Calling Kotlin from Java - Mini Project: Build & Apply

Calling Kotlin from Java

📖 Scenario: You are working on a project where Kotlin and Java code need to work together. You want to call a Kotlin function from Java code.

🎯 Goal: Build a simple Kotlin function and call it from Java code, then print the result.

📋 What You'll Learn

Create a Kotlin file with a function named greet that takes a String parameter called name and returns a greeting message.

Create a Java class with a main method that calls the Kotlin greet function with the name "Alice".

Print the greeting message returned by the Kotlin function in the Java main method.

💡 Why This Matters

🌍 Real World

Many Android apps and JVM projects use both Kotlin and Java together. Knowing how to call Kotlin from Java helps you work on mixed codebases.

💼 Career

Understanding interoperability between Kotlin and Java is important for Android developers and backend developers working on JVM platforms.

1

Create Kotlin function greet

Create a Kotlin file named Greeting.kt and write a function called greet that takes a String parameter name and returns the string "Hello, " plus the name.

Kotlin

// Your code here

Hint

Use a Kotlin function with a String parameter and return a greeting using string templates.

2

Create Java class with main method

Create a Java class named Main with a main method. Inside the method, declare a String variable named name and set it to "Alice".

Kotlin

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

// Your code here

Hint

Write a Java class with a main method and declare a String variable.

3

Call Kotlin greet function from Java

In the Java main method, call the Kotlin function greet with the variable name and store the result in a String variable called message.

Kotlin

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

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        // Your code here
    }
}

Hint

Use the Kotlin file name GreetingKt to call the greet function from Java.

4

Print the greeting message in Java

In the Java main method, print the message variable using System.out.println.

Kotlin

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

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        String message = GreetingKt.greet(name);
        // Your code here
    }
}

Hint

Use System.out.println(message); to print the greeting.

Practice

1. Which statement is true about calling Kotlin functions from Java?

easy

A. You can call Kotlin class functions like normal Java methods.

B. You cannot call Kotlin functions from Java directly.

C. Kotlin functions must be rewritten in Java to be used.

D. Kotlin functions are only accessible in Kotlin files.

Solution

1. Step 1: Understand Kotlin-Java interoperability

   Kotlin classes compile to Java bytecode, so their functions appear as normal Java methods.

2. Step 2: Confirm Java can call Kotlin class functions

   Java code can create Kotlin objects and call their methods just like Java methods.

3. Final Answer:

   You can call Kotlin class functions like normal Java methods. -> Option A

4. Quick Check:

   Kotlin class functions = callable from Java [OK]

Hint: Kotlin classes compile to Java classes, so methods are callable [OK]

Common Mistakes:

• Thinking Kotlin functions need rewriting in Java
• Believing Kotlin functions are inaccessible from Java
• Confusing top-level functions with class functions

2. Which is the correct way to call a top-level Kotlin function fun greet() from Java?

easy

A. KotlinFileKt.greet();

B. greet();

C. new KotlinFile().greet();

D. Kotlin.greet();

Solution

1. Step 1: Understand top-level function compilation

   Top-level Kotlin functions compile into static methods inside a generated class named after the Kotlin file with 'Kt' suffix.

2. Step 2: Call the function from Java using generated class

   Java calls the function as a static method on the generated class, e.g., KotlinFileKt.greet().

3. Final Answer:

   KotlinFileKt.greet(); -> Option A

4. Quick Check:

   Top-level function call = KotlinFileKt.greet() [OK]

Hint: Top-level Kotlin functions appear in Java as static methods in FileNameKt class [OK]

Common Mistakes:

• Trying to call top-level function directly without class
• Assuming top-level functions need an instance
• Using wrong class name without 'Kt' suffix

3. Given Kotlin file Utils.kt with top-level function:

fun add(a: Int, b: Int): Int = a + b

What is the output of this Java code?

System.out.println(UtilsKt.add(3, 4));

medium

A. Runtime exception

B. 34

C. Compile error

D. 7

Solution

1. Step 1: Identify the Kotlin function behavior

   The Kotlin function adds two integers and returns the sum.

2. Step 2: Understand Java call to Kotlin top-level function

   Java calls UtilsKt.add(3, 4), which returns 3 + 4 = 7.

3. Final Answer:

   7 -> Option D

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Top-level Kotlin functions return expected results when called from Java [OK]

Common Mistakes:

• Confusing concatenation with addition
• Expecting compile error due to Kotlin-Java call
• Assuming runtime exception from Kotlin code

4. What is wrong with this Java code calling Kotlin class Person with function fun greet()?:

Person p = new Person();
p.greet;

medium

A. greet is a private function in Kotlin.

B. Person class cannot be instantiated in Java.

C. Missing parentheses to call the function.

D. Kotlin functions cannot be called from Java.

Solution

1. Step 1: Check Java method call syntax

   In Java, methods must be called with parentheses, even if no arguments.

2. Step 2: Identify the error in code

   Code uses p.greet; missing parentheses, causing syntax error.

3. Final Answer:

   Missing parentheses to call the function. -> Option C

4. Quick Check:

   Method call syntax requires () [OK]

Hint: Always use parentheses to call methods in Java [OK]

Common Mistakes:

• Omitting parentheses when calling methods
• Assuming Kotlin classes can't be instantiated in Java
• Thinking Kotlin functions are private by default

5. You have a Kotlin file MathUtils.kt with:

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

How should you call multiply from Java to get 20 when multiplying 4 and 5?

hard

A. MathUtilsKt.multiply(4, 5);

B. MathUtils.Companion.multiply(4, 5);

C. new MathUtils().multiply(4, 5);

D. MathUtils.multiply(4, 5);

Solution

1. Step 1: Understand @JvmStatic effect on Kotlin functions

   @JvmStatic makes the function a static method inside the companion object, callable via the Companion instance.

2. Step 2: Identify correct Java call

   Java calls MathUtils.Companion.multiply(4, 5) to get 20.

3. Final Answer:

   MathUtils.Companion.multiply(4, 5); -> Option B

4. Quick Check:

   @JvmStatic function call = MathUtils.Companion.multiply() [OK]

Hint: @JvmStatic makes Kotlin function callable as static method in Java companion object [OK]

Common Mistakes:

• Calling via generated Kt class ignoring @JvmStatic
• Trying to instantiate Kotlin object unnecessarily
• Calling via Companion object when not needed