Referring to Ask Ubuntu question How to set JAVA_HOME for OpenJDK?,

How to install Open JDK (Java developement kit) in Ubuntu (Linux)?

1. Open Terminal from Application Dash or press Ctrl+Alt+T

2. Update repository:

   sudo add-apt-repository ppa:openjdk-r/ppa  # only Ubuntu 17.4 and earlier
   sudo apt update
   

3. Optional: To search available distributions of openjdk, use the following command:

   apt search openjdk
   

4. Install the appropriate version with the following command:

   sudo apt install openjdk-8-jdk
   sudo apt install openjdk-8-source #this is optional, the jdk source code
   

5. For JAVA_HOME (Environment Variable) type command as shown below, in "Terminal" using your installation path...

   export JAVA_HOME=/usr/lib/jvm/java-8-openjdk
   

   (Note: /usr/lib/jvm/java-8-openjdk is symbolically used here just for demostration. You should use your path as per your installation.)

6. For PATH (Environment Variable) type command as shown below, in Terminal:

   export PATH=$PATH:$JAVA_HOME/bin

7. To check your installation:

   java -version

You can install and stay up to date with the latest Oracle Java 7 and all you have to do is manually add the PPA repository to the Software Sources.

From http://www.webupd8.org/2012/06/how-to-install-oracle-java-7-in-debian.html:

su -
echo "deb http://ppa.launchpad.net/webupd8team/java/ubuntu precise main" | tee -a     /etc/apt/sources.list
echo "deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu precise main" | tee -a /etc/apt/sources.list
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EEA14886
apt-get update
apt-get install oracle-java7-installer
exit

For Ubuntu, http://www.webupd8.org/2012/01/install-oracle-java-jdk-7-in-ubuntu-via.html. With this, I've managed to installed JDK on my ubuntu server.

try using this command in the terminal:

sudo apt install default-jre

then you can try using:

java -version

to check version