Many-to-many relation of a single entity - Hibernate

I have a simple User entity and would like to implement a ManyToMany relation with another users. The class looks like this:

@Entity
public class User {
    @Id
    @GeneratedValue
    private long id;

    private String username;

    @ManyToMany
    private List<User> friends;

    public long getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public List<User> getFriends() {
        return friends;
    }

    public void setFriends(List<User> friends) {
        this.friends = friends;
    }

    public void addFriend(User user){
        if (friends==null) friends = new ArrayList<User>();
        friends.add(user);
    }
}

This doesn't work how I want however, because I'd like to have only one row in the database for bidirectional relation, in this case it makes two rows, in both directions.

How can I achieve that?