You need to add the Backports repository where the OpenJDK 8 package is located.
Add the file /etc/apt/sources.list.d/backports.list
with this content:
# Backports repository
deb http://ftp.debian.org/debian jessie-backports main contrib non-free
(You may replace http://ftp.debian.org/debian with any closer mirror, or even http://httpredir.debian.org/debian.)