Now it is possible to install openjdk-11 this way:

sudo apt-get install openjdk-11-jdk

(Previously it installed openjdk-10, but not anymore)

1. You have to install the openjdk-8-jre:

    sudo apt-get install openjdk-8-jre
   

2. Next switch to the jre-8 version:

    $ sudo update-alternatives --config java
    There are 2 choices for the alternative java (providing /usr/bin/java).
   
    Selection    Path                                            Priority   Status
   
    * 0            /usr/lib/jvm/java-9-openjdk-amd64/bin/java       1500      auto mode
      1            /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java   1500      manual mode
      2            /usr/lib/jvm/java-9-openjdk-amd64/bin/java       1500      manual mode
   

3. Add JAVA_HOME variable:

    export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
   

4. check java version:

    $ java -version
    openjdk version "1.8.0_151"
    OpenJDK Runtime Environment (build 1.8.0_151-8u151-b12-0ubuntu0.17.10.2-b12)
    OpenJDK 64-Bit Server VM (build 25.151-b12, mixed mode)
   

Same procedure for Oracle java.

You can manipulate your own PATH. The usual suggestion is a JAVA_HOME environment variable. Add the lines to your .bashrc to keep through logout / reboot.

export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Have you checked this answer ? How to set default Java version?

It looks like it can answer your problem. You can have multiple Java versions running on your system and change between them with:

sudo update-alternatives --config java

Do this to uninstall previous versions of openJDK on Ubuntu

Copysudo apt-get remove openjdk

To remove all of the config files and dependencies.

Copysudo apt-get purge --auto-remove openjdk*

Then install openJDK 11

Copysudo apt-get install openjdk-11-jdk

You can run this command and chose version java:

sudo update-alternatives --config java