Custom Types

Which types are supported by default in ObjectBox, how to store types that are not, recommendations for storing enums.

ObjectBox - Supported Types

With ObjectBox you can store pretty much any type (class), given that it can be converted to any of the built-in types.

ObjectBox can store the following built-in types without a converter:

boolean, Boolean
int, Integer
short, Short
long, Long
float, Float
double, Double
byte, Byte
char, Character

// Strings
Map<String, String>

// integer arrays

// floating point arrays

// Stored as time (long) with millisecond precision.

// Stored as time (long) with nanosecond precision.
@Type(DatabaseType.DateNano) long, Long 

Flex properties

ObjectBox supports properties where the type is not known at compile time using Object in Java or Any? in Kotlin. These "flex properties" can store types like integers, floating point values, strings and byte arrays. Or lists and maps (using string keys) of those. Some limitations apply, see the FlexObjectConverter class documentation for details.

data class Customer(
    @Id var id: Long = 0,
    // Stores any supported type at runtime
    var tag: Any? = null,
    // Or explicitly use a String map
    var stringMap: MutableMap<String, Any>? = null
    // Or a list
    var flexList: MutableList<Any>? = null

val customerStrTag = Customer(tag = "string-tag")
val customerIntTag = Customer(tag = 1234)
box.put(customerStrTag, customerIntTag)

To store any other type or to override the default behavior, configure a converter like shown below.

Convert annotation and property converter

To add support for a custom type, you need to provide a conversion to one of the ObjectBox built-in types. For example, you could define a color in your entity using a custom Color class and map it to an Integer. Or you can map the popular org.joda.time.DateTime from Joda Time to a Long.

Here is an example mapping an enum to an integer:

public class User {
    public long id;
    @Convert(converter = RoleConverter.class, dbType = Integer.class)
    public Role role;
    public enum Role {
        DEFAULT(0), AUTHOR(1), ADMIN(2);
        final int id;
        Role(int id) {
   = id;

    public static class RoleConverter implements PropertyConverter<Role, Integer> {
        public Role convertToEntityProperty(Integer databaseValue) {
            if (databaseValue == null) {
                return null;
            for (Role role : Role.values()) {
                if ( == databaseValue) {
                    return role;
            return Role.DEFAULT;
        public Integer convertToDatabaseValue(Role entityProperty) {
            return entityProperty == null ? null :;

Things to look out for

If you define your custom type or converter inside a Java or Kotlin entity class, it must be static or respectively not an inner class.

Don’t forget to handle null values correctly – usually, you should return null if the input is null.

Database types in the sense of the converter are the primitive (built-in) types offered by ObjectBox, as mentioned in the beginning. It is recommended to use a primitive type that is easily convertible (int, long, byte array, String, …).

You must not interact with the database (such as using Box or BoxStore) inside the converter. The converter methods are called within a transaction, so for example, getting or putting entities to a box will fail.

Note: For optimal performance, ObjectBox will use a single converter instance for all conversions. Make sure the converter does not have any other constructor besides the parameter-less default constructor. Also, make it thread-safe, because it might be called concurrently on multiple entities.

List/Array types

You can use a converter with List types. For example, you could convert a List of Strings to a JSON array resulting in a single string for the database. At the moment it is not possible to use an array with converters (you can track this feature request).

ObjectBox (Java, Dart) has built-in support for String lists. ObjectBox for Java also has built-in support for String arrays.

How to convert Enums correctly

Enums are popular with data objects like entities. When persisting enums, there are a couple of best practices:

  • Do not persist the enum’s ordinal or name: Both are unstable, and can easily change the next time you edit your enum definitions.

  • Use stable ids: Define a custom property (integer or string) in your enum that is guaranteed to be stable. Use this for your persistence mapping.

  • Prepare for the unknown: Define an UNKNOWN enum value. It can serve to handle null or unknown values. This will allow you to handle cases like an old enum value getting removed without crashing your app.

Custom types in queries

QueryBuilder is unaware of custom types. You have to use the primitive DB type for queries.

So for the Role example above you would get users with the role of admin with the query condition .equal(UserProperties.Role, 2).

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