Quick Start - Java SDK

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Initialize Realm

Define Your Object Model
Open a Realm
Create, Read, Update, and Delete Objects
Watch for Changes
Complete Example
Output

Realm is now Atlas Device SDK – Learn More

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This Guide Does Not Use Device Sync

This guide can help you get started with device-local Realm. If your application needs to communicate with a backend App over the network using features like Atlas Device Sync, Realm Functions, or user management, you should follow the Quick Start with Sync guide.

This page contains information to quickly get Realm integrated into your app. Before you begin, ensure you have:

Installed the Java SDK

Initialize Realm

Before you can use Realm in your app, you must initialize the Realm library. Your application should initialize Realm just once each time the application runs.

To initialize the Realm library, provide an Android context to the Realm.init() static function. You can provide an Activity, Fragment, or Application context for initialization with no difference in behavior. You can initialize the Realm library in the onCreate() method of an application subclass to ensure that you only initialize Realm once each time the application runs.

Realm.init(this); // context, usually an Activity or Application

Realm.init(this) // context, usually an Activity or Application

Tip

Register Your Application Subclass in the Android Manifest

If you create your own Application subclass, you must add it to your application's AndroidManifest.xml to execute your custom application logic. Set the android.name property of your manifest's application definition to ensure that Android instantiates your Application subclass before any other class when a user launches your application.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.mongodb.example">
   <application
      android:name=".MyApplicationSubclass"
      ...
   />
</manifest>

Define Your Object Model

Your application's data model defines the structure of data stored within Realm. You can define your application's data model via Kotlin or Java classes in your application code with Realm Object Models.

To define your application's data model, add the following class definitions to your application code:

Task.java

import io.realm.RealmObject;
import io.realm.annotations.PrimaryKey;
import io.realm.annotations.Required;
public class Task extends RealmObject {
    @PrimaryKey private String name;
    @Required private String status = TaskStatus.Open.name();
    public void setStatus(TaskStatus status) { this.status = status.name(); }
    public String getStatus() { return this.status; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public Task(String _name) { this.name = _name; }
    public Task() {}
}

TaskStatus.java

public enum TaskStatus {
    Open("Open"),
    InProgress("In Progress"),
    Complete("Complete");
    String displayName;
    TaskStatus(String displayName) {
        this.displayName = displayName;
    }
}

Task.kt

enum class TaskStatus(val displayName: String) {
    Open("Open"),
    InProgress("In Progress"),
    Complete("Complete"),
}
open class Task() : RealmObject() {
    @PrimaryKey
    var name: String = "task"
    @Required
    var status: String = TaskStatus.Open.name
    var statusEnum: TaskStatus
        get() {
            // because status is actually a String and another client could assign an invalid value,
            // default the status to "Open" if the status is unreadable
            return try {
                TaskStatus.valueOf(status)
            } catch (e: IllegalArgumentException) {
                TaskStatus.Open
            }
        }
        set(value) { status = value.name }
}

Open a Realm

Use RealmConfiguration to control the specifics of the realm you would like to open, including the name or location of the realm, whether to allow synchronous reads or writes to a realm on the UI thread, and more.

String realmName = "My Project";
RealmConfiguration config = new RealmConfiguration.Builder().name(realmName).build();
Realm backgroundThreadRealm = Realm.getInstance(config);

val realmName: String = "My Project"
val config = RealmConfiguration.Builder().name(realmName).build()
val backgroundThreadRealm : Realm = Realm.getInstance(config)

Create, Read, Update, and Delete Objects

Once you have opened a realm, you can modify the objects within that realm in a write transaction block.

Important

Synchronous Reads and Writes on the UI Thread

By default, you can only read or write to a realm in your application's UI thread using asynchronous transactions. That is, you can only use Realm methods whose name ends with the word Async in the main thread of your Android application unless you explicitly allow the use of synchronous methods.

