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RealmDictionary - Java SDK

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  • Usage
  • Notifications

New in version 10.6.0.

You can use the RealmDictionary data type to manage a collection of unique String keys paired with values. RealmDictionary implements Java's Map interface, so it works just like the built-in HashMap class, except managed RealmDictionary instances persist their contents to a realm. RealmDictionary instances that contain Realm objects store references to those objects. When you delete a Realm object from a realm, any references to that object in a RealmDictionary become null values.

Usage

To create a field of type RealmDictionary, define an object property of type RealmDictionary<T>, where T defines the values you would like to store in your RealmDictionary. Currently, RealmDictionary instances can only use keys of type String.

The following table shows which methods you can use to complete common collection tasks with RealmDictionary:

Task
Method
Add an object to a RealmDictionary
put() (or the [] operator in Kotlin)
Add multiple objects to a RealmDictionary
Check if the dictionary contains an specific key
Check if the dictionary contains a specific value
import io.realm.RealmDictionary;
import io.realm.RealmObject;
public class Frog extends RealmObject {
    String name;
    RealmDictionary<Frog> nicknamesToFriends;
    // realm-required empty constructor
    public Frog() {}
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public RealmDictionary<Frog> getNicknamesToFriends() { return nicknamesToFriends; }
    public void setNicknamesToFriends(RealmDictionary<Frog> nicknamesToFriends) { this.nicknamesToFriends = nicknamesToFriends; }
}
Frog frog = realm.createObject(Frog.class);
frog.setName("George Washington");
// get the RealmDictionary field from the object we just created
RealmDictionary<Frog> dictionary = frog.getNicknamesToFriends();
// add key/value to the dictionary
Frog wirt = realm.createObject(Frog.class);
wirt.setName("Wirt");
dictionary.put("tall frog", wirt);
// add multiple keys/values to the dictionary
Frog greg = realm.createObject(Frog.class);
greg.setName("Greg");
Frog beatrice = realm.createObject(Frog.class);
beatrice.setName("Beatrice");
dictionary.putAll(Map.of("small frog", greg, "feathered frog", beatrice));
// check for the presence of a key
Assert.assertTrue(dictionary.containsKey("small frog"));
// check for the presence of a value
Assert.assertTrue(dictionary.containsValue(greg));
// remove a key
dictionary.remove("feathered frog");
Assert.assertFalse(dictionary.containsKey("feathered frog"));
// deleting a Realm object does NOT remove it from the dictionary
int sizeOfDictionaryBeforeDelete = dictionary.size();
greg.deleteFromRealm();
// deleting greg object did not reduce the size of the dictionary
Assert.assertEquals(sizeOfDictionaryBeforeDelete, dictionary.size());
// but greg object IS now null:
Assert.assertEquals(dictionary.get("small frog"), null);
import io.realm.RealmDictionary
import io.realm.RealmObject
open class Frog
    : RealmObject() {
    var name: String? = null
    var nicknamesToFriends: RealmDictionary<Frog> = RealmDictionary<Frog>()
}
val frog =
    realm.createObject(Frog::class.java)
frog.name = "George Washington"
// get the RealmDictionary field from the object we just created
val dictionary = frog.nicknamesToFriends
// add key/value to the dictionary
val wirt =
    realm.createObject(Frog::class.java)
wirt.name = "Wirt"
dictionary["tall frog"] = wirt
// add multiple keys/values to the dictionary
val greg =
    realm.createObject(Frog::class.java)
greg.name = "Greg"
val beatrice =
    realm.createObject(Frog::class.java)
beatrice.name = "Beatrice"
dictionary.putAll(mapOf<String, Frog>(
    Pair("small frog", greg),
    Pair("feathered frog", beatrice)))
// check for the presence of a key
Assert.assertTrue(dictionary.containsKey("small frog"))
// check for the presence of a value
Assert.assertTrue(dictionary.containsValue(greg))
// remove a key
dictionary.remove("feathered frog")
Assert.assertFalse(dictionary.containsKey("feathered frog"))
// deleting a Realm object does NOT remove it from the dictionary
val sizeOfDictionaryBeforeDelete = dictionary.size
greg.deleteFromRealm()
// deleting greg object did not reduce the size of the dictionary
Assert.assertEquals(
    sizeOfDictionaryBeforeDelete.toLong(),
    dictionary.size.toLong()
)
// but greg object IS now null:
Assert.assertEquals(dictionary["small frog"], null)

Notifications

To subscribe to changes to a RealmDictionary, pass a MapChangeListener implementation to the RealmSet.addChangeListener method. Your MapChangeListener implementation must define an onChange() method, which accepts a reference to the changed RealmDictionary and a set of changes as parameters. You can access the keys added to the dictionary as well as the keys removed from the dictionary through the MapChangeSet parameter.

AtomicReference<Frog> frog = new AtomicReference<Frog>();
realm.executeTransaction(r -> {
    frog.set(realm.createObject(Frog.class));
    frog.get().setName("Jonathan Livingston Applesauce");
});
MapChangeListener<String, Frog> mapChangeListener =
    new MapChangeListener<String, Frog>() {
        @Override
        public void onChange(RealmMap<String, Frog> map,
                             MapChangeSet<String> changes) {
            for (String insertion : changes.getInsertions()) {
                Log.v("EXAMPLE",
                        "Inserted key:  " + insertion +
                                ", Inserted value: " + map.get(insertion).getName());
            }
        }
    };
frog.get().getNicknamesToFriends().addChangeListener(mapChangeListener);
realm.executeTransaction(r -> {
    // get the RealmDictionary field from the object we just created
    RealmDictionary<Frog> dictionary = frog.get().getNicknamesToFriends();
    // add key/value to the dictionary
    Frog wirt = realm.createObject(Frog.class);
    wirt.setName("Wirt");
    dictionary.put("tall frog", wirt);
    // add multiple keys/values to the dictionary
    Frog greg = realm.createObject(Frog.class);
    greg.setName("Greg");
    Frog beatrice = realm.createObject(Frog.class);
    beatrice.setName("Beatrice");
    dictionary.putAll(Map.of("small frog", greg, "feathered frog", beatrice));
});
var frog: Frog? = null
realm.executeTransaction { r: Realm? ->
    frog = realm.createObject(Frog::class.java)
    frog?.name = "Jonathan Livingston Applesauce"
}
val mapChangeListener: MapChangeListener<String, Frog>
        = MapChangeListener<String, Frog> { map, changes ->
    for (insertion in changes.insertions) {
        Log.v("EXAMPLE",
                "Inserted key:  $insertion, Inserted value: ${map[insertion]!!.name}")
    }
}
frog?.nicknamesToFriends?.addChangeListener(mapChangeListener)
realm.executeTransaction { r: Realm? ->
    // get the RealmDictionary field from the object we just created
    val dictionary = frog!!.nicknamesToFriends
    // add key/value to the dictionary
    val wirt = realm.createObject(Frog::class.java)
    wirt.name = "Wirt"
    dictionary["tall frog"] = wirt
    // add multiple keys/values to the dictionary
    val greg = realm.createObject(Frog::class.java)
    greg.name = "Greg"
    val beatrice = realm.createObject(Frog::class.java)
    beatrice.name = "Beatrice"
    dictionary.putAll(mapOf<String, Frog>(
            Pair("small frog", greg),
            Pair("feathered frog", beatrice)))
}

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