se você realmente quiser instalar o JDK7, adicione-o ao seu arquivo /etc/apt/sources.list
,
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
emitindo o seguinte comando,
sudoedit /etc/apt/sources.list
execute,
sudo apt-get update
execute apt-cache search openjdk-7-jre
para verificar.