Problems with listings

Problems with listings

Im trying to "decorate" source code with listings, but im having some
warnings and bad boxes in mi document. However the document compiles and
looks like i want.
I noticed the warning is because the "}" and "{" simbols in the code.
Overfull \hbox (6.0pt too wide) in paragraph at lines 31--31
[][][][][][][] []
LaTeX Font Info: Try loading font information for OMS+cmss on input line 32.
LaTeX Font Info: No file OMScmss.fd. on input line 32.
LaTeX Font Warning: Font shape OMS/cmss/m/n' undefined (Font)
usingOMS/cmsy/m/n' instead (Font) for symbol `textbraceleft' on input line
32.
\documentclass[a4paper]{article}
\usepackage[spanish,activeacute]{babel}
\usepackage{amsmath}
\usepackage{listings}
\usepackage{color}
\usepackage{xcolor}
\usepackage{caption}
\lstset{
basicstyle=\footnotesize,
numbers=left,
numberstyle=\tiny,
frame=tb,
columns=fullflexible,
showstringspaces=false
}
\renewcommand{\familydefault}{\sfdefault} %this is because default font
looks blurry
\DeclareCaptionFont{white}{\color{white}}
\DeclareCaptionFormat{listing}{\colorbox{black}{\parbox{\textwidth}{#1#2#3}}}
\captionsetup[lstlisting]{format=listing,labelfont=white,textfont=white}
\begin{document}
Some Code:
\begin{lstlisting}[caption=some code, label=source, language=java]
public void hello() {
aaaa();
}
\end{lstlisting}
\end{document}