Explain and implement a Kotlin higher-order function

What's really being asked

This question assesses your understanding of a core functional programming concept in Kotlin: higher-order functions and lambdas. The interviewer is checking two things: can you articulate the definition clearly (the 'what'), and can you apply it practically by writing code (the 'how'). It separates candidates who just use map and filter from those who understand how they are built. For a senior role, this is table stakes.

The full answer

A strong answer has two parts, addressing the theory and then the practice. First, define a higher-order function as a function that either takes one or more functions as parameters, or returns a function. Mention that this is possible because Kotlin treats functions as first-class citizens, meaning they can be stored in variables or passed as arguments. Second, for the implementation of filterAndTransform, you should create a new mutable list, iterate through the input list, apply the predicate lambda to each element, and if the predicate returns true, apply the transform lambda to that element and add the result to your new list. Finally, return the new list.

The mistakes people make

A major red flag is implementing the function correctly but giving a weak or incorrect definition of a higher-order function. This suggests you can copy patterns but lack deep understanding. Another wrong turn is to implement the logic using hardcoded filtering or transformation logic, completely ignoring the passed-in lambdas. Fumbling the syntax for function types, like (Int) -> Boolean, also shows a lack of fluency. Finally, do not simply chain Kotlin's built-in .filter() and .map() functions. While more idiomatic for a caller, the prompt asks you to implement the underlying mechanism to prove you understand it.

What usually comes next

Expect questions that build on this. An interviewer might ask you to make the function more generic using type parameters <T> and <R> instead of Int and String. They might also ask you to rewrite it as an extension function on List. Another common follow-up is to discuss performance implications, such as creating intermediate lists if you were to chain filter and map, and how asSequence() can help avoid that for large lists by enabling lazy evaluation.

A concrete example

A correct implementation would be a function filterAndTransform that takes a List<Int>, a predicate of type (Int) -> Boolean, and a transform of type (Int) -> String. Inside, it initializes a mutableListOf<String>. It then iterates over the input list. For each number, it calls the predicate function. If the predicate returns true, it calls the transform function with the number and adds the resulting string to the mutable list. Finally, it returns the list.