Kotlin annotations for Java callers (@JvmStatic, @JvmField) - Mini Project: Build & Apply

Using Kotlin Annotations for Java Callers (@JvmStatic, @JvmField)

📖 Scenario: You are creating a Kotlin class that will be used by Java code. To make the Kotlin code easier and more natural to call from Java, you will use special Kotlin annotations.These annotations help Java callers access Kotlin properties and functions as if they were regular Java static members or fields.

🎯 Goal: Build a Kotlin class with a companion object that uses @JvmStatic and @JvmField annotations so Java code can call the static function and access the field directly.

📋 What You'll Learn

Create a Kotlin class named Utils with a companion object

Inside the companion object, create a function greet that returns the string "Hello from Kotlin"

Annotate the greet function with @JvmStatic

Inside the companion object, create a constant string field VERSION with value "1.0"

Annotate the VERSION field with @JvmField

Print the result of calling Utils.greet() and the value of Utils.VERSION

💡 Why This Matters

🌍 Real World

Kotlin is often used in Android apps and backend services, but many projects still use Java. These annotations help Kotlin code work smoothly with existing Java code.

💼 Career

Understanding Kotlin-Java interoperability is important for Android developers and backend engineers working in mixed Kotlin and Java codebases.

1

Create the Kotlin class with a companion object

Create a Kotlin class called Utils with an empty companion object inside it.

Kotlin

class Utils {
    companion object {
        // Your code here
    }
}

Hint

Remember, a companion object is like a static holder inside a Kotlin class.

2

Add a function with @JvmStatic annotation

Inside the companion object of Utils, create a function called greet that returns the string "Hello from Kotlin". Annotate this function with @JvmStatic.

Kotlin

class Utils {
    companion object {
        // Add @JvmStatic function greet here
    }
}

Hint

Use @JvmStatic just before the function declaration inside the companion object.

3

Add a constant field with @JvmField annotation

Inside the companion object of Utils, create a constant string field called VERSION with the value "1.0". Annotate this field with @JvmField.

Kotlin

class Utils {
    companion object {
        @JvmStatic
        fun greet(): String {
            return "Hello from Kotlin"
        }
        // Add @JvmField VERSION here
    }
}

Hint

Use @JvmField just before the val declaration inside the companion object.

4

Print the function call and field value

Write two println statements to print the result of calling Utils.greet() and the value of Utils.VERSION.

Kotlin

class Utils {
    companion object {
        @JvmStatic
        fun greet(): String {
            return "Hello from Kotlin"
        }

        @JvmField
        val VERSION: String = "1.0"
    }
}

// Print the greet message and version
// Your code here

Hint

Call Utils.greet() and Utils.VERSION inside println to display their values.

Practice

1. What is the main purpose of the @JvmStatic annotation in Kotlin when called from Java?

easy

A. To convert Kotlin lambdas into Java anonymous classes

B. To expose Kotlin properties as public fields in Java

C. To make a Kotlin function in a companion object appear as a static method in Java

D. To mark a Kotlin class as a Java interface

Solution

1. Step 1: Understand the role of @JvmStatic

   @JvmStatic makes functions inside Kotlin companion objects look like static methods in Java, so Java code can call them without an instance.

2. Step 2: Differentiate from @JvmField

   @JvmField exposes properties as fields, not functions. The question asks about functions, so @JvmStatic is correct.

3. Final Answer:

   To make a Kotlin function in a companion object appear as a static method in Java -> Option C

4. Quick Check:

   @JvmStatic = static method in Java [OK]

Hint: Remember: @JvmStatic = static method in Java [OK]

Common Mistakes:

• Confusing @JvmStatic with @JvmField
• Thinking @JvmStatic applies to properties
• Assuming it changes Kotlin class types

2. Which of the following is the correct way to expose a Kotlin property as a public field accessible directly from Java?

easy

A. Annotate the property with @JvmStatic

B. Annotate the property with @JvmField

C. Declare the property as val without annotations

D. Use @JvmOverloads annotation

Solution

1. Step 1: Identify the annotation for exposing fields

   @JvmField exposes Kotlin properties as public fields for direct Java access, bypassing getters/setters.

