Extensions resolved statically in Kotlin - Mini Project: Build & Apply

Understanding Extensions Resolved Statically in Kotlin

📖 Scenario: You are working on a Kotlin project where you want to add new functions to existing classes without changing their source code. Kotlin extensions let you do this, but they are resolved statically, which means the function called depends on the type of the variable, not the actual object it points to.Let's explore this behavior with a simple example.

🎯 Goal: Build a Kotlin program that shows how extension functions are resolved statically by creating two classes and extension functions for them, then calling these functions through variables of different types.

📋 What You'll Learn

Create two classes: Base and Derived where Derived inherits from Base.

Create an extension function printMessage() for Base that prints "Base extension".

Create an extension function printMessage() for Derived that prints "Derived extension".

Create a function showMessage() that takes a Base parameter and calls printMessage() on it.

Create a Derived object and call showMessage() passing it as a Base type.

Print the output to observe which extension function is called.

💡 Why This Matters

🌍 Real World

Extension functions let you add new features to existing classes without changing their code, useful when working with libraries or APIs you cannot modify.

💼 Career

Understanding how Kotlin resolves extension functions helps avoid bugs and write clearer, more predictable code in Android and Kotlin backend development.

1

Create classes Base and Derived

Create a class called Base and a class called Derived that inherits from Base.

Kotlin

# Create class Base and class Derived inheriting Base
// Your code here

Hint

Use open keyword to allow inheritance for Base.

2

Create extension functions printMessage() for Base and Derived

Create an extension function called printMessage() for Base that prints "Base extension" and another extension function called printMessage() for Derived that prints "Derived extension".

Kotlin

open class Base
class Derived : Base()

// Create extension function printMessage() for Base
// Create extension function printMessage() for Derived
// Your code here

Hint

Define extension functions outside the classes using fun ClassName.functionName() syntax.

3

Create function showMessage() that calls printMessage()

Create a function called showMessage() that takes a parameter obj of type Base and calls obj.printMessage() inside it.

Kotlin

open class Base
class Derived : Base()

fun Base.printMessage() {
    println("Base extension")
}

fun Derived.printMessage() {
    println("Derived extension")
}

// Create function showMessage() that takes Base and calls printMessage()
// Your code here

Hint

Call the extension function on the parameter inside the function body.

4

Create a Derived object and call showMessage()

Create a variable called derivedObj of type Derived and assign it a new Derived() object. Then call showMessage(derivedObj) and print the output.

Kotlin

open class Base
class Derived : Base()

fun Base.printMessage() {
    println("Base extension")
}

fun Derived.printMessage() {
    println("Derived extension")
}

fun showMessage(obj: Base) {
    obj.printMessage()
}

// Create Derived object derivedObj and call showMessage(derivedObj)
// Your code here

Hint

Remember that extension functions are resolved by the variable type, not the actual object type.

Practice

1. What does it mean that Kotlin extension functions are resolved statically?

easy

A. Extension functions override member functions dynamically.

B. The extension function called depends on the variable's declared type, not the actual object type.

C. The extension function is chosen based on the runtime type of the object.

D. Extension functions can change the original class's source code.

Solution

1. Step 1: Understand static resolution

   Static resolution means decisions are made at compile time, not runtime.

2. Step 2: Apply to extension functions

   Kotlin chooses the extension function based on the variable's declared type, ignoring the actual object type.

3. Final Answer:

   The extension function called depends on the variable's declared type, not the actual object type. -> Option B

4. Quick Check:

   Extensions resolved statically = based on declared type [OK]

Hint: Remember: extensions use variable type, not object type [OK]

Common Mistakes:

• Thinking extension functions override member functions dynamically
• Assuming extension functions change the original class
• Confusing static with dynamic dispatch

