This talk is to help the kotliners understand the caveats of the default interoperability features provided by Kotlin and provides a few tips for interoperability to make Java developers life easier.

1. Dear otliners
Java Developers are Humans Too
@Vivek_Chanddru

2. Quick Check

3. A Kotlin Enthusiast
Rooting for Java Developers

4. Why is there so much hype?
• Unveiled on July, 2011 by JetBrains and released Feb, 2016
• Received First Class support at Google I/O 2017
• A much better Java
• Provides support similar to other modern languages
• A boon for Java programmers writing verbose code

5. Kotlin Adoption

6. Kotlin Adoption

7. Kotlin Adoption
We are here now

8. Kotlin Adoption
• Very soon, Kotlin is going to overtake Java (For Android)
• Libraries are popping up with Kotlin support
• New libraries are developed using Kotlin
• Sample apps and code are in Kotlin
• Medium is filled with wonders of Kotlin

9. Kotlin is growing and people
are Adopting it
So Why Bother about Java Developers?

10. Problems in Adopting Kotlin
• Have to learn an all new language 😫
• Team members not willing to learn a new language
• Company is not willing to spend time and money on R&D
• Fear of breaking what is already working fine
• Hard to find Kotlin experts to get the work done

11. Most of it is not their fault!
A lot of factors come into play that stops their adoption

12. What goes on in a Java developer's mind?
• The world is moving on leaving them behind
• See a new blog? It is in Kotlin
• An awesome library that they want? It is developed and
optimized for Kotlin
• Left with the default implementation of Kotlin
interoperability support

13. You can support them!
With very minimal work on your side

14. How Kotlin helps with interoperability
• Kotlin works out of box with Java (mostly)
• Java classes and methods can be used directly from Kotlin
• Kotlin classes and functions can be used from Java
• With very little effort, you can make Java developers’ life much
easier and write better looking code

15. Tips for Interoperability
The typical Do's and Don'ts

16. Think about access scopes
What you think is invisible is available in plain sight!
1

17. Visibility Modifiers - Kotlin
• Private fields in Kotlin stay private in Java
• Private top-level declarations turns out to be package-local
declarations
• Protected members in Java are accessible to other classes in same
package
• Internal members become public after some name-mangling
• Public stay public

18. Be mindful of the class name
Name the class to make more sense
2

19. fun calculateAge(dob: Date): Int {
//some magic happens here
return 42
}
Kotlin - AmazingUtils.kt

20. fun calculateAge(dob: Date): Int {
//some magic happens here
return 42
}
void runAgeDemo(Date date) {
AmazingUtilsKt
.calculateAge(date);
}
Kotlin - AmazingUtils.kt
Java

21. @file:JvmName("Utils")
fun calculateAge(dob: Date): Int {
//some magic happens here
return 42
}
Kotlin - AmazingUtils.kt

22. Utils.calculateAge(date);
void runAgeDemo(Date date) {
}
@file:JvmName("Utils")
fun calculateAge(dob: Date): Int {
//some magic happens here
return 42
}
Kotlin - AmazingUtils.kt
Java

23. @file:JvmName(“Utils")
@file:JvmMultifileClass
fun calculateAge(dob: Date): Int {
//some magic happens here
return 42
}
@file:JvmName("Utils")
@file:JvmMultifileClass
fun isEligibleToVote(age: Int): Boolean {
//some magic happens here
return age >= 18
}
A.kt
B.kt

24. void runAgeDemo(Date date) {
int age = Utils.calculateAge(date);
boolean isEligible = Utils.isEligibleToVote(age);
}
Java

25. Have fun() with names
Name the methods to make more sense
3

26. fun String?.isNullorEmpty(): Boolean {
return this != null && this.isEmpty()
}
Kotlin

27. Does not show the receiver type. Might want
function name to give more context
fun String?.isNullorEmpty(): Boolean {
return this != null && this.isEmpty()
}
Kotlin
void nullOrEmptyDemo(String s) {
boolean nullorEmpty =
Utils.isNullorEmpty(s);
}
Java

