\begin{tikzpicture}
\begin{umlseqdiag}
\umlactor[no ddots]{user}
\umlobject[class=A]{a}
\umlobject[class=B]{b}
\begin{umlcall}{a}{b}
\end{umlcall}
\end{umlseqdiag}
\end{tikzpicture}