The openJDK version 8 is only available by default since Kubuntu 14.10. Howeer, if one runs still the 14.04 LTS Linux, some small hands on are required, as in this case a new personal package archive (ppa) needs to be added to the repository. The commands for that are simply
sudo add-apt-repository ppa:openjdk-r/ppa sudo apt-get update sudo apt-get install openjdk-8-jdk openjdk-8-demo openjdk-8-doc openjdk-8-jre-headless openjdk-8-source
After installing the new Java version, it needs to be set as default. This can be easily done by typing
sudo update-alternatives --config java
and there you can choose then the corresponding version.