79 lines
4.0 KiB
Plaintext
79 lines
4.0 KiB
Plaintext
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The `kotlin.collections` package offers many functions to interact with collections.
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In particular, functions `find(predicate)`, `findLast(predicate)`, `firstOrNull(predicate)` and "`lastOrNull(predicate)` return the element that matches the given predicate.
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== Why is this an issue?
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The functions `find(predicate)`, `findLast(predicate)`, `firstOrNull(predicate)` and "`lastOrNull(predicate)`
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can be improperly used to check the presence of an element that matches the given predicate.
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In such cases the code is more difficult to read and understand than it would be with the functions `any(predicate)`, `none(predicate)` or `contains(element)`.
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=== What is the potential impact?
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The pattern of using `find(predicate)`, `findLast(predicate)`, `firstOrNull(predicate)` and "`lastOrNull(predicate)` combined with a null check,
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to check the presence of an element is not immediately clear to readers.
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For example, the expression `list.find { it > 5 } != null` is more difficult to understand than `list.any { it > 5 }`.
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The additional comparison operator increases the complexity of the expression and introduces confusion about the intent of the code.
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== How to fix it
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Replace the use of `find(predicate)`, `findLast(predicate)`, `firstOrNull(predicate)` and "`lastOrNull(predicate)` with `any(predicate)`, `none(predicate)` or `contains(element)`.
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There are four possible scenarios:
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1. The predicate is a simple binary expression checking for equality, and the found object is compared to not be `null`:
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- Replace `find { it == element } != null` with `contains(element)`.
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- Replace `findLast { it == element } != null` with `contains(element)`.
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- Replace `firstOrNull { x -> x == element } != null` with `contains(element)`.
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- Replace `lastOrNull { x -> x == element } != null` with `contains(element)`.
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2. The predicate is a simple binary expression checking for equality, and the found object is compared to be `null`:
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- Replace `find { it == element } == null` with `!contains(element)`.
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- Replace `findLast { it == element } == null` with `!contains(element)`.
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- Replace `firstOrNull { x -> x == element } == null` with `!contains(element)`.
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- Replace `lastOrNull { x -> x == element } == null` with `!contains(element)`.
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3. The predicate is any binary expression not checking for equality, and the found object is compared to not be `null`:
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- Replace `find { it > 5 } != null` with `any { it > 5 }`.
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- Replace `findLast { it != 5 } != null` with `any { it != 5 }`.
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- Replace `firstOrNull { x -> x < 5 } != null` with `any { x -> x < 5 }`.
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- Replace `lastOrNull { x -> x != 5 } != null` with `any { x -> x != 5 }`.
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4. The predicate is any binary expression not checking for equality, and the found object is compared to be `null`:
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- Replace `find { it > 5 } == null` with `none { it > 5 }`.
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- Replace `findLast { it != 5 } == null` with `none { it != 5 }`.
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- Replace `firstOrNull { x -> x < 5 } == null` with `none { x -> x < 5 }`.
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- Replace `lastOrNull { x -> x != 5 } == null` with `none { x -> x != 5 }`.
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=== Code examples
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==== Noncompliant code example
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[source,kotlin,diff-id=1,diff-type=noncompliant]
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----
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fun example(list: List<Int>) {
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list.find { it > 5 } != null // Noncompliant
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list.findLast { it > 5 } == null // Noncompliant
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list.firstOrNull { it == 5 } != null // Noncompliant
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list.lastOrNull { x -> x == 5 } != null // Noncompliant
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list.find { x -> 5 == 4 } != null // Noncompliant, note that this case cannot be fixed using contains
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}
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----
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==== Compliant solution
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[source,kotlin,diff-id=1,diff-type=compliant]
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----
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fun example(list: List<Int>) {
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list.any { it > 5 } // Compliant
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list.none { it > 5 } // Compliant
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list.contains(5) // Compliant
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!list.contains(5) // Compliant
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list.any { x -> 5 == 4 } // Compliant, note that this case cannot be fixed using contains
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}
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----
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== Resources
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=== Documentation
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* https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/[Kotlin API Docs, Package kotlin.collections]
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