For anyone running a JDK on Ubuntu and want to upgrade to JDK11, I'd recommend installing via sdkman. SDKMAN is a tool for switching JVMs, removing and upgrading.
SDKMAN is a tool for managing parallel versions of multiple Software Development Kits on most Unix based systems. It provides a convenient Command Line Interface (CLI) and API for installing, switching, removing and listing Candidates.
Install SDKMAN
$ curl -s "https://get.sdkman.io" | bash
$ source "$HOME/.sdkman/bin/sdkman-init.sh"
$ sdk version
Install Java (11.0.3-zulu)
$ sdk install java