Although Java 11 is now available in .deb format (yay!) the packaging is incomplete as it is not integrated with Debian's update-alternatives mechanism. An example of how to do this can be found in the Debian java-package package. The .deb should probably also interwork with the Debian update-java-alternatives script as well.
https://wiki.debian.org/Teams/JavaPackaging
https://salsa.debian.org/java-team/java-package
https://wiki.debian.org/Teams/JavaPackaging
https://salsa.debian.org/java-team/java-package