JDK 7 or 8 On Ubuntu

I have successfully installed and updated Ubuntu on VM...now I am getting the promt hadoop@ubuntu:~$ ....but the slides in simplilearn are showing [hadoop@simplilearn] # and when I use sudo apt-get install openjdk-7-jdk gives error 'has no installation candidate' . Do I have to do anything before the command to install java 7.?? I understand Jdk 7 is more stable , so need suggestion to get rid of this error.

And when I start the Ubuntu on the VM why it is always opening the command...

JDK 7 or 8 On Ubuntu