Tips and tricks for adopting Kotlin

Kotlin is the new cool kid in town, but there is not a lot of information out there on the process of adopting it as a company.
In this talk we are going to show why and how we started using Kotlin in N26, focusing on a service written from scratch by the Barcelona team, and discussing some of the lessons learned in the process.
At the end we are going to close with a quick 5 step guide on how your Kotlin migration can be successful and a Q&A.

2. Tips and tricks for adopting Kotlin Uriel Salischiker - @urielsalis N26, Barcelona - May 2019
3. Uriel Salischiker Backend Engineer @ N26 @urielsalis - kotlin@urielsalis.me
4. Uriel Salischiker Backend Engineer @ N26 @urielsalis - kotlin@urielsalis.me
5. The first bank you will love
6. The first bank you will love We are hiring! https://n26.com/en/careers
7. z Introduction
8. Tips and tricks for adopting Kotlin Share your own at kotlin@urielsalis.me This is my own experience
9. Our journey adopting Kotlin In our production environments
10. What we like about Kotlin
11. Why Kotlin? Null safety Tips and tricks for adopting Kotlin
12. Why Kotlin? Null safety 一 Aimed at removing null references Tips and tricks for adopting Kotlin
13. Why Kotlin? Null safety 一 Aimed at removing null references 一 Avoiding The Billion Dollar Mistake Tips and tricks for adopting Kotlin
14. Why Kotlin? Null safety 一 Aimed at removing null references 一 Avoiding The Billion Dollar Mistake 一 Clearer APIs Tips and tricks for adopting Kotlin
15. Why Kotlin? Null safety var str: String = "I’m a string!" str = null // Fails to compile var str2: String? = "I’m a nullable string!" str2 = null // Works! Tips and tricks for adopting Kotlin
16. Why Kotlin? Null safety var str: String = "I’m a string!" str = null // Fails to compile var str2: String? = "I’m a nullable string!" str2 = null // Works! Tips and tricks for adopting Kotlin Optional<String> strOptional = Optional.of("I’m a string!"); Optional<String> strOptional = Optional.ofNullable("I’m a string!"); String contents = strOptional.orElse("I was null :c"); String contents2 = strOptional2.orElse("I was null :c");
17. Why Kotlin? Immutability Tips and tricks for adopting Kotlin
18. Why Kotlin? Immutability 一 Immutability by default Tips and tricks for adopting Kotlin
19. Why Kotlin? Immutability 一 Immutability by default Tips and tricks for adopting Kotlin 一 Clear mutability on properties
20. Why Kotlin? Immutability Tips and tricks for adopting Kotlin val immutableList = List<String>() val mutableList = MutableList<String>() Collections
21. Why Kotlin? Immutability Tips and tricks for adopting Kotlin val immutableList = List<String>() val mutableList = MutableList<String>() immutableList.add("string") //Fails mutableList.add("string") //Works Collections
22. Why Kotlin? Immutability Tips and tricks for adopting Kotlin val finalStr = "string" var mutableStr = "string" Properties
23. Why Kotlin? Immutability Tips and tricks for adopting Kotlin val finalStr = "string" var mutableStr = "string" finalStr = "something else" //Fails mutableStr = "something else" // Works Properties
24. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 一 Reducing boilerplate 一 Human comprehension is the key
25. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin
26. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters
27. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters
28. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor
29. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor • Equals
30. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor • Equals • Hashcode
31. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor • Equals • Hashcode • ToString
32. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor • Equals • Hashcode • ToString • Builder
33. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor • Equals • Hashcode • ToString • Builder • Copy
34. Why Kotlin? Data classes Tips and tricks for adopting Kotlin 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; } 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; 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; } @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 data class VideoGame( val name: String, val publisher: String, var reviewScore: Int ) Kotlin • Getters • Setters • Constructor • Equals • Hashcode • ToString • Builder • Copy • ComponentN
35. Why Kotlin? Data classes Tips and tricks for adopting Kotlin
36. Why Kotlin? Data classes data class Pet( val name: String, var office: String? = “Berlin” ) Tips and tricks for adopting Kotlin
37. Why Kotlin? Data classes data class Pet( val name: String, var office: String? = “Berlin” ) Tips and tricks for adopting Kotlin @Data @AllArgsConstructor public class Pet { @NonNull private final String name; private boolean office = "Berlin"; @Builder @SuppressWarnings("unused") private Pet(String name) { this.name = name; } }
38. Why Kotlin? Data classes Tips and tricks for adopting Kotlin
39. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String)
40. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String)
41. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR")
42. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA")
43. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee)
44. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee)
45. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) {
46. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps
47. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like
48. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like // Map.Entry<Key, Value> entry: map.entries()
49. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like // Map.Entry<Key, Value> entry: map.entries() }
50. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like // Map.Entry<Key, Value> entry: map.entries() }
51. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like // Map.Entry<Key, Value> entry: map.entries() }
52. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like // Map.Entry<Key, Value> entry: map.entries() } data class Result(val result: Int, val status: Status)
53. Why Kotlin? Data classes Tips and tricks for adopting Kotlin data class Fee(val amount: Int, val country: String) val ukFee = Fee(10, "GBR") val usaFee = ukFee.copy(country = "USA") val fees = listOf(ukFee, usaFee) for ((amount, country) in fees) { // This is also done automatically for maps // No need to do something like // Map.Entry<Key, Value> entry: map.entries() } data class Result(val result: Int, val status: Status) val (result, status) = ...