This restriction exists for the benefit of your application users: performing read and write operations on the UI thread can lead to unresponsive or slow UI interactions, so it's usually best to handle these operations either asynchronously or in a background thread. However, if your application requires the use of synchronous realm reads or writes on the UI thread, you can explicitly allow the use of synchronous methods with the following SyncConfiguration options:

SyncConfiguration config = new SyncConfiguration.Builder(app.currentUser(), PARTITION)
        .allowQueriesOnUiThread(true)
        .allowWritesOnUiThread(true)
        .build();
Realm.getInstanceAsync(config, new Realm.Callback() {
    @Override
    public void onSuccess(Realm realm) {
        Log.v(
                "EXAMPLE",
                "Successfully opened a realm with reads and writes allowed on the UI thread."
        );
    }
});

val config = SyncConfiguration.Builder(app.currentUser(), PARTITION)
    .allowQueriesOnUiThread(true)
    .allowWritesOnUiThread(true)
    .build()
Realm.getInstanceAsync(config, object : Realm.Callback() {
    override fun onSuccess(realm: Realm) {
        Log.v("EXAMPLE", "Successfully opened a realm with reads and writes allowed on the UI thread.")
    }
})

To create a new Task, instantiate an instance of the Task class and add it to the realm in a write block:

Task Task = new Task("New Task");
backgroundThreadRealm.executeTransaction (transactionRealm -> {
    transactionRealm.insert(Task);
});

val task : Task = Task()
task.name = "New Task"
backgroundThreadRealm.executeTransaction { transactionRealm ->
    transactionRealm.insert(task)
}

You can retrieve a live collection of all items in the realm:

// all Tasks in the realm
RealmResults<Task> Tasks = backgroundThreadRealm.where(Task.class).findAll();

// all tasks in the realm
val tasks : RealmResults<Task> = backgroundThreadRealm.where<Task>().findAll()

You can also filter that collection using a filter:

// you can also filter a collection
RealmResults<Task> TasksThatBeginWithN = Tasks.where().beginsWith("name", "N").findAll();
RealmResults<Task> openTasks = Tasks.where().equalTo("status", TaskStatus.Open.name()).findAll();

// you can also filter a collection
val tasksThatBeginWithN : List<Task> = tasks.where().beginsWith("name", "N").findAll()
val openTasks : List<Task> = tasks.where().equalTo("status", TaskStatus.Open.name).findAll()

To modify a task, update its properties in a write transaction block:

Task otherTask = Tasks.get(0);
// all modifications to a realm must happen inside of a write block
backgroundThreadRealm.executeTransaction( transactionRealm -> {
    Task innerOtherTask = transactionRealm.where(Task.class).equalTo("_id", otherTask.getName()).findFirst();
    innerOtherTask.setStatus(TaskStatus.Complete);
});

val otherTask: Task = tasks[0]!!
// all modifications to a realm must happen inside of a write block
backgroundThreadRealm.executeTransaction { transactionRealm ->
    val innerOtherTask : Task = transactionRealm.where<Task>().equalTo("name", otherTask.name).findFirst()!!
    innerOtherTask.status = TaskStatus.Complete.name
}

Finally, you can delete a task by calling the deleteFromRealm() method in a write transaction block:

Task yetAnotherTask = Tasks.get(0);
String yetAnotherTaskName = yetAnotherTask.getName();
// all modifications to a realm must happen inside of a write block
backgroundThreadRealm.executeTransaction( transactionRealm -> {
    Task innerYetAnotherTask = transactionRealm.where(Task.class).equalTo("_id", yetAnotherTaskName).findFirst();
    innerYetAnotherTask.deleteFromRealm();
});

val yetAnotherTask: Task = tasks.get(0)!!
val yetAnotherTaskName: String = yetAnotherTask.name
// all modifications to a realm must happen inside of a write block
backgroundThreadRealm.executeTransaction { transactionRealm ->
    val innerYetAnotherTask : Task = transactionRealm.where<Task>().equalTo("name", yetAnotherTaskName).findFirst()!!
    innerYetAnotherTask.deleteFromRealm()
}

Watch for Changes

You can watch a realm, collection, or object for changes by attaching a custom OrderedRealmCollectionChangeListener with the addChangeListener() method:

