Andrey Breslav
57 lines
1.5 KiB
Plaintext
57 lines
1.5 KiB
Plaintext
In Kotlin, one can attach metadata to declarations in the form of _annotations_. To declare an annotation, put the *annotation* annotation (no pun intended :)) in front of a normal class:
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{jet}
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annotation class fancy {}
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{jet}
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(Note that, by convention, annotation classes are named with the lowercase fitst letter; the reason will be clear from the examples below.)
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Now we can annotate a declaration or an expression with the new {{fancy}} annotation. In general, one puts square brackets around the annotation name:
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{jet}
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[fancy] class Foo {
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[fancy] fun baz([fancy] foo : Int) : Int {
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return [fancy] 1
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}
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}
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{jet}
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{note:title=Annotations are under development}{note}
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When annotating a declaration (e.g. a function or a class), the square brackets may be omitted:
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{jet}
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fancy class Foo() {
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fancy fun baz(fancy foo: Int) {
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return [fancy] 1
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}
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}
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{jet}
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Note that square brackets are required for expressions.
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Annotation classes may have constructors that take parameters. For example:
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{jet}
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annotation class special(val why : String)
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special("example") class Foo {}
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{jet}
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As you can see, to pass arguments to an annotation one simply calls its constructor.
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On the JVM one can re-use Java annotations:
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{jet}
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import org.junit.Test
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import org.junit.Assert.*
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class Tests {
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Test fun simple() {
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assertEquals(42, getTheAnswer())
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}
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}
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{jet}
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If you want your Java annotations to look like modifiers, you can rename them on import:
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{jet}
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import org.junit.Test as test
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class Tests {
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test fun simple() {
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...
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}
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}
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{jet}
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