Realm with pre populated data into assets?

Since Realm Java 0.91.0 there has been an assetFile(String) option on the RealmConfiguration that automatically will copy a file from assets and use that if needed (e.g. if the Realm is opened the first time or has been deleted for some reason):

RealmConfiguration config = new RealmConfiguration.Builder()
  .assetFile("path/to/file/in/assets") // e.g "default.realm" or "lib/data.realm"
  .deleteRealmIfMigrationNeeded()
  .build()

The above will copy the file from assets the first time the Realm is opened or if it has been deleted due to migrations (remember to update the asset Realm in that case).

OLD ANSWER:

It is possible to bundle a Realm database in the assets folder, but then you just need to copy it from there when starting the app the first time.

We have an example of how to copy the files here: https://github.com/realm/realm-java/blob/master/examples/migrationExample/src/main/java/io/realm/examples/realmmigrationexample/MigrationExampleActivity.java#L101-Lundefined

copyBundledRealmFile(this.getResources().openRawResource(R.raw.default_realm), "default.realm");

private String copyBundledRealmFile(InputStream inputStream, String outFileName) {
    try {
        File file = new File(this.getFilesDir(), outFileName);
        FileOutputStream outputStream = new FileOutputStream(file);
        byte[] buf = new byte[1024];
        int bytesRead;
        while ((bytesRead = inputStream.read(buf)) > 0) {
            outputStream.write(buf, 0, bytesRead);
        }
        outputStream.close();
        return file.getAbsolutePath();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}

Since Realm 0.89.0 RealmConfiguration.initialData(Realm.Transaction) can now be used to populate a Realm file before it is used for the first time.

RealmConfiguration conf = new RealmConfiguration.Builder(context)
.initialData(new Realm.Transaction() {
                @Override
                public void execute(Realm realm) {
                  realm.beginTransaction();
                  realm.createObject(....)
                  realm.commitTransaction();
                }
            }).deleteRealmIfMigrationNeeded().name("mRealm.db").build();
Realm realm = Realm.getInstance(conf);

[EDIT] See Stan's answer below. Apparently Realm now supports this directly so you can ignore this answer (unless you're using older Realm versions).

We had a similar need, and also wanted support for a read-only realm database shared with an iOS version of the app.

We created a simple library and have open-sourced it. It includes the copy code given in @christian-melchior's answer, as well as some optional extra tracking for read-only realm database(s) bundled with the APK. Comments and PRs welcomed. See:

https://github.com/eggheadgames/android-realm-asset-helper

Realm has a special parameter in its RealmConfiguration.Builder called assetFile. You could use it like:

realmConfiguration = new RealmConfiguration.Builder()
                    .assetFile("dataBase/default.realm") // your app's packaged DB
                    ...
                    .build();

just set yer assets DB path and file name and you are good to go without any android-realm-asset-helper lib or copy-file-from-assets code. In this example my app packaged DB-file lies in "assets/dataBase/default.realm".
Note, version below 2 has a bit another way to call assetFile, you should pass context additionally:

realmConfiguration = new RealmConfiguration.Builder(context)
        .assetFile(context, "dataBase/default.realm")
        .build();