[java] How to install JDK 11 under Ubuntu?

First check the default-jdk package, good chance it already provide you an OpenJDK >= 11.
ref: https://packages.ubuntu.com/search?keywords=default-jdk&searchon=names&suite=all&section=all

Ubuntu 18.04 LTS +

So starting from Ubuntu 18.04 LTS it should be ok.

sudo apt update -qq
sudo apt install -yq default-jdk

note: don't forget to set JAVA_HOME

export JAVA_HOME=/usr/lib/jvm/default-java
mvn -version

Ubuntu 16.04 LTS

For Ubuntu 16.04 LTS, only openjdk-8-jdk is provided in the official repos so you need to find it in a ppa:

sudo add-apt-repository -y ppa:openjdk-r/ppa
sudo apt update -qq
sudo apt install -yq openjdk-11-jdk

note: don't forget to set JAVA_HOME

export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
mvn -version