Accessing Kotlin extension functions from Java

Defining a custom naming convention for the calling class in Java

You can utilize Kotlin's @JvmName annotation to define a custom class name for Java.

// At the top of your Kotlin file
@file:JvmName("DemoUtils") // Giving Java a name it can pronounce

fun String.exclaim(): String = "$this!" 
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And in Java, you refer to DemoUtils instead of StringExtensionsKt.

// In Java
String excited = DemoUtils.exclaim("Hello, World");
// Ah! A name I recognize.
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Matching understandings between Kotlin and Java

In the generated Java class, the receiver type (the class being extended in Kotlin) appears as the first parameter of the associated static method.

// Kotlin
fun String.multiply(times: Int): String = repeat(times)
// 'this' in Kotlin-land is 'String' in Java-land.
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This function in Java requires the String object as the first parameter:

// Java
String repeated = StringExtensionsKt.multiply("hi", 3);
// 'this' is now 'hi'. 'hi' is happy to be first.
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Borrowing Kotlin's awesome sauce in Java classes

Use static imports to leverage convenience in your Java classes, mimicking Kotlin's dot-syntax.

// Java
import static your.package.StringExtensionsKt.*;

public class JavaUsage {
  // Borrow Kotlin's magic and use it here.
  public void printGreeting() {
    System.out.println(greet("Java User"));
  }
}
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Java's nullability annotations keep Kotlin's null safety

Annotate parameters in Java with @NotNull or @Nullable. Kotlin respects these annotations. It informs the Kotlin compiler of nullability. Check this Kotlin function:

// Kotlin
fun String?.orEmpty(): String = this ?: "" // Defusing Kotlin's nullable bomb.
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Java, remember that null is a possibility:

// Java
String maybeNull = possiblyNullString.orEmpty(); // null or not null. That's the question.
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Predictable patterns of generated classes

Kotlin creates classes for Java to use following an understandable convention. A MyExtensions.kt file results in:

// Java
import static com.example.MyExtensionsKt.*; 
// Clever Kotlin. Changing MyExtensions.kt into MyExtensionsKt. See what it did there? 
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A convention you can trust unless @JvmName is playing.

Kotlin to Java tricks

Experience enhancement by knowing these interoperability tricks:

• Use @JvmOverloads for functions with default params.
• @JvmStatic helps you access methods from Java without an instance within companion objects.
• @JvmField directly exposes fields with the same visibility in Java.