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## Kotlin

[Kotlin](https://kotlinlang.org/) is an alternative language for the JVM. Its compact syntax and
native support for annotation processing make it a good fit for the mapper.

We have a full example at [DataStax-Examples/object-mapper-jvm/kotlin].

### Writing the model

You can use Kotlin [data classes] for your entities. Data classes are usually
[immutable](../../entities/README.md#mutability), but you don't need to declare that explicitly with
[@PropertyStrategy]: the mapper detects that it's processing Kotlin code, and will assume `mutable =
false` by default:

```kotlin
@Entity
data class Product(@PartitionKey val id: Int?, val description: String?)
```

Data classes may also be made mutable (by declaring the components with `var` instead of `val`). If
you choose that approach, you'll have to annotate your entities with [@PropertyStrategy], and also
declare a default value for every component in order to generate a no-arg constructor:

```kotlin
@Entity
@PropertyStrategy(mutable = true)
data class Product(@PartitionKey var id: Int? = null, var description: String? = null)
```

All of the [property annotations](../../entities/README.md#property-annotations) can be declared directly on
the components.

If you want to take advantage of [null saving strategies](../../daos/null_saving/README.md), your components
should be nullable.

The other mapper interfaces are direct translations of the Java versions:

```kotlin
@Dao
interface ProductDao {
  @Insert
  fun insert(product: Product)
}
```

Known limitation: because of a Kotlin bug ([KT-4779]), you can't use default interface methods. They
will appear as abstract methods to the mapper processor, which will generate an error since they are
not properly annotated. As a workaround, you can use a companion object method that takes the DAO as
an argument (as shown in [UserDao.kt]), or query provider methods.

### Building

#### Gradle

See the example's [build.gradle].

You enable Kotlin support with [kotlin][gradle_kotlin] and [kotlin_kapt][gradle_kapt], and declare
the mapper processor with the `kapt` directive.

#### Maven

Configure [dual compilation][maven_kotlin_java] of Kotlin and Java sources. In addition, you'll need
an additional execution of the [kotlin-maven-plugin:kapt][maven_kapt] goal with the mapper processor
before compilation:

```xml
<plugin>
  <groupId>org.jetbrains.kotlin</groupId>
  <artifactId>kotlin-maven-plugin</artifactId>
  <version>${kotlin.version}</version>
  <executions>
    <execution>
      <id>kapt</id>
      <goals><goal>kapt</goal></goals>
      <configuration>
        <sourceDirs>
          <sourceDir>src/main/kotlin</sourceDir>
          <sourceDir>src/main/java</sourceDir>
        </sourceDirs>
        <annotationProcessorPaths>
          <annotationProcessorPath>
            <groupId>org.apache.cassandra</groupId>
            <artifactId>java-driver-mapper-processor</artifactId>
            <version>${java-driver.version}</version>
          </annotationProcessorPath>
        </annotationProcessorPaths>
      </configuration>
    </execution>
    <execution>
      <id>compile</id>
      <goals><goal>compile</goal></goals>
      ...
    </execution>
  </executions>
</plugin>
```

[maven_kotlin_java]: https://kotlinlang.org/docs/reference/using-maven.html#compiling-kotlin-and-java-sources
[maven_kapt]: https://kotlinlang.org/docs/reference/kapt.html#using-in-maven
[gradle_kotlin]: https://kotlinlang.org/docs/reference/using-gradle.html
[gradle_kapt]: https://kotlinlang.org/docs/reference/kapt.html#using-in-gradle
[data classes]: https://kotlinlang.org/docs/reference/data-classes.html
[KT-4779]: https://youtrack.jetbrains.com/issue/KT-4779

[DataStax-Examples/object-mapper-jvm/kotlin]: https://github.com/DataStax-Examples/object-mapper-jvm/tree/master/kotlin
[build.gradle]: https://github.com/DataStax-Examples/object-mapper-jvm/blob/master/kotlin/build.gradle
[UserDao.kt]: https://github.com/DataStax-Examples/object-mapper-jvm/blob/master/kotlin/src/main/kotlin/com/datastax/examples/mapper/killrvideo/user/UserDao.kt

[@PropertyStrategy]: https://docs.datastax.com/en/drivers/java/4.17/com/datastax/oss/driver/api/mapper/annotations/PropertyStrategy.html
