ObjectBox and Kotlin

ObjectBox comes with full Kotlin support for Android. This allows entities to be modeled in Kotlin classes (regular and data classes). With Kotlin support you can build faster apps even faster.

Kotlin Entities

ObjectBox supports regular and data classes for entities. However, @Id properties must be var (not val) because ObjectBox assigns the ID after putting a new entity. They also should be of non-null type Long with the special value of zero for marking entities as new.

See the Getting Started or Entity Annotations guide for examples.

Defining Relations in Kotlin Entities

When defining relations in Kotlin, keep in mind that relation properties must be var. Otherwise they can not be initialized as described in the relations docs. To avoid null checks use a lateinit modifier. When using a data class this requires the relation property to be moved to the body.

See the Relations page for examples.

Using the provided extension functions

To simplify your code, you might want to use the Kotlin extension functions provided by ObjectBox. The library containing them is added automatically if the Gradle plugin detects a Kotlin project.

To add it manually, modify the dependencies section in your app's build.gradle file:

dependencies {
    implementation "io.objectbox:objectbox-kotlin:$objectboxVersion"
}

Now have a look at what is possible with the extensions compared to standard Kotlin idioms:

Get a box:

// Regular:
val box = store.boxFor(DataClassEntity::class.java)
​
// With extension:
val box: Box<DataClassEntity> = store.boxFor()

Build a query:

// Regular:
val query = box.query().run {
    equal(property, value)
    order(property)
    build()
}
​
// With extension:
val query = box.query {
    equal(property, value)
    order(property)
}

Use the in filter of a query:

// Regular:
val query = box.query().`in`(property, array).build()
​
// With extension:
val query = box.query().inValues(property, array).build()

Modify a ToMany:

// Regular:
toMany.apply { 
    reset()
    add(entity)
    removeById(id)
    applyChangesToDb()
}
​
// With extension:
toMany.applyChangesToDb(resetFirst = true) { // default is false
    add(entity)
    removeById(id)
}

Something missing? Let us know what other extension functions you want us to add.

Next Steps

• Check out the Kotlin example on GitHub.

• Continue with Getting Started.