public class Group {
    private long _id;
    private List<Contact> _contacts;
    // irrelevant stuff..

    // getters/setters
}

public class Contact {
    private long _id;
    private List<Group> _groups;
    // irrelevant stuff...

    // getters/setters
}

// How the fuck can I implement DAOs for Group and Contact to avoid both: circular reference and code duplication.