While Kotlin is designed to work well with Java by default, we'll still need to some work to get clean and idiomatic code in both languages. In this talk we'll cover both how to make your Java code more Kotlin friendly and how to make your Kotlin code nicer to use from Java.

1. Kotlin - Java
Interop
Matej Vidaković

2. Doesn’t it just work?

3. Calculator calculator = new Calculator(CalculatorKt.triple(2));
if (calculator.getHasEvenValue()) {
calculator.
calculate$module(Calculator.Companion.randomMultiplier(), 0);
} else {
calculator.
calculate$module(Calculator.Companion.getDEFAULT_MULTIPLIER(), 0);
}

4. Kotlin - Java differences
● null safety
● extension functions
● properties
● companion objects
● checked exceptions
● primitive types
● static members
● visibility modifiers

5. fun Int.triple(): Int = this * 3
class Calculator(val value: Int) {
val hasEvenValue: Boolean = value % 2 == 0
internal fun calculate(multiplier: Int, offset: Int = 0)
= value * multiplier + offset
companion object {
val DEFAULT_MULTIPLIER = 1
fun randomMultiplier() = Random.nextInt()
}
}

6. Top level functions
@file:JvmName("IntUtils")
@file:JvmMultifileClass
package hr.matvidako
CalculatorKt.triple(2)
IntUtils.triple(2)

7. Renaming getters
@get:JvmName("hasEvenValue")
val hasEvenValue: Boolean = value % 2 == 0
calculator.hasEvenValue()
calculator.getHasEvenValue()

8. Internal methods
@JvmName("calculate")
internal fun calculate(multiplier: Int, offset: Int = 0)
calculator.calculate$module(Calculator.Companion.randomMultiplier(), 0)
calculator.calculate(Calculator.Companion.randomMultiplier(), 0)

9. Default param values
@JvmOverloads @JvmName("calculate")
internal fun calculate(multiplier: Int, offset: Int = 0)
calculator.calculate(Calculator.Companion.randomMultiplier(), 0)
calculator.calculate(Calculator.Companion.randomMultiplier())

10. Static methods
@JvmStatic
fun randomMultiplier() = Random.nextInt()
calculator.calculate(Calculator.randomMultiplier())
calculator.calculate(Calculator.Companion.randomMultiplier())

11. Exposing property as field
@JvmField
val DEFAULT_MULTIPLIER = 1
calculator.calculate(Calculator.DEFAULT_MULTIPLIER)
calculator.calculate(Calculator.getDEFAULT_MULTIPLIER());

12. Calculator calculator = new Calculator(IntUtils.triple(2));
if (calculator.hasEvenValue()) {
calculator.calculate(Calculator.randomMultiplier());
} else {
calculator.calculate(Calculator.DEFAULT_MULTIPLIER);
}

13. @file:JvmName("IntUtils") @file:JvmMultifileClass
package hr.matvidako
fun Int.triple(): Int = this * 3
class Calculator(val value: Int) {
@get:JvmName("hasEvenValue")
val hasEvenValue: Boolean = value % 2 == 0
@JvmOverloads @JvmName("calculate")
internal fun calculate(multiplier: Int, offset: Int = 0)
= value * multiplier + offset
companion object {
@JvmField
val DEFAULT_MULTIPLIER = 1
@JvmStatic
fun randomMultiplier() = Random.nextInt()
}
}

14. That’s a lot of annotations…

15. Kotlin friendly Java
● add Nullability annotations
● avoid using Kotlin keywords in method / field names
○ `as`, `in`, `when`
● use standard get, set method prefixes
○ allows property syntax in Kotlin
● put params eligible for SAM conversion last
○ allows use of trailing lambda syntax

16. In conclusion...

17. Thank you