54. Why Kotlin? One language Multiple platforms Tips and tricks for adopting Kotlin
55. Why Kotlin? One language Multiple platforms 一 Compiles to native code, javascript, JVM Tips and tricks for adopting Kotlin
56. Why Kotlin? One language Multiple platforms 一 Compiles to native code, javascript, JVM 一 Client and Server can share code Tips and tricks for adopting Kotlin
57. Why Kotlin? One language Multiple platforms 一 Compiles to native code, javascript, JVM 一 Client and Server can share code 一 Official Android language Tips and tricks for adopting Kotlin
58. Why Kotlin? One language Multiple platforms 一 Compiles to native code, javascript, JVM 一 Client and Server can share code 一 Official Android language 一 Supports iOS (more targets in the future) Tips and tricks for adopting Kotlin
59. Why Kotlin? Open source Tips and tricks for adopting Kotlin
60. Why Kotlin? Open source 一 Fully open source Tips and tricks for adopting Kotlin
61. Why Kotlin? Open source 一 Fully open source 一 Backed by Jetbrains Tips and tricks for adopting Kotlin
62. Why Kotlin? Open source 一 Fully open source 一 Backed by Jetbrains 一 Language proposals KEEP Tips and tricks for adopting Kotlin
63. “Adoption of a language depends more of its ecosystem than its features” Empirical analysis of programming languages adoption Meyerovich LA, Rabkin AS November 2013 Tips and tricks for adopting Kotlin
64. Tips and tricks for adopting Kotlin
65. Why Kotlin? Interoperability with Java Tips and tricks for adopting Kotlin
66. Why Kotlin? Interoperability with Java Tips and tricks for adopting Kotlin
67. Why Kotlin? Interoperability with Java 一 100% compatible with Java Tips and tricks for adopting Kotlin
68. Why Kotlin? Interoperability with Java 一 100% compatible with Java 一 Easier to migrate in steps Tips and tricks for adopting Kotlin
69. Why Kotlin? Interoperability with Java 一 100% compatible with Java 一 Easier to migrate in steps Tips and tricks for adopting Kotlin Note This doesn’t work as well with code generators like Lombok
70. Why Kotlin? Cleaner functional programming Tips and tricks for adopting Kotlin
71. Why Kotlin? Cleaner functional programming 一 Extensive base library Tips and tricks for adopting Kotlin
72. Why Kotlin? Cleaner functional programming 一 Extensive base library 一 Collection processing Tips and tricks for adopting Kotlin
73. Why Kotlin? Cleaner functional programming 一 Extensive base library 一 Collection processing 一 Sequences: Better streams Tips and tricks for adopting Kotlin
74. Why Kotlin? Cleaner functional programming 一 Extensive base library 一 Collection processing 一 Sequences: Better streams 一 Ranges Tips and tricks for adopting Kotlin
75. Why Kotlin? Cleaner functional programming 一 Extensive base library 一 Collection processing 一 Sequences: Better streams 一 Ranges Tips and tricks for adopting Kotlin Tip Libraries like Arrow make this even better
76. Why Kotlin? Tips and tricks for adopting Kotlin
77. Tips and tricks for adopting Kotlin Why NOT Kotlin?
78. 一 Lombok Tips and tricks for adopting Kotlin Why NOT Kotlin?