28. @file:JvmName("Utils")
@JvmName("isStringNullorEmpty")
fun String?.isNullorEmpty(): Boolean {
return this != null && this.isEmpty()
}
Kotlin

29. @file:JvmName("Utils")
@JvmName("isStringNullorEmpty")
fun String?.isNullorEmpty(): Boolean {
return this != null && this.isEmpty()
}
Gives more context on what the method is
and what it does.
Kotlin
void nullOrEmptyDemo(String s) {
boolean nullorEmpty =
Utils.isStringNullorEmpty(s);
}
Java

30. Rename the Accessors
Name the getters and setters the way you want
4

31. class Person {
var name: String? = null
var age: Int? = null
}
Kotlin

32. void personDemo(Person person) {
person.getName();
person.getAge();
person.setName("Vivek");
}
class Person {
var name: String? = null
var age: Int? = null
}
Java
Kotlin

33. var name: String? = null
@JvmName("getPersonName")
get() {
return field?.toLowerCase()
}
set(value) {
field = value?.toUpperCase()
}
var age: Int? = null
class Person {
}
Kotlin

34. class Person {
var name: String? = null
@JvmName("getPersonName")
get() {
return field?.toLowerCase()
}
set(value) {
field = value?.toUpperCase()
}
@get:JvmName("getpersonAge")
@set:JvmName("setPersonAge")
var age: Int? = null
}
Kotlin

35. void personDemo(Person person) {
person.getPersonName();
person.getpersonAge();
person.setName("Vivek");
}
class Person {
var name: String? = null
@JvmName("getPersonName")
get() {
return field?.toLowerCase()
}
set(value) {
field = value?.toUpperCase()
}
@get:JvmName("getpersonAge")
@set:JvmName("setPersonAge")
var age: Int? = null
}
Java
Kotlin

36. Never forget your Companions
Companion functions need a little tweak to look nice
5

37. class AmazingClass {
companion object {
fun simpleUtilFunction() {
//Do some magic here
}
}
}
Kotlin

38. void companionDemo() {
AmazingClass
.Companion
.simpleUtilFunction();
}
class AmazingClass {
companion object {
fun simpleUtilFunction() {
//Do some magic here
}
}
}
Kotlin
Java

39. class AmazingClass {
companion object {
fun simpleUtilFunction() {
//Do some magic here
}
}
}
@JvmStatic
Kotlin

40. void companionDemo() {
AmazingClass.simpleUtilFunction();
}
class AmazingClass {
companion object {
fun simpleUtilFunction() {
//Do some magic here
}
}
}
@JvmStatic
Kotlin
Java

41. Call properties by their names
There is no need of an accessor for everything
6

42. Accessing Properties by Name
• There are three scenarios where the properties of a
class can be accessed directly without getters and
setters from Java
• When the property is a `const`
• When the property is a `lateinit`
• When the property is annotated with `@JvmField`

43. class AmazingClass {
companion object {
const val BASE_URL_GOOGLE
= "www.google.com"
val BASE_URL
= "www.droidcon.at"
}
}
Kotlin

44. void companionDemo() {
system.out.println(AmazingClass.BASE_URL_GOOGLE);
system.out.println(AmazingClass.Companion.getBASE_URL()
);
}
class AmazingClass {
companion object {
const val BASE_URL_GOOGLE
= "www.google.com"
val BASE_URL
= "www.droidcon.at"
}
}
Kotlin
Java

45. void companionDemo() {
system.out.println(AmazingClass.BASE_URL_GOOGLE);
system.out.println(AmazingClass.Companion.getBASE_URL()
);
}
class AmazingClass {
companion object {
const val BASE_URL_GOOGLE
= "www.google.com"
val BASE_URL
= "www.droidcon.at"
}
}
Kotlin
Java

