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Kotlin - Annotations
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  Declaring an annotation

Annotations are means of attaching metadata to code. To declare an annotation, put the annotation modifier in front of a class:

annotation class Strippable

Annotations can have meta-anotations:


   @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
    annotation class Strippable

Annotations, like other classes, can have constructors:

annotation class Strippable(val importanceValue: Int)

But unlike other classes, is limited to the following types:

  • types that correspond to Java primitive types (Int, Long etc.);
  • strings
  • classes ( Foo:: class)
  • enums
  • other annotations
  • arrays of the types listed above
  Meta-annotations

When declaring an annotation, meta-info can be included using the following meta-annotations:

  • `@Target`: specifies the possible kinds of elements which can be annotated with the annotation (classes, functions, properties, expressions etc.)
  • `@Retention` specifies whether the annotation is stored in the compiled class files and whether it's visible through reflection at runtime (by default, both are true.)
  • `@Repeatable` allows using the same annotation on a single element multiple times.
  • `@MustBeDocumented` specifies that the annotation is part of the public API and should be included in the class or method signature shown in the generated API documentation.

    • Example:

    @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,
        AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
@Retention(AnnotationRetention.SOURCE)
@MustBeDocumented
annotation class Fancy



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