Extensions resolved statically in Kotlin

Introduction

Extensions in Kotlin add new functions to classes without changing them. They are resolved based on the type known at compile time, not the actual object type at runtime.

When you want to add a function to a class you cannot modify.

When you want to write cleaner code by calling new functions directly on objects.

When you want to provide utility functions for existing classes.

When you want to avoid inheritance but still extend functionality.

Syntax

Kotlin

fun ClassName.newFunction() {
    // function body
}

Extension functions look like normal functions but are called on an instance of the class.

They do not actually modify the class or its instances.

Examples

Adds an exclamation mark to a String.

Kotlin

fun String.addExclamation() = this + "!"

Checks if an integer is even.

Kotlin

fun Int.isEven() = this % 2 == 0

Shows that extension functions are resolved by the declared type, not the actual object type.

Kotlin

open class Base
class Derived : Base()

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

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

Sample Program

Even though base holds a Derived object, the extension function called is for Base because extensions are resolved statically by the variable's declared type.

Kotlin

open class Base
class Derived : Base()

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

fun main() {
    val base: Base = Derived()
    println(base.foo())
}

Important Notes

Extension functions do not override member functions.

If a member function and an extension function have the same signature, the member function is called.

Extensions are resolved at compile time based on the variable's declared type, not the runtime type.

Summary

Extensions add functions to classes without changing them.

They are resolved statically by the declared type, not dynamically by the actual object.

This means the extension function called depends on the variable type, not the instance 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