Realm models must extend the RealmObject base class, they define the schema of the underlying database.

Supported field types are boolean, byte, short, int, long, float, double, String, Date, byte[], links to other RealmObjects, and RealmList<T extends RealmModel>.

public class Person extends RealmObject {
    @PrimaryKey //primary key is also implicitly an @Index 
                //it is required for `copyToRealmOrUpdate()` to update the object.
    private long id;

    @Index //index makes queries faster on this field
    @Required //prevents `null` value from being inserted
    private String name; 

    private RealmList<Dog> dogs; //->many relationship to Dog

    private Person spouse; //->one relationship to Person

    private Calendar birthday; //calendars are not supported but can be ignored

    // getters, setters

If you add (or remove) a new field to your RealmObject (or you add a new RealmObject class or delete an existing one), a migration will be needed. You can either set deleteIfMigrationNeeded() in your RealmConfiguration.Builder, or define the necessary migration. Migration is also required when adding (or removing) @Required, or @Index, or @PrimaryKey annotation.

Relationships must be set manually, they are NOT automatic based on primary keys.

Since 0.88.0, it is also possible to use public fields instead of private fields/getters/setters in RealmObject classes.

It is also possible to implement [RealmModel](<>) instead of extending RealmObject, if the class is also annotated with @RealmClass.

public class Person implements RealmModel {
    // ...

In that case, methods like person.deleteFromRealm() or person.addChangeListener() are replaced with RealmObject.deleteFromRealm(person) and RealmObject.addChangeListener(person).

Limitations are that by a RealmObject, only RealmObject can be extended, and there is no support for final, volatile and transient fields.

It is important that a managed RealmObject class can only be modified in a transaction. A managed RealmObject cannot be passed between threads.