すみません、なぜこれがエラーになるのかわかりません。
apt-get install --reinstall sun-java6-jre もしてみましたが同じ結果です。
解決のヒントなどもらえると嬉しいです。よろしくお願いします。

$ sudo apt-get install java6-runtime-headless sun-java6-jre
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package java6-runtime-headless is a virtual package provided by:
　sun-java6-jre 6.22-1
　openjdk-6-jre-headless 6b18-1.8.3-2
　default-jre-headless 1:1.6-40
You should explicitly select one to install.

E: Package 'java6-runtime-headless' has no installation candidate
$ cat /etc/apt/sources.list | grep "^deb"
deb http://ftp.jp.debian.org/debian/ squeeze main non-free contrib
deb-src http://ftp.jp.debian.org/debian/ squeeze main non-free contrib
deb http://security.debian.org/ squeeze/updates main contrib non-free
deb-src http://security.debian.org/ squeeze/updates main contrib non-free
$ dpkg -l | grep jre
ii sun-java6-jre 6.22-1 Sun Java(TM) Runtime Environment (JRE) 6 (architecture independent files)