Kotlin annotations for Java callers (@JvmStatic, @JvmField) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of @JvmStatic annotated companion object function call from Java

Consider this Kotlin code with a companion object and a function annotated with @JvmStatic. What will be the output when called from Java as Example.Companion.greet() and Example.greet()?

Kotlin

class Example {
    companion object {
        @JvmStatic
        fun greet() = "Hello from @JvmStatic"

        fun greetNormal() = "Hello from normal companion"
    }
}

A Example.Companion.greet() prints "Hello from @JvmStatic"; Example.greet() does not compile

B Example.Companion.greet() prints "Hello from normal companion"; Example.greet() prints "Hello from @JvmStatic"

C Example.Companion.greet() prints "Hello from @JvmStatic"; Example.greet() prints "Hello from @JvmStatic"

D Example.Companion.greet() does not compile; Example.greet() prints "Hello from @JvmStatic"

💡 Hint

Remember that @JvmStatic creates a static method in the containing class for Java callers.

The @JvmStatic annotation generates a static method in the containing class, so Java can call Example.greet() directly. The companion object still has the instance method, so Example.Companion.greet() also works and returns the same string.

❓ Predict Output

intermediate

Effect of @JvmField on Kotlin property visibility in Java

Given this Kotlin class with a property annotated with @JvmField, what is the output or behavior when accessing Example.CONSTANT from Java?

Kotlin

class Example {
    @JvmField
    val CONSTANT = "Kotlin constant"

    val normalConstant = "Normal constant"
}

A Java can access Example.CONSTANT directly as a field; Example.normalConstant requires getter method

B Java cannot access Example.CONSTANT directly; both require getter methods

C Java can access both Example.CONSTANT and Example.normalConstant directly as fields

D Java cannot access either Example.CONSTANT or Example.normalConstant

💡 Hint

Think about how @JvmField changes the generated bytecode for properties.

The @JvmField annotation exposes the Kotlin property as a public field in Java, so Java code can access Example.CONSTANT directly. The normal property generates a private field with a public getter, so Java must use the getter method.

🔧 Debug

advanced

Why does Java code fail to call Kotlin companion function without @JvmStatic?

This Kotlin code defines a companion object function without @JvmStatic. Java code tries to call Example.greet() but fails. Why?

Kotlin

class Example {
    companion object {
        fun greet() = "Hello"
    }
}

A Because the function requires an instance of Example to be called

B Because Kotlin companion objects cannot be called from Java at all

C Because the function is private by default and not visible to Java

D Because without @JvmStatic, the function is only inside the companion object instance, not as a static method in Example class

💡 Hint

Check how Kotlin companion object functions are compiled and accessed from Java.

Without @JvmStatic, the function is compiled as an instance method inside the companion object class. Java must call Example.Companion.greet(), not Example.greet(). The latter does not exist as a static method.

📝 Syntax

advanced

Which Kotlin property declaration with @JvmField is valid?

Select the valid Kotlin property declaration using @JvmField that compiles without error.

A

"class Example {
    @JvmField
    var count: Int = 0
}"

B

"class Example {
    @JvmField
    private val name: String = "Test"
}"

C

"class Example {
    @JvmField
    val age: Int
}"

D

"class Example {
    @JvmField
    fun getValue() = 42
}"

💡 Hint

Remember @JvmField can only be applied to certain property types and visibility.

@JvmField can only be applied to public properties (val or var) without custom getters/setters. Option A is valid: public var with initializer. Option A is invalid because property is private. Option A is invalid because val without initializer or getter is not allowed. Option A is invalid because @JvmField cannot be applied to functions.

🚀 Application

expert

How to expose Kotlin top-level constant as a Java static field?

You want to expose a Kotlin top-level constant so Java code can access it as a static field without calling a getter method. Which Kotlin declaration achieves this?

A

@JvmField
val MAX_COUNT = 100

B const val MAX_COUNT = 100

C

@JvmStatic
val MAX_COUNT = 100

D val MAX_COUNT = 100

💡 Hint

Consider how Kotlin compiles const val and @JvmField at top-level.

const val creates a compile-time constant that is exposed as a static final field in the generated Java class, accessible directly from Java. @JvmField cannot be applied to top-level properties. @JvmStatic applies only to members inside objects or companion objects. Plain val generates a private field with getter.