What is a Kotlin extension function? Write one for String.
Tests adding functionality to classes without inheritance. Define extension functions as static utilities, explain their use case (e.g., third-party libs), then write hasWhitespace using an idiomatic method like any. A red flag is manually looping.
What's really being asked
This question tests your practical and theoretical knowledge of a core Kotlin feature. The interviewer is checking if you understand that extension functions are syntactic sugar for static utility methods, not a form of inheritance. They are looking for a concise, idiomatic implementation, which demonstrates fluency in the Kotlin standard library, not just basic language syntax.
The full answer
First, define an extension function as a mechanism to add new functions to any class, even one you don't own (like from a third-party library or the JDK), without modifying its source code. Second, explain that they are resolved statically at compile time, not dynamically at runtime. Third, provide the correct syntax for the function signature: fun String.hasWhitespace(): Boolean. Finally, implement the logic idiomatically using a standard library function, like this.any { it.isWhitespace() }, which is far superior to a manual loop.
The mistakes people make
A major red flag is stating that extension functions modify the original class or are a form of inheritance. They do not; they are compiled down to static methods that take the receiver object as the first argument. Another common mistake is writing a verbose, C-style for loop to check for whitespace. While technically correct, it signals a lack of familiarity with idiomatic Kotlin and the powerful standard library functions available. A candidate might write for (char in this) { if (Character.isWhitespace(char)) return true } return false;. This works, but it's not the Kotlin way.
What usually comes next
Expect questions about limitations and edge cases. For instance: "What happens if a class has a member function and an extension function with the same signature?" (The member function always wins). Or, "Can you have extension properties?" (Yes, but they cannot have a backing field; they must be computed properties with a custom getter/setter). Another good follow-up is "How would you make this extension function available to your entire project?" (By defining it at the top level of any Kotlin file).
A concrete example
Here is a correct and idiomatic implementation:
fun String.hasWhitespace(): Boolean {
return this.any { it.isWhitespace() }
}// Usage: val withSpace = "hello world" val noSpace = "helloworld"
println(withSpace.hasWhitespace()) // Prints: true
println(noSpace.hasWhitespace()) // Prints: false
Interview question
Which statement accurately describes the underlying mechanism of Kotlin extension functions?
- a.They modify the original class's bytecode to inject new methods at runtime.
- b.They provide a mechanism for dynamic method dispatch, allowing custom implementations to override existing class methods.
- c.They are compiled into static utility methods where the receiver object is passed as the first parameter.Correct
- d.They enable a form of compile-time inheritance, allowing a class to implicitly extend another's functionality.
Why? this is the answer
The card explicitly states that extension functions are "syntactic sugar for static utility methods" and are "compiled down to static methods that take the receiver object as the first argument." They do not modify the original class or its bytecode, which is a common misconception.
Just read this? Test yourself on what you have been reading.
Read the original → kotlinlang.org
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