📌 Extensions in Kotlin

Extensions in Kotlin allow you to add new functionality to existing classes without modifying their source code. You can add methods or properties to classes using extensions.

This is particularly useful when working with third-party libraries or classes you don't own.

✅ 1. Extension Functions

An extension function allows you to add a new function to an existing class.
The syntax is:

kotlin
fun ClassName.(): ReturnType {    // Function body}

📌 Example:

kotlin
fun String.(): String {    return this.replace(" ", "")}fun () {    val str = "Hello Kotlin Extensions"    println(str.removeSpaces()) // Output: HelloKotlinExtensions}

🔎 Explanation:

• this refers to the object the function is called on (String in this case).

• The function removeSpaces() removes all spaces using replace().

✅ 2. Extension Properties

You can also add properties using extensions.
Note that extension properties cannot store state — they rely on getters.

📌 Example:

kotlin
val String.wordCount: Int    get() = this.split(" ").sizefun () {    val sentence = "Kotlin is a concise language"    println("Word Count: ${sentence.wordCount}") fun String?.(): Boolean {    return this == null || this.isEmpty()}fun () {    val str: String? = null    println(str.isNullOrEmpty()) // Output: true    val str2: String? = ""    println(str2.isNullOrEmpty()) // Output: true}

🔎 Explanation:

• String? indicates the function works with nullable strings.

• It checks for both null and empty values.

✅ 4. Extension Functions with Inheritance

Extension functions are not overridden if a subclass has the same function.

📌 Example:

kotlin
open class Animalclass Dog : Animal()fun Animal.() = println("Animal makes a sound")fun Dog.() = println("Dog barks")fun () {    val animal: Animal = Dog()    animal.sound() // Output: Animal makes a sound}

🔎 Explanation:

• Kotlin chooses the extension based on the reference type (Animal) instead of the actual object (Dog).

✅ 5. Using Extensions with Companion Objects

You can extend a companion object using extensions.

📌 Example:

kotlin
class MathUtils {    companion object {}}fun MathUtils.Companion.(num: Int): fun main() {    println(MathUtils.square(4)) // Output: 16}

🔎 Explanation:

• MathUtils.Companion is extended to add a square() function.

✅ 6. Conclusion

• Extension Functions → Add new functions to classes without modifying them.

• Extension Properties → Provide calculated properties using getters.

• Nullable Extensions → Handle null safely with extension functions.

• Extensions with Inheritance → Work based on reference type, not actual object.

• Companion Object Extensions → Extend companion objects for utility functions.