79. 一 Lombok 一 KAPT Tips and tricks for adopting Kotlin Why NOT Kotlin?
80. 一 Lombok 一 KAPT 一 Change of mentality and way of coding Tips and tricks for adopting Kotlin Why NOT Kotlin?
81. 一 Lombok 一 KAPT 一 Change of mentality and way of coding 一 Patterns and Anti-Patterns are not set Tips and tricks for adopting Kotlin Why NOT Kotlin?
82. “I’m just playing with Kotlin as a kid with a new game, so I find a lot of possibilities of writing the same code in different ways, but no opinions yet” - Pull request comment Yesterday Tips and tricks for adopting Kotlin
83. Why Not Kotlin? Tips and tricks for adopting Kotlin
84. Why Not Kotlin? Tips and tricks for adopting Kotlin - Developers are not as experienced
85. Why Not Kotlin? Tips and tricks for adopting Kotlin - Developers are not as experienced - Bad Kotlin code is uglier than Java
86. History of Kotlin @ N26 Tips and tricks for adopting Kotlin
87. Talks Tips and tricks for adopting Kotlin
88. Talks 一 Lighting talks Tips and tricks for adopting Kotlin
89. Talks 一 Lighting talks 一 Get Stuff Done Days projects Tips and tricks for adopting Kotlin
90. Talks 一 Lighting talks 一 Get Stuff Done Days projects 一 Initial gotchas Tips and tricks for adopting Kotlin
91. Tech radar Tips and tricks for adopting Kotlin (not our real one)
92. Tests Tips and tricks for adopting Kotlin
93. Tests 一 Migrating tests to kotlin 一 Kotlin Junit & mockito-kotlin Tips and tricks for adopting Kotlin
94. Tests 一 Migrating tests to kotlin 一 Kotlin Junit & mockito-kotlin Tips and tricks for adopting Kotlin Tip Check KotlinTest and MockK!
95. Converting code Tips and tricks for adopting Kotlin
96. Migrating services Tips and tricks for adopting Kotlin
97. Migrating services 一 Small services: Transaction Notifications Tips and tricks for adopting Kotlin
98. Migrating services 一 Small services: Transaction Notifications 一 Automatic conversion using Intellij IDEA Tips and tricks for adopting Kotlin
99. Migrating services 一 Small services: Transaction Notifications 一 Automatic conversion using Intellij IDEA 一 Manual changes to make it more idiomatic Tips and tricks for adopting Kotlin
100. New services Tips and tricks for adopting Kotlin
101. New services 一 New architecture Tips and tricks for adopting Kotlin
102. New services 一 New architecture 一 Shared libraries Tips and tricks for adopting Kotlin
103. New services 一 New architecture 一 Shared libraries 一 New problems Tips and tricks for adopting Kotlin
104. Fee Service
105. Design challenges
106. Final by default Tips and tricks for adopting Kotlin
107. Final by default 一 Lots of open everywhere Tips and tricks for adopting Kotlin
108. Final by default 一 Lots of open everywhere 一 “all-open” plugin Tips and tricks for adopting Kotlin
109. Codestyle Tips and tricks for adopting Kotlin
110. Codestyle 一 No set code style Tips and tricks for adopting Kotlin
111. Codestyle 一 No set code style 一 Java Static analysis tools Tips and tricks for adopting Kotlin
112. Codestyle 一 No set code style 一 Java Static analysis tools 一 Kotlin Static analysis tools Tips and tricks for adopting Kotlin
113. Codestyle 一 No set code style 一 Java Static analysis tools 一 Kotlin Static analysis tools 一 Team conventions Tips and tricks for adopting Kotlin
114. Jooq Tips and tricks for adopting Kotlin
115. Jooq 一 Custom converters Tips and tricks for adopting Kotlin
116. Jooq 一 Custom converters 一 Runs after kotlin Tips and tricks for adopting Kotlin
117. Jooq 一 Custom converters 一 Runs after kotlin 一 Requires code to compile Tips and tricks for adopting Kotlin
118. Jooq Tips and tricks for adopting Kotlin
119. Jooq table("pet") { "pet_id" withTinyType CatId::class "owner_id" withTinyType OwnerId::class "price" withCustomConverter CurrencyConverter::class "purchase_date" withInstantTinyType PurchaseDate::class "branch" withEnum Branch::class } Tips and tricks for adopting Kotlin
120. New patterns
121. Sealed classes Tips and tricks for adopting Kotlin
122. Sealed classes 一 State machines defined on the domain Tips and tricks for adopting Kotlin
123. Sealed classes Tips and tricks for adopting Kotlin
124. Sealed classes sealed class RegisterResponse() Tips and tricks for adopting Kotlin
