\begin{tikzpicture}%[show background grid]

\begin{tikzpicture}%[show background grid]
\begin{interface}{Person}{0,0}
\attribute{firstName : String}
\attribute{lastName : String}
\end{interface}

\begin{class}{Professor}{-5,-5}
\implement{Person}
\attribute{salary : Dollars}
\end{class}

\begin{class}{Student}{5,-5}
\implement{Person}
\attribute{major : String}
\end{class}
\end{tikzpicture}