jenv

set jdk

mkdir -p $HOME/.jenv/candidates/java
ln -s /Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home $HOME/.jenv/candidates/java/9

jenv default java 9
Default java version set to current

java -version
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)
jenv list
jenv default java 1.8.0_144

ref

What Else?