OpenJDK 8 for Trusty isn’t available in the official Ubuntu repositories.
As a workaround, you can install OpenJDK 8 from a PPA.
Open a terminal and execute:

sudo apt-add-repository ppa:openjdk-r/ppa
sudo apt-get update
sudo apt-get install openjdk-8-jdk

Or if you only need the JRE

sudo apt-get install openjdk-8-jre

It is usual that several major versions are installed in parallel.

To switch between Java 7 and Java 8 use this command:

sudo update-alternatives --config java

Source: how can i upgrade only jre from 7 to jre 8 on ubuntu 1404 – Ask Ubuntu