// all Tasks in the realm
RealmResults<Task> Tasks = uiThreadRealm.where(Task.class).findAllAsync();
Tasks.addChangeListener(new OrderedRealmCollectionChangeListener<RealmResults<Task>>() {
    @Override
    public void onChange(RealmResults<Task> collection, OrderedCollectionChangeSet changeSet) {
        // process deletions in reverse order if maintaining parallel data structures so indices don't change as you iterate
        OrderedCollectionChangeSet.Range[] deletions = changeSet.getDeletionRanges();
        for (OrderedCollectionChangeSet.Range range : deletions) {
            Log.v("QUICKSTART", "Deleted range: " + range.startIndex + " to " + (range.startIndex + range.length - 1));
        }
        OrderedCollectionChangeSet.Range[] insertions = changeSet.getInsertionRanges();
        for (OrderedCollectionChangeSet.Range range : insertions) {
            Log.v("QUICKSTART", "Inserted range: " + range.startIndex + " to " + (range.startIndex + range.length - 1));                            }
        OrderedCollectionChangeSet.Range[] modifications = changeSet.getChangeRanges();
        for (OrderedCollectionChangeSet.Range range : modifications) {
            Log.v("QUICKSTART", "Updated range: " + range.startIndex + " to " + (range.startIndex + range.length - 1));                            }
    }
});

// all tasks in the realm
val tasks : RealmResults<Task> = realm.where<Task>().findAllAsync()
tasks.addChangeListener(OrderedRealmCollectionChangeListener<RealmResults<Task>> { collection, changeSet ->
    // process deletions in reverse order if maintaining parallel data structures so indices don't change as you iterate
    val deletions = changeSet.deletionRanges
    for (i in deletions.indices.reversed()) {
        val range = deletions[i]
        Log.v("QUICKSTART", "Deleted range: ${range.startIndex} to ${range.startIndex + range.length - 1}")
    }
    val insertions = changeSet.insertionRanges
    for (range in insertions) {
        Log.v("QUICKSTART", "Inserted range: ${range.startIndex} to ${range.startIndex + range.length - 1}")
    }
    val modifications = changeSet.changeRanges
    for (range in modifications) {
        Log.v("QUICKSTART", "Updated range: ${range.startIndex} to ${range.startIndex + range.length - 1}")
    }
})

Complete Example

If you're running this project in a fresh Android Studio project, you can copy and paste this file into your application's MainActivity -- just remember to:

use a package declaration at the top of the file for your own project
update the import statements for Task and TaskStatus if you're using java

Task.java

import io.realm.RealmObject;
import io.realm.annotations.PrimaryKey;
import io.realm.annotations.Required;
public class Task extends RealmObject {
    @PrimaryKey private String name;
    @Required private String status = TaskStatus.Open.name();
    public void setStatus(TaskStatus status) { this.status = status.name(); }
    public String getStatus() { return this.status; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public Task(String _name) { this.name = _name; }
    public Task() {}
}

TaskStatus.java

public enum TaskStatus {
    Open("Open"),
    InProgress("In Progress"),
    Complete("Complete");
    String displayName;
    TaskStatus(String displayName) {
        this.displayName = displayName;
    }
}

