public class RealmManager {
    private static RealmManager instance;
    private final ThreadLocal<Realm> localRealm = new ThreadLocal<>();

    RealmManager(){}

    public synchronized static RealmManager getInstance(){
        if(instance == null){
            instance = new RealmManager();
        }
        return instance;
    }

    public Realm openLocalInstance() {
        Realm realm = Realm.getDefaultInstance();
        if(localRealm.get() == null) {
            localRealm.set(realm);
        }
        return realm;
    }

    public Realm getLocalInstance() {
        Realm realm = localRealm.get();
        if(realm == null) {
            throw new IllegalStateException("No open Realms were found on this thread.");
        }
        return realm;
    }

    public void closeLocalInstance() {
        Realm realm = localRealm.get();
        if(realm == null) {
            throw new IllegalStateException(
                "Cannot close a Realm that is not open.");
        }
        realm.close();
        if(Realm.getLocalInstanceCount(Realm.getDefaultConfiguration()) <= 0) {
        localRealm.set(null);
        }
    }

    public void storePreferenceDao(int userID, String rank){
        final PreferenceDao preferenceDao = new PreferenceDao();
        preferenceDao.setUserID(userID);
        preferenceDao.setRank(rank);
        openLocalInstance();
        getLocalInstance().executeTransactionAsync(new Realm.Transaction() {
            @Override
            public void execute(Realm realm) {
                realm.copyToRealmOrUpdate(preferenceDao);
            }
        }, new Realm.Transaction.OnSuccess(){
            @Override
            public void onSuccess(){
                System.out.println("Data is stored successfully!");
            }
        }, new Realm.Transaction.OnError(){
            @Override
            public void onError(Throwable error){
                System.out.println("There is an error in storePreferenceDao()");
            }
        });
        closeLocalInstance();
    }

RealmManager.getInstance().storePreferenceDao(123, "Alpaca");

public class MySingleton {
private static MySingleton sMySingleton;

//private constructor.
private MySingleton() {
    if (sMySingleton != null){
        throw new RuntimeException("Use getInstance() for the instance");
    }
}

public synchronized static MySingleton getInstance() {
    if (sMySingleton == null){
        sMySingleton = new MySingleton();
    }
    return sMySingleton;
}
}

public class BaseActivity extends AppCompatActivity {
public Realm realm;
  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_base);
        realm = Realm.getDefaultInstance();
    }

}

public class MainActivity extends BaseActivity {

       @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //Here you can directly access realm object and perform your task
        realm.where()//just example
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        if(realm!=null)
           realm.close();
       //Don't forget to close realm object
    }

}