Calling Kotlin from Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you call a Kotlin function from Java?

You call a Kotlin function from Java like a regular Java method. Kotlin compiles to JVM bytecode, so Java can use Kotlin classes and functions directly.

beginner

What is the default Kotlin class name seen from Java if no @JvmName is used?

If you have top-level Kotlin functions in a file named Example.kt, Java sees them in a class named ExampleKt by default.

intermediate

What annotation helps rename Kotlin functions or classes for Java callers?

The @JvmName annotation lets you specify a different name for Kotlin functions or classes when called from Java.

intermediate

How do Kotlin properties appear when accessed from Java?

Kotlin properties appear as getter and setter methods in Java. For example, a Kotlin property 'val name' becomes 'getName()' in Java.

intermediate

What is the purpose of the @JvmStatic annotation in Kotlin for Java callers?

@JvmStatic makes a Kotlin function or property static in the generated Java bytecode, so Java can call it without an instance.

If you have a Kotlin file named Utils.kt with a top-level function, how do you call it from Java?

A UtilsKt.functionName()

B Utils.functionName()

C new Utils().functionName()

D You cannot call it from Java

Top-level Kotlin functions are placed in a class named after the file with 'Kt' appended, so Java calls UtilsKt.functionName().

Which annotation changes the name of a Kotlin function for Java callers?

A @JvmStatic

B @JvmName

C @JvmOverloads

D @JvmField

@JvmName lets you rename Kotlin functions or classes for Java interoperability.

How does a Kotlin 'val' property appear in Java?

A As a public field

B As a setter method

C As a private field with no access

D As a getter method

A Kotlin 'val' property appears as a getter method in Java, e.g., getPropertyName().

What does the @JvmStatic annotation do in Kotlin?

A Makes a function static in Java bytecode

B Generates overloads

C Changes function visibility

D Makes a function inline

@JvmStatic makes the annotated function or property static in the generated Java bytecode.

Can Java call Kotlin extension functions directly?

A Yes, as instance methods

B No, extension functions are invisible to Java

C Yes, as static methods

D Only if annotated with @JvmName

Kotlin extension functions become static methods in Java and can be called like static methods.

Explain how Kotlin top-level functions are accessed from Java and how you can customize their names.

Think about the file name and the default class name Java sees.

Describe how Kotlin properties are represented in Java and how @JvmStatic affects calling Kotlin functions from Java.

Consider how Java accesses fields and static methods.

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