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