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Annotations in Kotlin

What are Annotations in Kotlin?​

Annotations are a way of attaching metadata to code. In Kotlin, annotations are used to give additional information to the compiler about the behavior of our code. They don't change the action of the code itself but rather give hints or instructions to the compiler.

Basic Syntax of Annotations​

Annotations are marked by the @ symbol in front of their name. They can be placed on classes, functions, properties, and even parameters. Here's a simple example:

@Deprecated("This function is deprecated")
fun oldFunction() {
    // Some code...
}

In the example above, the Deprecated annotation is used to signal that the function oldFunction is deprecated. The string "This function is deprecated" is an argument for the Deprecated annotation, providing a message to anyone who tries to use this function.

Annotation Targets​

By default, if you use an annotation on a property, it applies to the property itself. But what if you want to annotate the property's getter or setter, or its backing field? Kotlin provides a way to specify the target of an annotation using the @target: prefix. Here's how you can do it:

class MyClass {
    @get:JvmName("getMyProperty") 
    var myProperty: String = "Hello, World!"
}

In the above example, the JvmName annotation is applied to the getter of myProperty.

Built-in Annotations​

Kotlin provides a set of built-in annotations that you can use to control the behavior of your code. Here are a few examples:

  • @Deprecated: Marks the annotated program element as deprecated.
  • @JvmStatic: Makes the annotated static for Java users.
  • @JvmOverloads: Instructs the Kotlin compiler to generate overloads for this function that substitute default parameter values.
  • @JvmField: Instructs the Kotlin compiler not to generate getters/setters for this property and expose it as a field.

Defining Custom Annotations​

You can define your own annotations in Kotlin. Here's a simple example:

annotation class MyCustomAnnotation

You can then use this annotation like any other:

@MyCustomAnnotation 
class MyClass {
    // Some code...
}

Annotation Parameters​

Annotations can have parameters. Here's how you can define an annotation with parameters:

annotation class MyAnnotation(val myValue: String)

And here's how you can use it:

@MyAnnotation("Hello, World!") 
class MyClass {
    // Some code...
}

In the above example, the MyAnnotation annotation takes a single parameter of type String.

In conclusion, annotations in Kotlin are a powerful tool that can provide additional information to the compiler, and they can be used to control the behavior of your code. They don't change the logic of the code itself, but they do provide hints or instructions to the compiler. Whether you're using built-in annotations or defining your own, annotations are a key part of writing idiomatic Kotlin code.