I've installed Sun Java 6 manually since Canonical has removed it from its package repo (I use Ubuntu 12.04). Now I have the following problem: when I want to install a package which depends on Java via a virtual Java packages (java2-runtime or default-jre) the package manager complains that Java is not installed and tries to install the default JRE (OpenJDK on my system). Is there a way to manually mark the virtual package (e.g. java6-runtime) to be installed? This must be a common problem - how is it best addressed?
use the equivs package to create a dummy package which provides the missing java6-runtime dependency, which will make dpkg/apt think that this dependency is satisfied. Here are some basic instructions on the debian wiki.
Also note that openjdk-6-jre is the free version of java, which is still available from ubuntu, and generally just works, and might be a much more simple alternative.