MainActivity.java

import io.realm.OrderedCollectionChangeSet;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import android.util.Log;
import io.realm.OrderedRealmCollectionChangeListener;
import io.realm.Realm;
import io.realm.RealmConfiguration;
import io.realm.RealmResults;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
import com.mongodb.realm.examples.model.java.Task;
import com.mongodb.realm.examples.model.java.TaskStatus;
public class MainActivity extends AppCompatActivity {
    Realm uiThreadRealm;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Realm.init(this); // context, usually an Activity or Application
        String realmName = "My Project";
        RealmConfiguration config = new RealmConfiguration.Builder().name(realmName).build();
        uiThreadRealm = Realm.getInstance(config);
        addChangeListenerToRealm(uiThreadRealm);
        FutureTask<String> Task = new FutureTask(new BackgroundQuickStart(), "test");
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        executorService.execute(Task);
    }
    private void addChangeListenerToRealm(Realm realm) {
        // all Tasks in the realm
        RealmResults<Task> Tasks = uiThreadRealm.where(Task.class).findAllAsync();
        Tasks.addChangeListener(new OrderedRealmCollectionChangeListener<RealmResults<Task>>() {
            @Override
            public void onChange(RealmResults<Task> collection, OrderedCollectionChangeSet changeSet) {
                // process deletions in reverse order if maintaining parallel data structures so indices don't change as you iterate
                OrderedCollectionChangeSet.Range[] deletions = changeSet.getDeletionRanges();
                for (OrderedCollectionChangeSet.Range range : deletions) {
                    Log.v("QUICKSTART", "Deleted range: " + range.startIndex + " to " + (range.startIndex + range.length - 1));
                }
                OrderedCollectionChangeSet.Range[] insertions = changeSet.getInsertionRanges();
                for (OrderedCollectionChangeSet.Range range : insertions) {
                    Log.v("QUICKSTART", "Inserted range: " + range.startIndex + " to " + (range.startIndex + range.length - 1));                            }
                OrderedCollectionChangeSet.Range[] modifications = changeSet.getChangeRanges();
                for (OrderedCollectionChangeSet.Range range : modifications) {
                    Log.v("QUICKSTART", "Updated range: " + range.startIndex + " to " + (range.startIndex + range.length - 1));                            }
            }
        });
    }
        @Override
    protected void onDestroy() {
        super.onDestroy();
        // the ui thread realm uses asynchronous transactions, so we can only safely close the realm
        // when the activity ends and we can safely assume that those transactions have completed
        uiThreadRealm.close();
    }
    public class BackgroundQuickStart implements Runnable {
        @Override
        public void run() {
            String realmName = "My Project";
            RealmConfiguration config = new RealmConfiguration.Builder().name(realmName).build();
            Realm backgroundThreadRealm = Realm.getInstance(config);
            Task Task = new Task("New Task");
            backgroundThreadRealm.executeTransaction (transactionRealm -> {
                transactionRealm.insert(Task);
            });
            // all Tasks in the realm
            RealmResults<Task> Tasks = backgroundThreadRealm.where(Task.class).findAll();
            // you can also filter a collection
            RealmResults<Task> TasksThatBeginWithN = Tasks.where().beginsWith("name", "N").findAll();
            RealmResults<Task> openTasks = Tasks.where().equalTo("status", TaskStatus.Open.name()).findAll();
            Task otherTask = Tasks.get(0);
            // all modifications to a realm must happen inside of a write block
            backgroundThreadRealm.executeTransaction( transactionRealm -> {
                Task innerOtherTask = transactionRealm.where(Task.class).equalTo("_id", otherTask.getName()).findFirst();
                innerOtherTask.setStatus(TaskStatus.Complete);
            });
            Task yetAnotherTask = Tasks.get(0);
            String yetAnotherTaskName = yetAnotherTask.getName();
            // all modifications to a realm must happen inside of a write block
            backgroundThreadRealm.executeTransaction( transactionRealm -> {
                Task innerYetAnotherTask = transactionRealm.where(Task.class).equalTo("_id", yetAnotherTaskName).findFirst();
                innerYetAnotherTask.deleteFromRealm();
            });
            // because this background thread uses synchronous realm transactions, at this point all
            // transactions have completed and we can safely close the realm
            backgroundThreadRealm.close();
        }
    }
}

