\begin{tikzpicture}
\umlemptyclass{A1}
\umlemptyclass[x=2, y=-2]{A2}
\umlemptyclass[x=-2, y=-2.5]{A3}
\umlassoc[geometry=-|, arg1=x, mult1=1, pos1=0.2, arg2=y, mult2=*, pos2=1.9]{A1}{A2}
\umlassoc[geometry=|-|, arg1=a, mult1=1, pos1=0.5, arg2=b, mult2=*, pos2=1.5]{A1}{A3}
\end{tikzpicture}