Java and Kotlin are both official languages for Android development, and one of Kotlin’s biggest strengths is its seamless interoperability with Java. This allows developers to migrate projects gradually, use existing Java libraries, and leverage Kotlin’s modern features without abandoning Java entirely.

In this blog, we will explore how Kotlin interacts with Java, focusing on:

• Calling Java code from Kotlin
• Calling Kotlin code from Java
• Handling nullability
• Working with Java collections
• Overcoming common interoperability challenges

By the end of this guide, you’ll have a solid understanding of Java-Kotlin interoperability and how to make the most of both languages in a single project.

Why Java Interoperability Matters in Kotlin?

Since Java has been around for decades, a vast number of libraries, frameworks, and applications are built with it. Kotlin’s interoperability ensures that:

• You can migrate to Kotlin incrementally instead of rewriting entire projects.
• Existing Java libraries (e.g., Retrofit, Glide) can be used in Kotlin without modification.
• Teams can work with both languages in the same project.

Calling Java Code from Kotlin

Using Java classes in Kotlin is straightforward. Kotlin treats Java code almost as if it were native.

// Java class
public class User {
    private String name;
    public User(String name) { this.name = name; }
    public String getName() { return name; }
}

// Kotlin usage
val user = User("amol")
println(user.name) // Calls getName() seamlessly

Kotlin automatically maps Java getters and setters to properties, making the syntax cleaner.

Calling Kotlin Code from Java

The reverse is also possible: Java can call Kotlin code. However, some Kotlin features don’t translate directly, so annotations help.

class Utils {
    @JvmStatic
    fun printMessage(msg: String) {
        println(msg)
    }
}

// Java usage
Utils.printMessage("Hello from Java");

Here, @JvmStatic ensures the Kotlin function behaves like a regular Java static method.

Handling Nullability

One of Kotlin’s core advantages is null safety. When calling Java code, Kotlin treats platform types cautiously:

• A Java type like String might be nullable or non-nullable, and Kotlin lets you decide how to handle it.
• Use Kotlin’s safe call (?.) and elvis operator (?:) to protect against NullPointerException.

val length = javaUser.name?.length ?: 0

This guarantees safety when working with Java APIs that may return null.

Working with Java Collections

Kotlin distinguishes between mutable and immutable collections, while Java does not.

• A List<String> in Kotlin may map to a List<String> in Java but can cause confusion if mutability expectations differ.
• To avoid issues, be explicit when converting collections between Kotlin and Java using methods like toList() or toMutableList().

Common Interoperability Challenges and Solutions

• Default parameters in Kotlin — Java doesn’t support them. Use @JvmOverloads to generate overloaded versions.
• Companion objects — add @JvmStatic for Java-friendly access.
• Checked exceptions — Java requires them, Kotlin doesn’t. When calling Java code, handle exceptions properly.

By following these practices, you minimize friction between the two languages.

Conclusion

Kotlin’s interoperability with Java is one of its biggest advantages, allowing developers to:

• Gradually migrate projects
• Use existing Java libraries
• Leverage modern Kotlin features alongside Java

Understanding how to handle null safety, collections, and special Kotlin features in Java ensures smooth integration between the two languages. By following best practices and using annotations like @JvmOverloads and @JvmStatic, you can build efficient, maintainable, and error-free applications.

If you’re transitioning from Java to Kotlin, start small by calling Java code from Kotlin before diving deeper into full migration.

Happy migrating..!