Kotlin Basic Tutorial-annotations

Annotation definition Annotations

Annotation Class Fancy

constructor for annotations

can take parameters

Annotation Class Special (Val why:string)
Special ("Example") class Foo {}

using Annotations

@fancy class Foo {
    @fancy fun baz (@fancy foo:int): Int {
        return  1
    }
}

annotations in the main constructor

Class Foo @inject Constructor (dependency:mydependency)

annotations in property visitors

Class Foo {
    var x:mydependency?=null
        @inject Set
}

annotations in lambda expressions

Annotation class Suspendable
val f = @Suspendable {Fiber.sleep (10)}

introduction of annotations in Java

Kotlin can refer directly to annotations in Java.

Import org.junit.Test
Import org.junit.assert.*

class Tests {
    Test fun simple () {
        assertequals (42), GetTheAnswer ())
    }
}

Annotations can be renamed when importing

Import org.junit.Test as Test

class Tests {
  Test fun simple () {
    ...
  }
}

Specify parameters

Because annotations in Java have no concept of parameters, they are attributes. But the attributes have no order, so when you invoke the annotations, you must mention the names when you pass in the arguments:

Java public
@interface Ann {
    int intvalue ();
    String stringvalue ();
}

Kotlin
Ann (intvalue = 1, stringvalue = "abc") class C

Value parameter

If the definition in Java is a value parameter, then you can use no names:

Public @interface Annwithvalue {
    String value ();
}

Kotlin
annwithvalue ("abc") class C

Array

Kotlin using array instead of arrays

//Java public @interface annwitharrayvalue {string[] value ();}//Kotlin Annwitharrayvalue ("abc", "foo", "Bar") class C