Lately, I've been diving into the Kotlin book "Atom Kotlin," and I find the concept of extension functions truly remarkable. Despite being a small feature, when leveraged effectively, it has the potential to bring about significant improvements in your code.

Syntax of the extension function

fun ReceiverType.extensionFunction() { ... }

• Imagine you want to add single and double quote methods to strings. With extensions, it's a breeze:

fun String.singleQuote() = "'$this'"
fun String.doubleQuote() = "\"this\""

• Now you can use these functions like they were always part of the String class:

"Hi".singleQuote() // Now "Hi" is in single quotes!

Combo power: Chaining extensions

Need to apply multiple extensions one after another? No sweat! Just keep using the this keyword to reference the modified string. Like this:

fun String.strangeQuote() = this.singleQuote().singleQuote() // Double the quotes!

Simplifying Class Functionality

Extensions aren't just for other people's code. You can use them to improve your own classes too! Let's say you have a Book class:

class Book(val title: String)

With an extension, you can easily categorize books:

fun Book.categorize(category: String) =
"""title: "$title", category: $category"""

Now you can categorize books without writing out the title property every time.

Things to keep in mind:

• Extensions can only access public parts of the class they're extending.
• They're a kind of syntactic magic, making your code look cleaner and more natural.

Final thought

In short, extension functions are a powerful tool in Kotlin that lets you add functionality and improve code readability. It's like giving your code superpowers!

Refs: Atomic Kotlin by Bruce Eckel , Svetlana Isakova