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Install JDK 9 in MacOS using Homebrew:

brew update
brew cask install java

All Java version get installed here: /Library/Java/JavaVirtualMachines lets take a look.
ls -la /Library/Java/JavaVirtualMachinesList installed JDKs in your OSX:

$/usr/libexec/java_home -V

You should get result like this:

Matching Java Virtual Machines (5):
 9.0.1, x86_64: "Java SE 9.0.1" /Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home
 1.8.0_121, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home
 1.7.0_80, x86_64: "Java SE 7" /Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home
 1.6.0_65-b14-468, x86_64: "Java SE 6" /Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
 1.6.0_65-b14-468, i386: "Java SE 6" /Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/Library/Java/JavaVirtualMachines/jdk-9.0.1.jdk/Contents/Home</pre>

Install jenv: brew install jenv

Set JDK by following jenv commands: http://www.jenv.be/

Note:Tested on macOS Sierra.