MainActivity.kt

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import android.util.Log
import io.realm.*
import io.realm.annotations.PrimaryKey
import io.realm.annotations.Required
import io.realm.kotlin.where
import java.util.concurrent.ExecutorService
import java.util.concurrent.Executors
import java.util.concurrent.FutureTask
class MainActivity : AppCompatActivity() {
    lateinit var uiThreadRealm: Realm
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        Realm.init(this) // context, usually an Activity or Application
        val realmName: String = "My Project"
        val config = RealmConfiguration.Builder()
            .name(realmName)
            .build()
        uiThreadRealm = Realm.getInstance(config)
        addChangeListenerToRealm(uiThreadRealm)
        val task : FutureTask<String> = FutureTask(BackgroundQuickStart(), "test")
        val executorService: ExecutorService = Executors.newFixedThreadPool(2)
        executorService.execute(task)
    }
    fun addChangeListenerToRealm(realm : Realm) {
        // all tasks in the realm
        val tasks : RealmResults<Task> = realm.where<Task>().findAllAsync()
        tasks.addChangeListener(OrderedRealmCollectionChangeListener<RealmResults<Task>> { collection, changeSet ->
            // process deletions in reverse order if maintaining parallel data structures so indices don't change as you iterate
            val deletions = changeSet.deletionRanges
            for (i in deletions.indices.reversed()) {
                val range = deletions[i]
                Log.v("QUICKSTART", "Deleted range: ${range.startIndex} to ${range.startIndex + range.length - 1}")
            }
            val insertions = changeSet.insertionRanges
            for (range in insertions) {
                Log.v("QUICKSTART", "Inserted range: ${range.startIndex} to ${range.startIndex + range.length - 1}")
            }
            val modifications = changeSet.changeRanges
            for (range in modifications) {
                Log.v("QUICKSTART", "Updated range: ${range.startIndex} to ${range.startIndex + range.length - 1}")
            }
        })
    }
    override fun onDestroy() {
        super.onDestroy()
        // the ui thread realm uses asynchronous transactions, so we can only safely close the realm
        // when the activity ends and we can safely assume that those transactions have completed
        uiThreadRealm.close()
    }
    class BackgroundQuickStart : Runnable {
        override fun run() {
            val realmName: String = "My Project"
            val config = RealmConfiguration.Builder().name(realmName).build()
            val backgroundThreadRealm : Realm = Realm.getInstance(config)
            val task : Task = Task()
            task.name = "New Task"
            backgroundThreadRealm.executeTransaction { transactionRealm ->
                transactionRealm.insert(task)
            }
            // all tasks in the realm
            val tasks : RealmResults<Task> = backgroundThreadRealm.where<Task>().findAll()
            // you can also filter a collection
            val tasksThatBeginWithN : List<Task> = tasks.where().beginsWith("name", "N").findAll()
            val openTasks : List<Task> = tasks.where().equalTo("status", TaskStatus.Open.name).findAll()
            val otherTask: Task = tasks[0]!!
            // all modifications to a realm must happen inside of a write block
            backgroundThreadRealm.executeTransaction { transactionRealm ->
                val innerOtherTask : Task = transactionRealm.where<Task>().equalTo("name", otherTask.name).findFirst()!!
                innerOtherTask.status = TaskStatus.Complete.name
            }
            val yetAnotherTask: Task = tasks.get(0)!!
            val yetAnotherTaskName: String = yetAnotherTask.name
            // all modifications to a realm must happen inside of a write block
            backgroundThreadRealm.executeTransaction { transactionRealm ->
                val innerYetAnotherTask : Task = transactionRealm.where<Task>().equalTo("name", yetAnotherTaskName).findFirst()!!
                innerYetAnotherTask.deleteFromRealm()
            }
            // because this background thread uses synchronous realm transactions, at this point all
            // transactions have completed and we can safely close the realm
            backgroundThreadRealm.close()
        }
    }
}
enum class TaskStatus(val displayName: String) {
    Open("Open"),
    InProgress("In Progress"),
    Complete("Complete"),
}
open class Task() : RealmObject() {
    @PrimaryKey
    var name: String = "task"
    @Required
    var status: String = TaskStatus.Open.name
    var statusEnum: TaskStatus
        get() {
            // because status is actually a String and another client could assign an invalid value,
            // default the status to "Open" if the status is unreadable
            return try {
                TaskStatus.valueOf(status)
            } catch (e: IllegalArgumentException) {
                TaskStatus.Open
            }
        }
        set(value) { status = value.name }
}

Output

Running the above code should produce output resembling the following:

Successfully authenticated anonymously.
Updated range: 0 to 1
Deleted range: 0 to 1
Successfully logged out.

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