Calling Kotlin from Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling Kotlin function from Java

Consider a Kotlin class with a function called from Java. What will be the output when the Java code runs?

Kotlin

/* Kotlin file: Greeting.kt */
package example

class Greeting {
    fun greet(name: String): String {
        return "Hello, $name!"
    }
}

/* Java file: Main.java */
package example;

public class Main {
    public static void main(String[] args) {
        Greeting greeting = new Greeting();
        System.out.println(greeting.greet("Alice"));
    }
}

A Hello, Alice!

B greet

C Hello, name!

D Compilation error due to Kotlin-Java interop

💡 Hint

Remember Kotlin functions are callable from Java with the same method names.

The Kotlin class Greeting has a method greet that returns a greeting string. Java calls this method normally, so the output is the greeting with the passed name.

🧠 Conceptual

intermediate

Kotlin property access from Java

Given a Kotlin class with a property, how does Java access it?

Kotlin

/* Kotlin file: Person.kt */
package example

class Person(val name: String, var age: Int)

A Java accesses 'name' via getName() and 'age' via getAge() and setAge() methods.

B Java accesses 'name' via name() method and 'age' via age() method.

C Java cannot access Kotlin properties at all.

D Java accesses 'name' and 'age' directly as fields without methods.

💡 Hint

Kotlin properties generate getter and setter methods for Java.

Kotlin properties compile to private fields with public getter and setter methods. Java uses these methods to access properties.

🔧 Debug

advanced

Why does this Java code fail to call Kotlin top-level function?

You have a Kotlin file with a top-level function. Java code tries to call it but fails. Identify the cause.

Kotlin

/* Kotlin file: Utils.kt */
package example

fun multiply(a: Int, b: Int): Int = a * b

/* Java file: Main.java */
package example;

public class Main {
    public static void main(String[] args) {
        int result = Utils.multiply(3, 4);
        System.out.println(result);
    }
}

A The Kotlin function multiply must be inside a class to be called from Java.

B Java cannot call Kotlin top-level functions without @JvmName or using the generated class name.

C Java code must import kotlin.* to call Kotlin functions.

D The Kotlin function multiply is private by default and not visible to Java.

💡 Hint

Kotlin top-level functions compile to static methods in a generated class.

Kotlin top-level functions are compiled into a class named after the file with 'Kt' suffix by default. Java must call example.UtilsKt.multiply(...) or use @file:JvmName annotation to rename the class.

📝 Syntax

advanced

Correct Java syntax to call Kotlin companion object function

Given a Kotlin class with a companion object function, which Java code correctly calls it?

Kotlin

/* Kotlin file: Calculator.kt */
package example

class Calculator {
    companion object {
        fun add(a: Int, b: Int): Int = a + b
    }
}

A int sum = Calculator.Companion().add(5, 7);

B int sum = Calculator.add(5, 7);

C int sum = Calculator.add.Companion(5, 7);

D int sum = Calculator.Companion.add(5, 7);

💡 Hint

Kotlin companion object functions are accessed via Companion instance in Java.

In Java, companion object members are accessed through the static field 'Companion' of the class.

🚀 Application

expert

How to expose Kotlin function with default parameters to Java

Kotlin functions can have default parameter values. How can you call such a function from Java with default values?

Kotlin

/* Kotlin file: Printer.kt */
package example

class Printer {
    @JvmOverloads
    fun printMessage(message: String = "Hello", times: Int = 1) {
        repeat(times) {
            println(message)
        }
    }
}

A Java can call printMessage() without arguments only if the function is static.

B Java must always provide all arguments; default parameters are ignored.

C Java can call printMessage() with zero, one, or two arguments because @JvmOverloads generates overloads.

D Java cannot call Kotlin functions with default parameters at all.

💡 Hint

The @JvmOverloads annotation generates multiple Java methods with different parameters.

Kotlin's @JvmOverloads creates overloaded methods for Java, allowing calls with fewer arguments using defaults.