public class Album {

    private ObjectId _id;

    private String artist;
    private String title;
    private List<Song> tracks;

    public ObjectId getId() {
        return _id;
    }

    public void setId(ObjectId id) {
        this._id = id;
    }

    ... Standard Accessors/Mutators ...

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Album album = (Album) o;

        if (!_id.equals(album._id)) return false;

        return true;
    }

    @Override
    public int hashCode() {
        return _id.hashCode();
    }
}