77 lines
2.2 KiB
Plaintext
77 lines
2.2 KiB
Plaintext
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== Why is this an issue?
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An interface that declares only a single function should be marked as function interface.
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Function interfaces can be instantiated from lambda expressions directly and are, therefore, more comfortable to use.
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Also, consider using a function type instead of a function interface.
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In many situations, a function type is sufficient.
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A function interface is only required when the function must not be anonymous
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or when an object should implement multiple function interfaces at once.
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=== What is the potential impact?
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==== Complexity
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When an interface is declared _functional_, SAM conversion is enabled.
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This means that any lambda expression that matches the interface's single function's signature
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can be converted into an instance of the interface,
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without the need for an
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explicit class or singleton object to implement the interface.
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==== Wrong logic
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An interface can still be marked `@FunctionalInterface` in Kotlin,
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but this has no effect, and SAM conversion will not work.
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The annotation `java.lang.FunctionalInterface` is only a Java platform type with no special meaning in Kotlin.
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== How to fix it
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Replace `interface` with `fun interface`.
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If the interface is annotated with `@FunctionalInterface`, remove the annotation.
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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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interface IntMapper<T> { // Noncompliant
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fun map(value: Int): T
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}
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interface StringMapper<T> { // Noncompliant
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fun map(value: String): T
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}
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@FunctionalInterface // Noncompliant
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interface ProgressCallback {
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fun progressChanged(percent: Double)
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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 interface IntMapper<T> { // Compliant, function interface used
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fun map(value: Int): T
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}
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typealias StringMapper<T> = (value: String) -> T // Compliant, functional type used
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fun interface ProgressCallback { // Compliant, function interface used
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fun progressChanged(percent: Double)
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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/docs/fun-interfaces.html[Kotlin Docs, Functional (SAM) interfaces]
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=== Articles & blog posts
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* https://www.baeldung.com/kotlin/sam-conversions[Baeldung, SAM Conversions in Kotlin]
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