\documentclass[a4paper,12pt]{article}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[T1]{fontenc}
\usepackage[margin=2cm]{geometry}
\usepackage{minted}
\begin{document}
\renewcommand{\theFancyVerbLine}{
\sffamily\textcolor[rgb]{0.5,0.5,0.5}{\scriptsize\arabic{FancyVerbLine}}}
\inputminted[linenos,
numbersep=7pt,
gobble=0,
frame=lines,
framesep=2mm,
label=AbstractClass.java,
fontsize=\footnotesize, tabsize=4]{java}{AbstractClass.java}
\clearpage
\inputminted[linenos,
numbersep=7pt,
gobble=0,
frame=lines,
framesep=2mm,
label=ConcreteClass.java,
fontsize=\footnotesize, tabsize=4]{java}{ConcreteClass.java}
\end{document}