public JID getMemberForReservedNickname(@Nonnull final String nickname) {
        for (final Map.Entry<JID, String> entry : members.entrySet()) {
            if (entry.getValue().equalsIgnoreCase(nickname)) {
                return entry.getKey();
            }
        }
        return null;
    }
Returns the bare JID of the member for which a nickname is reserved. Returns null if no member registered the
nickname.

@param nickname The nickname for which to look up a member.
@return the bare JID of the member that has registered this nickname, or null if none.
xmppserver/src/main/java/org/jivesoftware/openfire/muc/MUCRoom.java