125. Sealed classes sealed class RegisterResponse() object SuccessfulResponse: RegisterResponse() class FailedResponse(val e: Exception): RegisterResponse() return when(response) { is SuccessfulResponse -> "All good!" is FailedResponse-> "All bad :C. $e" } Tips and tricks for adopting Kotlin
126. Sealed classes Tips and tricks for adopting Kotlin
127. Sealed classes sealed class Fee( Tips and tricks for adopting Kotlin
128. Sealed classes sealed class Fee( val id: FeeId Tips and tricks for adopting Kotlin
129. Sealed classes sealed class Fee( val id: FeeId ) Tips and tricks for adopting Kotlin
130. Sealed classes sealed class Fee( val id: FeeId ) Tips and tricks for adopting Kotlin
131. Sealed classes sealed class Fee( val id: FeeId ) data class ActiveFee(val id: FeeId): Fee(id) Tips and tricks for adopting Kotlin
132. Sealed classes sealed class Fee( val id: FeeId ) data class ActiveFee(val id: FeeId): Fee(id) data class DeactivatedFee(val id: FeeId): Fee(id) Tips and tricks for adopting Kotlin
133. Sealed classes sealed class Fee( val id: FeeId ) data class ActiveFee(val id: FeeId): Fee(id) data class DeactivatedFee(val id: FeeId): Fee(id) data class ProcessingFee(val id: FeeId): Fee(id) Tips and tricks for adopting Kotlin
134. Sealed classes sealed class Fee( val id: FeeId ) data class ActiveFee(val id: FeeId): Fee(id) data class DeactivatedFee(val id: FeeId): Fee(id) data class ProcessingFee(val id: FeeId): Fee(id) data class CompletedFee( val id: FeeId, val completed: Instant ): Fee(id) Tips and tricks for adopting Kotlin
135. Builders Tips and tricks for adopting Kotlin
136. Builders 一 Named builders Tips and tricks for adopting Kotlin
137. Builders 一 Named builders 一 Tests Tips and tricks for adopting Kotlin
138. Builders Tips and tricks for adopting Kotlin Ramon Nala and Benno
139. Builders Tips and tricks for adopting Kotlin
140. Builders data class Pet( val name: String, val office: String? = null ) val ramon = Pet("Ramon", "BCN") val benno = Pet("Benno") val nala = Pet(name = "Nala", office = "BLN") Tips and tricks for adopting Kotlin
141. Generics
142. Generics - Generic properties
143. Generics - Generic properties - Variance
144. Generics
145. Generics inline val <reified T> T.log: Logger get() = LoggerFactory.getLogger(T::class.java) class Foo { fun bar() { log.info("No need to create instances, only import") } }
146. Generics
147. Generics
148. Generics //Java Collection<Object> //Invariant Collection<? extends Object> // Covariant ??? //Contravariant
149. Generics
150. Generics
151. Generics interface Source<out T> { //Covariant fun nextT(): T } fun demo(strs: Source<String>) { val objects: Source<Any> = strs // This is OK, since T is an out-parameter // ... }
152. Generics
153. Generics
154. Generics interface Comparable<in T> { //Contravariant operator fun compareTo(other: T): Int } fun demo(x: Comparable<Number>) { x.compareTo(1.0) // 1.0 has type Double, which is a subtype of Number // Thus, we can assign x to a variable of type Comparable<Double> val y: Comparable<Double> = x // OK! }
155. Delegated properties 一 Lazy 一 Observable 一 Vetoable
156. Delegated properties
157. Delegated properties class Example(var p: String by Delegate()) class Delegate { operator fun getValue(thisRef: Any?, property: KProperty<*>): String { return "$thisRef, delegating '${property.name}'" } operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) { println("$value, '${property.name}' in $thisRef.") } } val e = Example() println(e.p) //Example@33a17727, delegating ‘p’
158. Delegated properties
159. Delegated properties val lazyValue: String by lazy { println("computed!") "Hello" } fun main() { println(lazyValue) println(lazyValue) } //computed //Hello //Hello
160. Delegated properties
161. Delegated properties class User { var name: String by Delegates.observable("<no name>") { prop, old, new -> println("$old -> $new") } } fun main() { val user = User() user.name = "first" user.name = "second" }
162. Delegated properties
163. Delegated properties class User(val map: Map<String, Any?>) { val name: String by map val age: Int by map } val user = User(mapOf( "name" to "John Doe", "age" to 25 )) println(user.name) // Prints "John Doe" println(user.age) // Prints 25
