public class User implements Comparable<User> {
  private String name;
  protected SortedSet<Group> groups;
  public User() { this.groups = new TreeSet<Group>(); }
  public synchronized String getName() { return name; }
  public synchronized void setName(String name) { this.name = name; }
  public synchronized boolean equals(Object obj) { [...] }
  public synchronized int hashCode() { [...] }
  public synchronized int compareTo(User otherUser) { [...] }
}

public class Group implements Comparable<Group> {
  private String name;
  public SortedSet<User> users;
  public Group() { this.users = new TreeSet<User>(); }
  public synchronized String getName() { return name; }
  public synchronized void setName(String name) { this.name = name; }
  public synchronized boolean equals(Object obj) { [...] }
  public synchronized int hashCode() { [...] }
  public synchronized int compareTo(Group otherGroup) { [...] }
}

  <entity name="User" class="foo.bar.User">
    <table name="USERS" />
    <attributes>
      <id name="name">
        <column name="NAME" unique="true" nullable="false" />
      </id>
      <many-to-many name="groups" mapped-by="users">
        <order-by>name</order-by>
        <join-table name="USER_GROUP_MAPPINGS">
          <join-column name="USER_NAME" nullable="false" />
          <inverse-join-column name="GROUP_NAME" nullable="false" />
        </join-table>
      </many-to-many>
    </attributes>
  </entity>
 
  <entity name="Group" class="foo.bar.Group">
    <table name="GROUPS" />
    <attributes>
      <id name="name">
        <column name="NAME" unique="true" nullable="false" />
      </id>
      <many-to-many name="users" mapped-by="groups" />
    </attributes>
  </entity>