Calling Java from Kotlin seamlessly - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling Java method from Kotlin

What is the output of this Kotlin code when calling a Java method?

Kotlin

public class JavaClass {
    public static String greet(String name) {
        return "Hello, " + name + "!";
    }
}

fun main() {
    println(JavaClass.greet("Alice"))
}

A Runtime error: NullPointerException

B Hello, null!

C Compilation error: Cannot find symbol greet

D Hello, Alice!

💡 Hint

Java static methods can be called directly from Kotlin using the class name.

The Kotlin code calls the static Java method greet with argument "Alice". The method returns "Hello, Alice!" which is printed.

❓ Predict Output

intermediate

Accessing Java fields from Kotlin

What will be printed when this Kotlin code accesses a Java field?

Kotlin

public class JavaData {
    public int number = 42;
}

fun main() {
    val data = JavaData()
    println(data.number)
}

A 42

B 0

C Compilation error: Cannot access field number

D Runtime error: IllegalAccessException

💡 Hint

Public Java fields are accessible directly from Kotlin.

The Kotlin code creates an instance of JavaData and prints the public field number which is 42.

🔧 Debug

advanced

Why does this Kotlin code fail to call Java method?

This Kotlin code tries to call a Java method but fails. What is the cause?

Kotlin

public class JavaExample {
    private String secret() {
        return "hidden";
    }
}

fun main() {
    val example = JavaExample()
    println(example.secret())
}

A Output: hidden

B Runtime error: NoSuchMethodException

C Compilation error: secret() has private access in JavaExample

D Compilation error: Cannot find symbol secret

💡 Hint

Kotlin respects Java access modifiers like private.

The secret() method is private in Java, so Kotlin cannot call it, causing a compilation error.

📝 Syntax

advanced

Correct Kotlin syntax to call Java static method

Which Kotlin code correctly calls a Java static method?

Kotlin

public class JavaUtil {
    public static String staticMethod() {
        return "Called";
    }
}

// Kotlin code options:

A println(JavaUtil.staticMethod)

B println(JavaUtil.staticMethod())

C println(JavaUtil().staticMethod())

D println(JavaUtil.Companion.staticMethod())

💡 Hint

Java static methods can be called directly on the class.

Java static methods are callable directly in Kotlin using the class name.

🚀 Application

expert

How to handle Java nullability in Kotlin safely?

Given a Java method that may return null, which Kotlin code safely handles the null value?

Kotlin

public class JavaNull {
    public static String getNullable() {
        return null;
    }
}

// Kotlin code options:

A

val result: String? = JavaNull.getNullable()
println(result ?: "default")

B

val result: String = JavaNull.getNullable()!!
println(result)

C

val result: String = JavaNull.getNullable()
println(result)

D

val result = JavaNull.getNullable() ?: throw Exception("Null")
println(result)

💡 Hint

Use Kotlin nullable types and the Elvis operator to handle possible nulls.

Option A safely assigns the nullable Java return to a nullable Kotlin variable and provides a default value if null.