164. Delegated properties
165. Delegated properties //Since Kotlin 1.1 fun example(computeFoo: () -> Foo) { val memoizedFoo by lazy(computeFoo) if (someCondition && memoizedFoo.isValid()) { //if someCondition is false, // memoizedFoo is never computed memoizedFoo.doSomething() } }
166. Delegated properties
167. Gotchas
168. Constants 一 Java -> static final 一 Kotlin -> ? Tips and tricks for adopting Kotlin
169. Constants 一 Java -> static final 一 Kotlin -> ? const val FOO = "foo" Tips and tricks for adopting Kotlin
170. Companion objects 一 Extra boilerplate 一 Performance penalty 一 Uglier code on Java Tips and tricks for adopting Kotlin
171. Companion objects Tips and tricks for adopting Kotlin class C { companion object { @JvmStatic fun foo() {} fun bar() {} } } C.foo(); // works fine C.bar(); // error: not a static method C.Companion.foo(); // instance method remains C.Companion.bar(); // the only way it works
172. DSL and Extension functions abuse 一 Leads to god objects 一 DSL for everything might not be as readable Tips and tricks for adopting Kotlin
173. DSL and Extension functions abuse val helloWorldInBrainf_ck = s[s { 7 }.r.a.l].r.s.o //-[------->+<]>-. .s[s.r.a { 5 }.l].r.a.a.o //-[->+++++<]>++. .a { 7 }.o.o //+++++++.. .a { 3 }.o[s.r.a { 5 }.l].r.a.o //+++.[->+++++<]>+. .s { 12 }.o //------------. .s { 3 }[s.r.a { 3 }.l].r.o //---[->+++<]>. .s[s { 3 }.r.a.l].r.s { 3 }.o //-[--->+<]>---. .a { 3 }.o //+++. .s { 6 }.o //------. .s { 8 }.o //--------. .s[s { 3 }.r.a.l].r.o //-[--->+<]>. Tips and tricks for adopting Kotlin
174. DSL and Extension functions abuse val v = village { house { "Alice" age 31 "Bob" age 45 500.gold } house { sword with strength value 24.2 sword with strength level 16.7 shield with defense value 15.3 } house() house { "Charles" age 52 2500.gold sword shield } } Tips and tricks for adopting Kotlin
175. DSL and Extension functions abuse class HouseBuilder { internal val people = mutableListOf<Person>() internal val items = mutableListOf<Item>() fun build(): House { return House(people, items) } infix fun String.age(age: Int) { people.add(Person(this, age)) } val Int.gold: Unit get() { items += Gold(this) return Unit } } class VillageBuilder { private val houses = mutableListOf<House>() operator fun House.unaryPlus() { houses += this } fun house(setup: HouseBuilder.() -> Unit = {}) { val houseBuilder = HouseBuilder() houseBuilder.setup() houses += houseBuilder.build() } fun build(): Village { return Village(houses) } } fun village(setup: VillageBuilder.() -> Unit): Village { val villageBuilder = VillageBuilder() villageBuilder.setup() return villageBuilder.build() } Tips and tricks for adopting Kotlin class ShieldContinuation(val house: HouseBuilder) object defense val HouseBuilder.shield: ShieldContinuation get() { this.items += Shield(0.0) return ShieldContinuation(this) } infix fun ShieldContinuation.with(d: defense): ShieldBuilder { house.items.removeAt(house.items.lastIndex) return ShieldBuilder(house) } class ShieldBuilder(val house: HouseBuilder) infix fun ShieldBuilder.value(defense: Double) { house.items += Shield(defense) } class SwordContinuation(val house: HouseBuilder) object strength val HouseBuilder.sword: SwordContinuation get() { this.items += Sword(0.0) return SwordContinuation(this) } infix fun SwordContinuation.with(d: strength): SwordBuilder { house.items.removeAt(house.items.lastIndex) return SwordBuilder(house) } class SwordBuilder(val house: HouseBuilder) infix fun SwordBuilder.value(strength: Double) { house.items += Sword(strength) } infix fun SwordBuilder.level(strength: Double) = value(strength)
176. Generics 一 Difficult to change mindset 一 * everywhere 一 out vs in confusion Tips and tricks for adopting Kotlin
177. Testing libraries Tips and tricks for adopting Kotlin
178. Testing libraries 一 All classes are final by default Tips and tricks for adopting Kotlin
179. Testing libraries 一 All classes are final by default 一 Used mockito Tips and tricks for adopting Kotlin
180. Testing libraries 一 All classes are final by default 一 Used mockito Tips and tricks for adopting Kotlin Tip Check MockK and KotlinTest!
