Apt-get won't overwrite the existing java versions.
To switch between installed java versions, use the update-java-alternatives command.
List all java versions:
update-java-alternatives --list
Set java version as default (needs root permissions):
sudo update-java-alternatives --set /path/to/java/version
...where /path/to/java/version is one of those listed by the previous command (e.g. /usr/lib/jvm/java-7-openjdk-amd64). Additional information:
update-java-alternatives is a convenience tool that uses Debian's alternatives system (update-alternatives) to set a bunch of links to the specified java version (e.g. java, javac, ...).