public boolean addMember(Principal user){
  if (groupMembers.contains(user))   return false;
  if (group.equals(user.toString()))   throw new IllegalArgumentException();
  groupMembers.addElement(user);
  return true;
}

public boolean addMember ( Principal user ) { if ( groupMembers . contains ( user ) ) return false ; if ( group . equals ( user . toString ( ) ) ) throw new IllegalArgumentException ( ) ; groupMembers . addElement ( user ) ; return true ; }
adds the specified member to the group .
