSocket(80))
 println("Running!")
59. #04 Let
60. #04 Let val user: User? = null
 
 if (user != null) {X
 // user not null
 }X
61. #04 Let var user: User? = null
 
 if (user != null) {X
 // user might be null
 }X
62. #04 Let var user: User? = null
 
 user?.let {X
 // it == user not null
 }X 
 
 if ( != null)
63. #04 Let var user: User? = null
 
 user?.let { 
 // it == user not null, only read once!
 }X
64. #04 Let class Foo {
 @Volatile var user: User? = null
 
 fun doSomething() {
 user?.let { user ->
 // user not null, only read once!
 }X
 }Y
 }Z 
 
 
 it ==
65. #04 Let class Foo {
 @Volatile var user: User? = null
 
 fun doSomething() {
 user?.apply { user ->
 // user not null, only read once!
 }X
 }Y
 }Z 
 
 
 
 let
66. #04 Let class Foo {
 @Volatile var user: User? = null
 
 fun doSomething() {
 user?.run { user ->
 // user not null, only read once!
 }X
 }Y
 }Z
67. #04 Let class Foo {
 @Volatile var user: User? = null
 
 fun doSomething() {
 user?.also { user ->
 // user not null, only read once!
 }X
 }Y
 }Z
68. #04 Let someMethod().let { result ->
 // use result multiple times
 }X
69. #05 Multiline String Literals
70. #05 Multiline String Literals val string = "foo"Z
71. #05 Multiline String Literals val string = "foo\nbar\nbaz"Z
72. #05 Multiline String Literals val string = "foo\n" + "bar\n" + "baz"Z
73. #05 Multiline String Literals val string = """foo bar baz"""Z
74. #05 Multiline String Literals val string = """
 foo
 bar
 baz""".trimIndent()
75. #05 Multiline String Literals val string = """
 foo
 bar
 baz""".trimIndent() 
 
 
 Margin
76. #05 Multiline String Literals val string = """|foo
 1|bar
 2|baz""".trimMargin() 
 
 Indent
77. #05 Multiline String Literals val foo = "FOO!"
 val bar = "BAR!"
 val baz = "BAZ!"
 
 val string = """|$foo
 1|$bar
 2|$baz""".trimMargin()
78. #06 Lazy but Speedy
79. #06 Lazy but Speedy class NamePrinter(val firstName: String, val lastName: String) {
 }Z
80. #06 Lazy but Speedy class NamePrinter(val firstName: String, val lastName: String) {
 fun printName() {
 println("$firstName $lastName")
 }Y
 }Z
81. #06 Lazy but Speedy class NamePrinter(val firstName: String, val lastName: String) {
 var fullName: String? = null
 
 fun printName() {
 if (fullName == null) {
 fullName = "$firstName $lastName"
 }X
 println(fullName)
 }Y
 }Z
82. #06 Lazy but Speedy class NamePrinter(val firstName: String, val lastName: String) {
 val fullName: String by lazy { "$firstName $lastName" }X
 
 fun printName() {
 println(fullName)
 }Y
 }Z 
 r ? = null
 
 
 if (fullName == null) {
 fullName =
 }X
83. #06 Lazy but Speedy class NamePrinter(val firstName: String, val lastName: String) {
 val fullName: String by lazy { "$firstName $lastName" }X
 
 fun printName() {
 println(fullName)
 }Y
 }Z
84. #06 Lazy but Speedy import kotlin.LazyThreadSafetyMode.NONE
 
 class NamePrinter(val firstName: String, val lastName: String) {
 val fullName: String by lazy(NONE) { "$firstName $lastName" }X
 
 fun printName() {
 println(fullName)
 }Y
 }Z
85. #07 Code Block Measurement
86. #07 Code Block Measurement val helloStart = System.currentTimeMillis()
 println("Hello, world!")
 val helloTook = System.currentTimeMillis() - helloStart
 println("Saying 'hello' took ${helloTook}ms")
87. #07 Code Block Measurement val helloTook = measureTimeMillis {
 println("Hello, world!")
 }X
 
 println("Saying 'hello' took ${helloTook}ms") val helloStart = System.currentTimeMillis()
 
 System.currentTimeMillis() - helloStart
88. #07 Code Block Measurement val helloTook = measureTimeNanos {
 println("Hello, world!")
 }X
 
 println("Saying 'hello'Ztook ${helloTook}ns")
89. #07 Code Block Measurement var helloTook = measureTimeNanos {
 println("Hello, world!")
 }X helloTook += measureTimeNanos {
 println("Hello, world!")
 }X
 
 println("Saying 'hello' twiceZtook ${helloTook}ns") l
90. #08 Deprecation Levels
91. #08 Deprecation Levels fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
92. #08 Deprecation Levels @Deprecated
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
93. #08 Deprecation Levels @Deprecated
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
94. #08 Deprecation Levels @Deprecated("Use strings.joinToString(sep).")
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
95. #08 Deprecation Levels @Deprecated("Use strings.joinToString(sep).")X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z join(", ", listOf("me", "you"))
96. #08 Deprecation Levels import kotlin.DeprecationLevel.ERROR
 