181. Jackson - Can’t serialize without a default constructor - Jackson-module-kotlin Tips and tricks for adopting Kotlin
182. Getters as properties 一 Might hide performance bugs Tips and tricks for adopting Kotlin
183. Reflection abuse 一 Easy reflection hides the performance penalty Tips and tricks for adopting Kotlin
184. Reflection abuse class A(val p: Int) fun main() { // prints "public final int A.getP()" println(A::p.javaGetter) // prints "private final int A.p" println(A::p.javaField) } Tips and tricks for adopting Kotlin
185. Reflection abuse val numberRegex = "\\d+".toRegex() println(numberRegex.matches("29")) val isNumber = numberRegex::matches println(isNumber("29")) Tips and tricks for adopting Kotlin
186. Immutability 一 We shouldn’t just use Mutable everywhere 一 Change of mindset Tips and tricks for adopting Kotlin
187. Performance cost 一 Hidden cost of lambdas • Inline to the rescue! 一 Local functions 一 Nullable primitives 一 Ranges(when there is indirection) Tips and tricks for adopting Kotlin
188. Code style - No set code style - Intellij doesn’t follow Kotlin guidelines completely by default(!) Tips and tricks for adopting Kotlin
189. Lessons learned
190. Kotlin champion Tips and tricks for adopting Kotlin
191. Kotlin champion 一 Designate a Kotlin champion Tips and tricks for adopting Kotlin
192. Kotlin champion 一 Designate a Kotlin champion 一 Kotlin Koans Tips and tricks for adopting Kotlin
193. Learn the basics Tips and tricks for adopting Kotlin
194. Learn the basics 一 Base concepts Tips and tricks for adopting Kotlin
195. Learn the basics 一 Base concepts 一 Common patterns Tips and tricks for adopting Kotlin
196. Learn the basics 一 Base concepts 一 Common patterns 一 Open source code Tips and tricks for adopting Kotlin
197. Tests Tips and tricks for adopting Kotlin
198. Tests 一 Start writing tests in Kotlin Tips and tricks for adopting Kotlin
199. Tests 一 Start writing tests in Kotlin 一 Migrate small services Tips and tricks for adopting Kotlin
200. Convincing Management Tips and tricks for adopting Kotlin
201. Convincing Management 一 The team is more productive with Kotlin Tips and tricks for adopting Kotlin
202. Convincing Management 一 The team is more productive with Kotlin 一 Increases app quality Tips and tricks for adopting Kotlin
203. Convincing Management 一 The team is more productive with Kotlin 一 Increases app quality 一 Team happiness! Tips and tricks for adopting Kotlin
204. Convincing Management 一 The team is more productive with Kotlin 一 Increases app quality 一 Team happiness! 一 The industry is moving to it Tips and tricks for adopting Kotlin
205. Convincing Management 一 The team is more productive with Kotlin 一 Increases app quality 一 Team happiness! 一 The industry is moving to it 一 Official Spring, Android & Gradle support Tips and tricks for adopting Kotlin
206. Effective Kotlin 一 Team needs to agree on patterns 一 With great flexibility comes great responsibility https://medium.com/@appmattus/effective-kotlin- 31215a6cf847 Tips and tricks for adopting Kotlin
207. Questions?
208. Questions?