Usually you install the openjdk like this:

sudo apt-get install openjdk-8-jdk

Maybe this is the problem. You can look this up at http://openjdk.java.net/install/index.html. If you do this you might have fixed this problem. But cant imagine why your command give this result.

Editors note: This answer is outdated as the PPA’s listed are not available anymore.

You can do this for;

Final Update

JDK

sudo apt-get install openjdk-8-jdk

JRE

sudo apt-get install openjdk-8-jre

Old Update

I found two repository but I do not recommend

• OpenJDK builds (all archs)

    ppa:openjdk-r/ppa
  

• OpenJDK 8 backport for trusty

    ppa:jochenkemnade/openjdk-8
  

Original Message

If you really want to use OpenJDK, you have to compile from source. There is not still any PPA for OpenJDK.

It has been requested at https://bugs.launchpad.net/ubuntu/+bug/1297065

I recommend you to use Webup8 Oracle Java8 Installer

Note: WebUpd8 team's PPA has been discontinued with effective from April 16, 2019. Thus this PPA doesn't have any Java files. More information can be found on PPA's page on Launchpad. Hence the below method no longer works and exists because of historical reasons.

sudo add-apt-repository ppa:webupd8team/java -y
sudo apt-get update
sudo apt-get install oracle-java8-installer

To automatically set up the Java 8 environment variables

sudo apt-get install oracle-java8-set-default

Check it

java -version

So you have to wait to use OpenJDK8

As of now, 30 April 2019 02:00 UTC, Open JDK 8 and its associated packages have been added back to the normal Ubuntu 19.04 Disco Dingo repositories.

So you can now do the usual

sudo apt install openjdk-8-jdk

without adding any PPAs or downloading .deb packages.

As an aside, I believe the package was removed from 19.04 (while originally being there) because it was (erroneously?) removed from Debian testing (related bug report) and then cleaned up downstream. It has been re-added today by its maintainer Matthias Klose aka doko.