Using homebrew It's super easy to install any JDK version from the adoptopenjdk builds:

Andreas-Mac-mini:installer a.leofreddi$ brew tap AdoptOpenJDK/openjdk
==> Tapping adoptopenjdk/openjdk
Cloning into '/Users/a.leofreddi/Applications/homebrew/Library/Taps/adoptopenjdk/homebrew-openjdk'...
remote: Enumerating objects: 24, done.
remote: Counting objects: 100% (24/24), done.
remote: Compressing objects: 100% (19/19), done.
remote: Total 24 (delta 12), reused 10 (delta 4), pack-reused 0
Unpacking objects: 100% (24/24), done.
Tapped 14 casks (67 files, 84.9KB).
Andreas-Mac-mini:installer a.leofreddi$ brew search adoptopenjdk
==> Casks
adoptopenjdk ✔                adoptopenjdk11                adoptopenjdk11-openj9         adoptopenjdk12                adoptopenjdk12-openj9         adoptopenjdk8 ✔               adoptopenjdk8-openj9          adoptopenjdk9 ✔
adoptopenjdk10                adoptopenjdk11-jre            adoptopenjdk11-openj9-jre     adoptopenjdk12-jre            adoptopenjdk12-openj9-jre     adoptopenjdk8-jre             adoptopenjdk8-openj9-jre
Andreas-Mac-mini:installer a.leofreddi$ brew cask install adoptopenjdk9

Then one can easily switch version using java_home:

export JAVA_HOME=`/usr/libexec/java_home -v 9`