46. class AmazingClass {
companion object {
const val BASE_URL_GOOGLE
= "www.google.com"
val BASE_URL
= "www.droidcon.at"
}
}
@JvmField
Kotlin

47. void companionDemo() {
system.out.println(AmazingClass.BASE_URL_GOOGLE);
system.out.println(AmazingClass.BASE_URL);
);
}
class AmazingClass {
companion object {
const val BASE_URL_GOOGLE
= "www.google.com"
val BASE_URL
= "www.droidcon.at"
}
}
@JvmField
Kotlin
Java

48.
class LazyClass {
lateinit var mandatoryVariable: String
var optionalVariable: String? = null
}
Kotlin

49. Java
void propertyAccessDemo() {
LazyClass lazyClass = new LazyClass();
lazyClass.mandatoryVariable = "abcd";
lazyClass.setMandatoryVariable("vivek");
}
class LazyClass {
lateinit var mandatoryVariable: String
var optionalVariable: String? = null
}
Kotlin

50. Java
void propertyAccessDemo() {
LazyClass lazyClass = new LazyClass();
lazyClass.optionalVariable = "hello";
//not available
}
class LazyClass {
lateinit var mandatoryVariable: String
var optionalVariable: String? = null
}
Kotlin

51. fun() with defaults
Functions with default params are not straight-forward
7

52. fun increaseValue(
givenValue: Int,
increaseBy: Int = 1): Int {
return givenValue + increaseBy
}
Kotlin

53. fun increaseValue(
givenValue: Int,
increaseBy: Int = 1): Int {
return givenValue + increaseBy
}
Kotlin
void increaseValueDemo() {
DemoKt.increaseValue(60,1);
}
Java

54. fun increaseValue(
givenValue: Int,
increaseBy: Int = 1): Int {
return givenValue + increaseBy
}
Kotlin
void increaseValueDemo() {
DemoKt.increaseValue(60,1);
}
Java
DemoKt.increaseValue(60);
This is an error. No methods match such signature

55. fun increaseValue(
givenValue: Int,
increaseBy: Int = 1): Int {
return givenValue + increaseBy
}
Kotlin
@JvmOverloads

56. fun increaseValue(
givenValue: Int,
increaseBy: Int = 1): Int {
return givenValue + increaseBy
}
Kotlin
@JvmOverloads

57. fun increaseValue(
givenValue: Int,
increaseBy: Int = 1): Int {
return givenValue + increaseBy
}
Kotlin
@JvmOverloads
void intelligentMethodDemo() {
DemoKt.increaseValue(60,5);
DemoKt.increaseValue(60);
}
Java

58. Handle with care
Exceptions are your worst enemies! Never forget them!
8

59. fun writeBytes(bytes: ByteArray) {
val file = File(“file_path")
val fos = FileOutputStream(file)
fos.write(bytes)
fos.close()
}
Kotlin
This line is going to throw
an exception

60. fun writeBytes(bytes: ByteArray) {
val file = File(“file_path")
val fos = FileOutputStream(file)
fos.write(bytes)
fos.close()
}
Kotlin
void exceptionDemo() {
byte[] bytes = new byte[];
DemoKt.writeBytes(bytes);
}
Java
Does not throw any compile time errors.
Silently crashes at Runtime

61. fun writeBytes(bytes: ByteArray) {
val file = File(“file_path")
val fos = FileOutputStream(file)
fos.write(bytes)
fos.close()
}
Kotlin
@Throws(IOException::class)
Document the exceptions that might
be thrown

62. fun writeBytes(bytes: ByteArray) {
val file = File(“file_path")
val fos = FileOutputStream(file)
fos.write(bytes)
fos.close()
}
Kotlin
@Throws(IOException::class)
void exceptionDemo() {
byte[] bytes = new byte[];
DemoKt.writeBytes(bytes);
}
Java
Shows a compile time error and provides an
option to surround with try catch block