 @Deprecated("Use strings.joinToString(sep).", level = ERROR)X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))
97. #08 Deprecation Levels import kotlin.DeprecationLevel.ERROR
 
 @Deprecated("Use strings.joinToString(sep).", level = ERROR)X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))
98. #08 Deprecation Levels import kotlin.DeprecationLevel.ERROR
 
 @Deprecated("Use strings.joinToString(sep).", level = ERROR)X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you")) Error:(10, 1) Kotlin: Using 'join(String, List<String>): String' is an error. Use strings.joinToString(sep).
99. #08 Deprecation Levels import kotlin.DeprecationLevel.HIDDEN
 
 @Deprecated("Use strings.joinToString(sep).", level = HIDDEN)X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you")) ERROR ERROR Error:(10, 1) Kotlin: Using 'join(String, List<String>): String' is an error. Use strings.joinToString(sep).
100. #08 Deprecation Levels import kotlin.DeprecationLevel.HIDDEN
 
 @Deprecated("Use strings.joinToString(sep).", level = HIDDEN)
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }
 
 join(", ", listOf("me", "you"))
101. #09 Deprecation Replacements
102. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).")X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z join(", ", listOf("me", "you"))
103. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).",
 replaceWith = ReplaceWith("strings.joinToString(sep)"))X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))
104. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).",
 replaceWith = ReplaceWith("strings.joinToString(sep)"))X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))
105. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).",
 replaceWith = ReplaceWith("strings.joinToString(sep)"))X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))
106. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).",
 replaceWith = ReplaceWith("strings.joinToString(sep)"))X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))
107. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).",
 replaceWith = ReplaceWith("strings.joinToString(sep)"))X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 listOf("me", "you").joinToString(", ")
108. #09 Deprecation Replacements @Deprecated("Use strings.joinToString(sep).",
 replaceWith = ReplaceWith("strings.joinToString(sep)"))X
 fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z 
 
 
 
 
 
 join(", ", listOf("me", "you"))
109. #09 Deprecation Replacements @Deprecated("Use Guava's Joiner.",
 replaceWith = ReplaceWith("Joiner.on(sep).join(strings)",
 imports = "com.google.common.base.Joiner"))X fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you")) strings.joinToString(sep) 
 .joinToString(
110. #09 Deprecation Replacements @Deprecated("Use Guava's Joiner.",
 replaceWith = ReplaceWith("Joiner.on(sep).join(strings)",
 imports = "com.google.common.base.Joiner"))X fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 join(", ", listOf("me", "you"))X 
 
 
 
 Joiner.on(", ").join(listOf("me", "you"))
111. #09 Deprecation Replacements import com.google.common.base.Joiner
 @Deprecated("Use Guava's Joiner.",
 replaceWith = ReplaceWith("Joiner.on(sep).join(strings)",
 imports = "com.google.common.base.Joiner"))X fun join(sep: String, strings: List<String>): String { 
 // ...
 }Z
 
 Joiner.on(", ").join(listOf("me", "you")) 
 
 
 
 
 
 join(", ", listOf("me", "you"))X
112. #10 Erasing Erasure
113. #10 Erasing Erasure class Foo : Callable<String>, Callable<Int> {
 }
114. #10 Erasing Erasure class Foo : Callable<String>, Callable<Int> {
 }
115. #10 Erasing Erasure fun sort(strings: List<String>) {
 // ...
 }Y 
 fun sort(ints: List<Int>) {
 // ...
 }Z
116. #10 Erasing Erasure fun sort(strings: List) {
 // ...
 }Y 
 fun sort(ints: List) {
 // ...
 }Z
117. #10 Erasing Erasure fun sort(strings: List<String>) {
 // ...
 }Y 
 fun sort(ints: List<Int>) {
 // ...
 }Z
118. #10 Erasing Erasure fun sortStrings(strings: List<String>) {
 // ...
 }Y 
 fun sortInts(ints: List<Int>) {
 // ...
 }Z
119. #10 Erasing Erasure fun sort(strings: List<String>) {
 // ...
 }Y 
 fun sort(ints: List<Int>) {
 // ...
 }Z
120. #10 Erasing Erasure @JvmName("sortStrings")
 fun sort(strings: List<String>) {
 // ...
 }Y
 @JvmName("sortInts")
 fun sort(ints: List<Int>) {
 // ...
 }Z
121. kotlinlang.org
122. jakewharton jakewharton jakewharton twitter.com/ github.com/ .com 10 Kotlin Tricks In 10(ish) Minutes