2. Step 2: Eliminate other options

   @JvmStatic applies to functions, not properties. @JvmOverloads is for default parameters. val alone does not expose as field.

3. Final Answer:

   Annotate the property with @JvmField -> Option B

4. Quick Check:

   @JvmField = public field in Java [OK]

Hint: Use @JvmField to expose properties as fields [OK]

Common Mistakes:

• Using @JvmStatic on properties
• Assuming val exposes public field automatically
• Confusing @JvmOverloads with field exposure

3. Given the Kotlin code below, what will be the output when called from Java?

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

        fun greetInstance() = "Hello from instance"
    }
}

// Java code:
System.out.println(Example.greet());
System.out.println(Example.Companion.greetInstance());

medium

A. Compilation error\nHello from instance

B. Hello from static\nCompilation error

C. Compilation error\nCompilation error

D. Hello from static\nHello from instance

Solution

1. Step 1: Understand @JvmStatic effect on Java calls

   @JvmStatic makes greet() callable as a static method on Example class directly from Java.

2. Step 2: Understand companion object function call

   greetInstance() is not annotated, so Java must call it via Example.Companion instance.

3. Final Answer:

   Hello from static\nHello from instance -> Option D

4. Quick Check:

   @JvmStatic allows direct static call, others via Companion [OK]

Hint: @JvmStatic = call directly on class; else use Companion [OK]

Common Mistakes:

• Assuming all companion functions are static in Java
• Trying to call unannotated functions as static
• Ignoring the Companion object in Java calls

4. Identify the error in the Kotlin code below if the goal is to allow Java code to access count as a public field:

class Counter {
    companion object {
        var count = 0
    }
}

medium

A. Missing @JvmField on count

B. Missing @JvmStatic on count

C. Companion object cannot have properties

D. count must be declared as val not var

Solution

1. Step 1: Understand property exposure to Java

   To expose a Kotlin property as a public field in Java, it must be annotated with @JvmField.

2. Step 2: Check annotations and property modifiers

   @JvmStatic applies to functions, not properties. Companion objects can have properties. var is allowed.

3. Final Answer:

   Missing @JvmField on count -> Option A

4. Quick Check:

   Expose property as field = use @JvmField [OK]

Hint: Use @JvmField on properties for Java field access [OK]

Common Mistakes:

• Using @JvmStatic on properties
• Thinking companion objects can't have properties
• Confusing var and val for Java exposure

5. You want to create a Kotlin utility class with a companion object that has a constant integer MAX accessible as a static field from Java, and a function printMax() callable as a static method from Java. Which is the correct Kotlin code?

hard

A. class Utils {
companion object {
@JvmField
val MAX = 100
@JvmStatic
fun printMax() = println(MAX)
}
}

B. class Utils {
companion object {
val MAX = 100
fun printMax() = println(MAX)
}
}

C. class Utils {
companion object {
@JvmStatic
val MAX = 100
@JvmField
fun printMax() = println(MAX)
}
}

D. class Utils {
companion object {
const val MAX = 100
fun printMax() = println(MAX)
}
}

Solution

1. Step 1: Expose constant as static field

   Use @JvmField on val MAX to expose it as a static field in Java.

2. Step 2: Expose function as static method

   Use @JvmStatic on printMax() to make it callable as a static method from Java.

3. Step 3: Verify other options

   class Utils { companion object { val MAX = 100 fun printMax() = println(MAX) } } lacks annotations, so Java calls require Companion. class Utils { companion object { @JvmStatic val MAX = 100 @JvmField fun printMax() = println(MAX) } } swaps annotations incorrectly. class Utils { companion object { const val MAX = 100 fun printMax() = println(MAX) } } uses const val but no @JvmStatic on function.

4. Final Answer:

   The code with @JvmField on MAX and @JvmStatic on printMax() -> Option A

5. Quick Check:

   @JvmField for fields, @JvmStatic for static methods [OK]

Hint: Use @JvmField for fields and @JvmStatic for methods [OK]

Common Mistakes:

• Swapping @JvmField and @JvmStatic annotations
• Omitting annotations and expecting static access
• Using const val without @JvmStatic on functions