2. Which of the following is the correct syntax to declare an extension function greet for class Person in Kotlin?

easy

A. fun greet() Person { println("Hello") }

B. fun greet(Person) { println("Hello") }

C. extension fun Person.greet() { println("Hello") }

D. fun Person.greet() { println("Hello") }

Solution

1. Step 1: Recall extension function syntax

   Extension functions use the syntax: fun ClassName.functionName() { ... }

2. Step 2: Match syntax to options

   fun Person.greet() { println("Hello") } matches the correct syntax: fun Person.greet() { println("Hello") }

3. Final Answer:

   fun Person.greet() { println("Hello") } -> Option D

4. Quick Check:

   Extension syntax = fun ClassName.functionName() [OK]

Hint: Extension syntax: fun ClassName.functionName() [OK]

Common Mistakes:

• Using 'extension' keyword which doesn't exist
• Incorrect parameter placement
• Missing receiver type before function name

3. Consider the code:

open class Base
class Derived : Base()

fun Base.foo() = "Base"
fun Derived.foo() = "Derived"

fun printFoo(b: Base) {
    println(b.foo())
}

fun main() {
    val d: Derived = Derived()
    printFoo(d)
}

What is the output when main() runs?

medium

A. Compilation error

B. Derived

C. Base

D. Runtime exception

Solution

1. Step 1: Analyze parameter type in printFoo

   printFoo takes a parameter of type Base, so inside printFoo, b is of type Base.

2. Step 2: Understand extension resolution

   Extension functions are resolved statically by the declared type, so b.foo() calls Base.foo(), not Derived.foo().

3. Final Answer:

   Base -> Option C

4. Quick Check:

   Extension call depends on declared type = Base [OK]

Hint: Extension call uses parameter type, not actual object type [OK]

Common Mistakes:

• Expecting Derived.foo() to be called dynamically
• Confusing extension with member function dispatch
• Assuming runtime type affects extension resolution

4. Given the code below, what is the error and how to fix it?

open class Animal
class Dog : Animal()

fun Animal.sound() = "Some sound"
fun Dog.sound() = "Bark"

fun main() {
    val animal: Animal = Dog()
    println(animal.sound())
}

medium

A. No error; output is "Some sound"

B. Compilation error: extension functions cannot be overloaded

C. Runtime error: unresolved extension function

D. Output is "Bark" but should be "Some sound"

Solution

1. Step 1: Check for compilation errors

   Extension functions can be overloaded by receiver type; no syntax error here.

2. Step 2: Understand output based on static resolution

   Variable animal is declared as Animal, so animal.sound() calls Animal.sound(), outputting "Some sound".

3. Final Answer:

   No error; output is "Some sound" -> Option A

4. Quick Check:

   Extension resolved by declared type = Animal.sound() [OK]

Hint: Extensions overload by receiver type; call depends on variable type [OK]

Common Mistakes:

• Expecting Dog.sound() to be called dynamically
• Thinking extension functions cause runtime errors
• Confusing extension overloading with overriding

5. You want to add an extension function describe() to a class hierarchy where Cat inherits from Animal. You want describe() to print "Animal description" when called on an Animal variable, and "Cat description" when called on a Cat variable. Given Kotlin's static resolution of extensions, how should you design your code to achieve this behavior?

hard

A. Declare two extension functions: one for Animal and one for Cat; call describe() on variables typed accordingly.

B. Override describe() as a member function in Cat and Animal classes.

C. Use a when expression inside a single extension function on Animal to check the runtime type.

D. Use reflection to call the correct extension function at runtime.

Solution

1. Step 1: Understand static resolution impact

   Extension functions are chosen by variable type, so different extensions for Animal and Cat can coexist.

2. Step 2: Design with two extensions and variable types

   Define describe() extensions for Animal and Cat separately, then call describe() on variables declared as Animal or Cat to get different outputs.

3. Final Answer:

   Declare two extension functions: one for Animal and one for Cat; call describe() on variables typed accordingly. -> Option A

4. Quick Check:

   Different extensions + variable types = desired output [OK]

Hint: Use separate extensions and variable types to get different outputs [OK]

Common Mistakes:

• Trying to override extensions as member functions
• Relying on runtime type for extension dispatch
• Using reflection unnecessarily