What is a Kotlin extension function? Write one for String.

What's really being asked

This question tests your practical knowledge of a key Kotlin feature. The interviewer is looking for more than a textbook definition. They want to see that you understand how extension functions work under the hood (static dispatch), why they are used (extending final classes or third-party code), and that you can write a clean, idiomatic one on the spot. It's a test of both conceptual understanding and practical coding fluency in Kotlin.

The full answer

A strong answer has two parts. First, the definition: explain that an extension function allows you to add new functions to any existing class without inheriting from it. Crucially, state that this does not actually modify the original class; it's syntactic sugar. Under the hood, the Kotlin compiler generates a static Java method that takes the receiver object (the instance the function is called on) as its first parameter. Second, the implementation: write the function correctly. A concise, idiomatic implementation is best.

The mistakes people make

The biggest red flag is stating that extension functions modify the original class. They don't. They are resolved statically at compile time, not dynamically at runtime like virtual methods in inheritance. This means you cannot override an extension function. Another mistake is writing a verbose implementation for hasWhitespace, like using a manual for-loop and if-check, instead of using a built-in Kotlin standard library function like any. This suggests a lack of familiarity with idiomatic Kotlin. Finally, some candidates just write the code without explaining the concept, which misses half the question.

What usually comes next

Expect questions about the limitations. For example, "Can an extension function access private or protected members of the class it extends?" (No, it can only access public members). Or, "What happens if a class has a member function and you define an extension function with the same signature?" (The member function always wins). Another good one is, "What's the difference between an extension function and the Decorator pattern?" (Extensions are compile-time and simpler; Decorator is a runtime pattern providing more flexibility via object composition).

A concrete example

Here is a correct and idiomatic implementation: fun String.hasWhitespace(): Boolean = this.any { it.isWhitespace() } This is a single-expression function. It uses the any standard library function, which iterates through the characters of the String (this) and returns true as soon as the provided lambda ({ it.isWhitespace() }) returns true for any character. It's concise and leverages the power of the Kotlin standard library.