Problem Analysis

When Java code attempts to call Kotlin functions, compilation errors often occur due to differences in how the two languages handle class and method visibility. The most common error is "cannot find symbol," which typically indicates that the Java compiler cannot locate the Kotlin class or method.

Solution Implementation

Create a Kotlin Module

First, establish a Kotlin module within your project structure. Most IDEs like IntelliJ IDEA or Android Studio provide templates for Kotlin module creation.

Define a Kotlin Class with Public Methods

Create a Kotlin class containing methods intended for Java consumption. Use explicit visibility modifiers:

class Calculator {
    fun displayMessage() {
        println("Message from Kotlin module")
    }
    
    fun computeSum(firstValue: Int, secondValue: Int): Int {
        return firstValue + secondValue
    }
}

Configure Build Dependencies

Ensure the Java module has a dependancy on the Kotlin module. In Gradle, add to your build.gradle:

dependencies {
    implementation project(':kotlin-module-name')
}

Java Implementation

In your Java class, import and use the Kotlin class normally:

public class Application {
    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        calculator.displayMessage();
        
        int total = calculator.computeSum(7, 3);
        System.out.println("Computed total: " + total);
    }
}

Build Configuration Verification

Verify that your build system properly processes Kotlin files before Java compilation. For Maven, ensure the kotlin-maven-plugin executes during the compile phase.