\documentclass[a4paper,12pt]{report}
\makeatletter
\newcommand{\textarrow}[2][1]
  { \settowidth{\@tempdima}{#2}
    \stackrel{#2}
             {\makebox[#1\@tempdima][l]{\rightarrowfill}}
  }
\makeatother

\begin{document}


\begin{eqnarray*}
 y = f(x) & \textarrow[1.5]{Transformation} & Y = F(X)\\
 y = f(x) & \textarrow{\ensuremath{x\mapsto X}} & Y = F(X)
\end{eqnarray*}

\end{document}
