Docs Menu

Docs Home → Develop Applications → Atlas Device SDKs

Quick Start - Java SDK

On this page

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

Tip

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:

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.

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:

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.

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:

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

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

You can also filter that collection using a filter:

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

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

Watch for Changes

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

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

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.
← Quick Starts - Java SDK
Quick Start with Sync - Java SDK →

On this page