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Kotlin’s Interoperability with Java
Presented by: David Greenhalgh

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How Kotlin Interoperates with Java

Why Kotlin?
• Concise
• Pragmatic
• Flexible
• Type-safe
Why Kotlin?
• Concise
• Pragmatic
Java
person.getName();
Kotlin
person.name

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

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

JVM Bytecode
Decompiled Java Bytecode

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

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())
}

Another Java Method
return "BLARGH";
}
public String determineFriendshipLevel() {
return null;
}
}
Platform Type

NullPointerException in Kotlin
...
val friendshipLevel = adversary.determineFriendshipLevel()
println(friendshipLevel.toLowerCase())
}
Nullability Annotations
@NotNull
return "BLARGH";
}
@Nullable
public String determineFriendshipLevel() {
return null;
}
}

IntelliJ Nullable Type Awareness
Java Field with Getter and Setter
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}

Property Reference from Kotlin
Jhava().name
}
Type Mapping
Some Kotlin types map directly to Java types
• String to String

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

Integers in Kotlin
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?

A Standalone Kotlin Function (Hero.kt)
...
}
fun makeProclamation() = "Greetings, beast!"
Calling a File-level Function from Java
public static void main(String[] args) {
System.out.println(HeroKt.makeProclamation());
}

Customizing Class Name (Hero.kt)
@file:JvmName("Hero")
...
}
fun makeProclamation() = "Greetings, beast!"
Using the Customized Class Name
public static void main(String[] args) {
System.out.println(Hero.makeProclamation());
}

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

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

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Kotlin's Interoperability with Java

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