Why is this an issue?

The filter(predicate) function is used to extract a subset of elements from a collection that match a given predicate. Many collection functions such as any(), count(), first(), and more, come with an optional condition predicate.

It is not recommended to invoke the filter(predicate) function prior to these terminal operations. Instead, the predicate variant of the terminal operation should be used as a replacement.

What is the potential impact?

Using filter(predicate) before terminal operations can result in unnecessary iterations over the collection, which could negatively impact the performance of the code, especially with large collections. By directly using the predicate variant of the function, you can streamline the code and improve its efficiency and readability.

How to fix it

Replace the filter(predicate) call with the predicate variant of the terminal operation. As of Kotlin API version 1.8, the list of terminal operations supporting a predicate is:

• any()
• none()
• count()
• first(), firstOrNull()
• last(), lastOrNull()
• single(), singleOrNull()

Code examples

Noncompliant code example

val list = listOf(5,2,9,6,8,2,5,7,3)
val hasElementsGreater5 = list.filter { it > 5 }.any() // Noncompliant
val countElementsGreater5 = list.filter { it > 5 }.count() // Noncompliant
val lastElementGreater5 = list.filter { it > 5 }.lastOrNull() // Noncompliant

Compliant solution

val list = listOf(5,2,9,6,8,2,5,7,3)
val hasElementsGreater5 = list.any { it > 5 } // Compliant
val countElementsGreater5 = list.count { it > 5 } // Compliant
val lastElementGreater5 = list.lastOrNull { it > 5 } // Compliant

Resources

Documentation

• Kotlin API Docs, Package kotlin.collections