Kotlin's Interoperability with Java
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The document presents a talk by David Greenhalgh on Kotlin's interoperability with Java, highlighting its concise, pragmatic, and type-safe features. It explains how Kotlin and Java can work together seamlessly, allowing for gradual transitions from Java to Kotlin while leveraging existing Java frameworks. Additionally, it discusses Kotlin's compilation process and practical examples of calling Java methods from Kotlin, as well as the implications of nullability and type mapping between the two languages.
![A Standalone Kotlin Function (Hero.kt)
fun main(args: Array<String>) {
...
}
fun makeProclamation() = "Greetings, beast!"
Calling a File-level Function from Java
public static void main(String[] args) {
System.out.println(HeroKt.makeProclamation());
}](https://image.slidesharecdn.com/2018kotlinjavainteroperability-200206191318/85/Kotlin-s-Interoperability-with-Java-21-320.jpg)
![Customizing Class Name (Hero.kt)
@file:JvmName("Hero")
fun main(args: Array<String>) {
...
}
fun makeProclamation() = "Greetings, beast!"
Using the Customized Class Name
public static void main(String[] args) {
System.out.println(Hero.makeProclamation());
}](https://image.slidesharecdn.com/2018kotlinjavainteroperability-200206191318/85/Kotlin-s-Interoperability-with-Java-22-320.jpg)
Kotlin's Interoperability with Java
- 1. NEW BEST FRIENDS: Kotlin’s Interoperability with Java Presented by: David Greenhalgh
- 2. TODAY’S FORMAT Housekeeping/Intro Kotlin Talk by David Live Q&A Lunch & Learn / Book Giveaway
- 3. BIG NERD RANCH Travel to your office Discuss Kotlin Enjoy lunch!
- 4. BUILD BRILLIANCE BIG NERD RANCH • Immersive BOOTCAMPS • Onsite TEAM TRAINING • THE FRONTIER subscription • Digital PRODUCT DEVELOPMENT 4
- 7. BIG NERD RANCH Travel to your office Discuss Kotlin Enjoy lunch!
- 8. How Kotlin Interoperates with Java
- 9. Why Kotlin? • Concise • Pragmatic • Flexible • Type-safe Why Kotlin? • Concise • Pragmatic Java person.getName(); Kotlin person.name
- 10. Why Kotlin? • Type-safe Objects may not hold null values unless declared using a nullable type String may not be null String? may be null Why Kotlin? • Full interoperability with Java
- 11. Interoperability Kotlin code and Java code works together directly • Call Java methods from Kotlin • Call Kotlin functions from Java The Kotlin Compilation Process Kotlin compiles down to Java bytecode
- 12. JVM Bytecode Decompiled Java Bytecode
- 13. Interoperability Kotlin code and Java code works together directly • Leverage legacy Java frameworks (e.g. Android) • Gradually transition your codebase from Java to Kotlin An Example • Calling a Java method from Kotlin • Nullability and platform types • Accessing Java fields from Kotlin • Type mapping • Kotlin file anatomy
- 14. A Java Method public class Jhava { public String utterGreeting() { return "BLARGH"; } } Calling a Java Method from Kotlin fun main(args: Array<String>) { val adversary = Jhava() println(adversary.utterGreeting()) }
- 15. Another Java Method public class Jhava { public String utterGreeting() { return "BLARGH"; } public String determineFriendshipLevel() { return null; } } Platform Type
- 16. NullPointerException in Kotlin fun main(args: Array<String>) { ... val friendshipLevel = adversary.determineFriendshipLevel() println(friendshipLevel.toLowerCase()) } Nullability Annotations public class Jhava { @NotNull public String utterGreeting() { return "BLARGH"; } @Nullable public String determineFriendshipLevel() { return null; } }
- 17. IntelliJ Nullable Type Awareness Java Field with Getter and Setter public class Jhava { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }
- 18. Property Reference from Kotlin fun main(args: Array<String>) { Jhava().name } Type Mapping Some Kotlin types map directly to Java types • String to String
- 19. Type Mapping Other Kotlin types do not map directly to Java types • Kotlin's Int to Java's int Primitive Types • Java represents basic data types using "primitve types" (non-objects) • Kotlin represents all data using objects
- 20. Integers in Kotlin fun main(args: Array<String>) { val adversary = Jhava() val adversaryHitPoints: Int = adversary.hitPoints adversaryHitPoints.dec() println(adversaryHitPoints.javaClass) } >>> int Kotlin File Anatomy • Java files contain exactly one class • Kotlin files may contain classes, functions, and variables at top-level • How can you call standalone Kotlin functions from Java?
- 21. A Standalone Kotlin Function (Hero.kt) fun main(args: Array<String>) { ... } fun makeProclamation() = "Greetings, beast!" Calling a File-level Function from Java public static void main(String[] args) { System.out.println(HeroKt.makeProclamation()); }
- 22. Customizing Class Name (Hero.kt) @file:JvmName("Hero") fun main(args: Array<String>) { ... } fun makeProclamation() = "Greetings, beast!" Using the Customized Class Name public static void main(String[] args) { System.out.println(Hero.makeProclamation()); }
- 23. Customizing Compiled Java Code JVM annotations exist to customize the Java code that is compiled from your Kotlin source • @JvmName • @JvmOverloads • @JvmStatic • @JvmField Refrain: Your Upgrade Path Kotlin compiles down to Java bytecode
- 24. Risks of Sticking with Java • Slow moving language development • Less type-safe • Less concise • Less interest from developers Next Steps • Start a new project in Kotlin • Upgrade selectively • Acquire Kotlin resources