> But my Java version is 7. Is something going wrong here?
The default runtime is still Java 7. You can switch to Java 8 by using
update-java-alternatives. For example:
sudo update-java-alternatives --set java-1.8.0-openjdk-amd64
> But for Java 8 I need to install unstable packages I understood. Or am
> I mistaken?
openjdk-8 as found in unstable has been rebuilt for Jessie and is
available from the jessie-backports repository. This backport is
perfectly suitable for a stable environment.
> That explains it: the Debian distribution of Java is different as the
> Oracle one.
Indeed, Oracle bundles several things that you'll find in separate
packages in Debian (openjdk-<n>-jdk, visualvm, openjfx, icedtea-web)