MainActivity.java

public class MainActivity extends AppCompatActivity {
    public Realm mRealm;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mRealm = Realm.getDefaultInstance();
        mRealm.beginTransaction();

        long id = autoIncrementId(User.class);
        User u = mRealm.createObject(User.class, id);
        u.setName("name" + id);

        Bread b1 = mRealm.createObject(Bread.class);
        b1.setName("croissant");
        b1.setIsEdible(true);
        b1.setPrice(230);

        Bread b2 = mRealm.createObject(Bread.class);
        b2.setName("chocolate tart");
        b2.setIsEdible(false);
        b2.setPrice(500);

        RealmList<Bread> breads = new RealmList<>();
        breads.add(b1);
        breads.add(b2);

        u.setBreads(breads);

        mRealm.commitTransaction();
        mRealm.close();
    }

    public long autoIncrementId(Class c){
        Number n = mRealm.where(c).max("id");
        long id = (n != null) ? n.longValue() + 1 : 0;
        return id;
    }
}