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Define a Realm Object Model - Java SDK

On this page

  • Define a Realm Object
  • Extend RealmObject
  • Implement RealmModel
  • Lists
  • Define an Embedded Object Field
  • Annotations
  • Primary Key
  • Required Fields
  • Optional Fields
  • Default Field Values
  • Index a Field
  • Ignore a Field
  • Rename a Field
  • Rename a Class
  • Omit Classes from your Realm Schema

To define a Realm object in your application, create a subclass of RealmObject or implement RealmModel.

Important

  • All Realm objects must provide an empty constructor.

  • All Realm objects must use the public visibility modifier in Java or the open visibility modifier in Kotlin.

Note

Class names are limited to a maximum of 57 UTF-8 characters.

The following code block shows a Realm object that describes a Frog. This Frog class can be stored in Realm because it extends the RealmObject class.

The following code block shows a Realm object that describes a Frog. This Frog class can be stored in Realm because it implements the RealmModel class and uses the @RealmClass annotation:

Tip

Using RealmObject Methods

When you create a Realm object by extending the RealmObject class, you can access RealmObject class methods dynamically on instances of your Realm object. Realm objects created by implementing RealmModel can access those same methods statically through the RealmObject class:

Realm objects can contain lists of non-Realm-object data types:

Tip

Realm provides the ability to nest objects within other objects. This has several advantages:

  • If using Sync, objects translate into MongoDB documents that follow a denormalized data model.

  • When you delete an object that contains another object, the delete operation removes both objects from the realm, so unused objects don't accumulate in your realm file, taking up valuable space on user's mobile devices.

To embed an object, set the embedded property of the @RealmClass annotation to true on the class that you'd like to nest within another class:

Then, any time you reference that class from another class, Realm will embed the referenced class within the enclosing class, as in the following example:

Use annotations to customize your Realm object models.

New in version 10.6.0: Realm automatically indexes primary key fields. Previously, Realm only indexed String primary keys automatically.

Realm treats fields marked with the @PrimaryKey annotation as primary keys for their corresponding object schema. Primary keys are subject to the following limitations:

  • You can define only one primary key per object schema.

  • Primary key values must be unique across all instances of an object in a realm. Attempting to insert a duplicate primary key value results in a RealmPrimaryKeyConstraintException.

  • Primary key values are immutable. To change the primary key value of an object, you must delete the original object and insert a new object with a different primary key value.

  • Embedded objects cannot define a primary key.

You can create a primary key with any of the following types:

  • String

  • UUID

  • ObjectId

  • Integer or int

  • Long or long

  • Short or short

  • Byte or byte[]

Non-primitive types can contain a value of null as a primary key value, but only for one object of a particular type, since each primary key value must be unique. Attempting to insert an object with an existing primary key into a realm will result in a RealmPrimaryKeyConstraintException.

Realm automatically indexes primary key fields, which allows you to efficiently read and modify objects based on their primary key.

You cannot change the primary key field for an object type after adding any object of that type to a realm. If you are using Sync, you cannot change the primary key field for an object after defining the primary key in your backend schema.

Embedded objects cannot contain primary keys.

You may optionally define a primary key for an object type as part of the object schema with the @PrimaryKey annotation:

Fields marked with Java object types and Kotlin nullable types (ending with ?) are nullable by default. All other types (primitives, non-nullable Kotlin object types) are required by default. You can mark a nullable field with the @Required annotation to prevent that field from holding a null value. RealmLists are never nullable, but you can use the @Required annotation to prevent objects in a list from holding a null value, even if the base type would otherwise allow it. You cannot mark a RealmList of RealmObject subtypes as required.

You can make any of the following types required:

  • String

  • UUID

  • ObjectId

  • Integer

  • Long

  • Short

  • Byte or byte[]

  • Boolean

  • Float

  • Double

  • Date

  • RealmList

Primitive types such as int and the RealmList type are implicitly required. Fields with the RealmObject type are always nullable, and cannot be made required.

Important

Kotlin Types and Nullability

In Kotlin, types are non-nullable by default unless you explicitly add a ? suffix to the type. You can only annotate nullable types. Using the @Required annotation on non-nullable types will fail compilation.

To assign a default value to a field, use the built-in language features to assign default values.

Note

Default Values and Nullability

While default values ensure that a newly created object cannot contain a value of null (unless you specify a default value of null), they do not impact the nullability of a field. To make a field non-nullable, see Required Fields.

Indexes support the efficient execution of queries in Realm. Without indexes, Realm must perform a collection scan, i.e. scan every document in a collection, to select those documents that match a query. If an appropriate index exists for a query, Realm can use the index to limit the number of documents that it must inspect.

Indexes are special data structures that store a small portion of a realm's data in an easy to traverse form. The index stores the value of a specific field ordered by the value of the field. The ordering of the index entries supports efficient equality matches and range-based query operations.

Adding an index can speed up some queries at the cost of slightly slower write times and additional storage and memory overhead. Indexes require space in your realm file, so adding an index to a property will increase disk space consumed by your realm file. Each index entry is a minimum of 12 bytes.

You can index fields with the following types:

  • String

  • UUID

  • ObjectId

  • Integer or int

  • Long or long

  • Short or short

  • Byte or byte[]

  • Boolean or bool

  • Date

  • RealmAny

Realm creates indexes for fields annotated with @Index.

To index a field, use the @Index annotation:

If you don't want to save a field in your model to a realm, you can ignore a field.

Ignore a field from a Realm object model with the @Ignore annotation:

Note

The SDK ignores static and transient Fields

Fields marked static or transient are always ignored, and do not need the @Ignore annotation.

By default, Realm uses the name defined in the model class to represent fields internally. In some cases you might want to change this behavior:

  • To make it easier to work across platforms, since naming conventions differ.

  • To change a field name in Kotlin without forcing a migration.

Choosing an internal name that differs from the name used in model classes has the following implications:

  • Migrations must use the internal name when creating classes and fields.

  • Schema errors reported will use the internal name.

Use the @RealmField annotation to rename a field:

Alternatively, you can also assign a naming policy at the module or class levels to change the way that Realm interprets field names.

You can define a naming policy at the module level, which will affect all classes included in the module:

You can also define a naming policy at the class level, which overrides module level settings:

By default, Realm uses the name defined in the model class to represent classes internally. In some cases you might want to change this behavior:

  • To support multiple model classes with the same simple name in different packages.

  • To make it easier to work across platforms, since naming conventions differ.

  • To use a class name that is longer than the 57 character limit enforced by Realm.

  • To change a class name in Kotlin without forcing a migration.

Use the @RealmClass annotation to rename a class:

By default, your application's Realm Schema includes all classes that extend RealmObject. If you only want to include a subset of classes that extend RealmObject in your Realm Schema, you can include that subset of classes in a module and open your realm using that module:

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