63. Working with Function Types
Function types which return Unit might not be what you want
9

64. The pitfalls of Function Types
• Fan of higher order functions and lambdas?
• Kotlin higher-order functions are a very nice feature to use but should be used
carefully.
• If your function type returns `Unit`, then, when used from Java, it has to return
`Unit.INSTANCE` which is unidiomatic
• Alternatives are to use a SAM (Single Abstract Method) interface in Java or Kotlin
• This issue will soon be fixed in one of the Kotlin releases and until then, you can
use Function Types bearing in mind the inconveniences caused to Java consumers

65. fun lambdaExample(block: (Int) -> Unit) {
//do some magic
block(3)
}
Kotlin

66. fun lambdaExample(block: (Int) -> Unit) {
//do some magic
block(3)
}
Kotlin
DemoKt.lambdaExample(integer -> {
system.out.println(integer);
return Unit.INSTANCE;
});
DemoKt.lambdaExample(integer -> {
system.out.println(integer);
return null;
});
Java

67. interface Callback {
fun doSomething(value: Int)
}
fun lambdaExampleInterface(
callback: Callback) {
callback.doSomething(3)
}
Kotlin
We have a SAM interface
written in Kotlin

68. Java
void lambdaDemo() {
DemoKt.lambdaExampleInterface(
integer -> {
system.out.println(integer);
});
}
interface Callback {
fun doSomething(value: Int)
}
fun lambdaExampleInterface(
callback: Callback) {
callback.doSomething(3)
}
Kotlin

69. interface Callback {
fun doSomething(value: Int)
}
fun lambdaExampleInterface(
callback: Callback) {
callback.doSomething(3)
}
Kotlin
Kotlin
lambdaExampleInterface(object : Callback {
override fun doSomething(value: Int) {
//do something here
}
})

70. Java
interface Callback {
void doSomething(int value)
}
Kotlin
fun lambdaExampleInterface(
callback: Callback) {
callback.doSomething(3)
}
We have a SAM interface
written in Java

71. Java
interface Callback {
void doSomething(int value)
}
Kotlin
fun lambdaExampleInterface(
callback: Callback) {
callback.doSomething(3)
}
Java
void lambdaDemo() {
DemoKt.lambdaExampleInterface(
integer -> {
system.out.println(integer);
});
}

72. Java
interface Callback {
void doSomething(int value)
}
Kotlin
fun lambdaExampleInterface(
callback: Callback) {
callback.doSomething(3)
}
Kotlin
lamdaExampleInterface(Callback { it ->
//do something here
})

73. Exposing immutable collections
Java does not treat collections as immutable.
10

74. Collections - Handle with Care
• In Kotlin, collections are by default immutable (list, set, map etc)
• Meaning, you cannot add or modify values in the collection
• If a mutable version is required, the request should be explicit to
create a mutable version of the collection
• When exposing immutable collections through public APIs, Java
consumers can receive them and make changes to it, causing
inconsistency

75. fun getPersonList(): List<Person> {
val personList: List<Person> =
fetchPersonList()
return personList
}
Kotlin

76. fun getPersonList(): List<Person> {
val personList: List<Person> =
fetchPersonList()
return personList
}
Kotlin
void collectionDemo() {
DemoKt.getPersonList().add(
new Person("Vivek",30));
}
Java

77. fun getCurrentPersonList(): List<Person> {
val personList = getPersonList()
val currentPersonList =
mutableListOf<Person>()
Collections.copy(
currentPersonList
,personList)
return currentPersonList
}
Kotlin

78. fun getCurrentPersonList(): List<Person> {
val personList = getPersonList()
val currentPersonList =
mutableListOf<Person>()
Collections.copy(
currentPersonList
,personList)
return currentPersonList
}
Kotlin
void collectionDemo() {
DemoKt.getCurrentPersonList().add(
new Person("Vivek",30));
}
Java

79. Thank you!
Start shooting your questions my way