\begin{figure}
\centering
    \tikzstyle{class} = [rectangle, draw, rounded corners]
    \begin{tikzpicture}[
        level distance=1cm, 
        growth parent anchor=south,
        child anchor=north,
        sibling distance=3cm
    ]

    \node [class] (Walker) {Walker} [->]
    child {
        node [class] (SymbolLookup) {SymbolLookup}
    }
    child {
        node [class] (ProcessState) {ProcessState}
    }
    child {
        node [class] (StepperGroup) {StepperGroup}
        child {
            node [class] (FrameStepper) {FrameStepper}    
        }
    }
    child {
        node [class] (Frame) {Frame}
    }
    ;
    
\end{tikzpicture}
\caption{Object Ownership}
\label{fig:object-ownership}

\end{figure}