To install OpenJDK, run this Homebrew command:

brew install openjdk@8

Note

You may be directed to create symlinks and/or update your PATH to make OpenJDK visible to your system, for instance:

For the system Java wrappers to find this JDK, symlink it with
  sudo ln -sfn /usr/local/opt/openjdk@8/libexec/openjdk.jdk /Library/Java/JavaVirtualMachines/openjdk-8.jdk

openjdk@8 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.

If you need to have openjdk@8 first in your PATH, run:
  echo 'export PATH="/usr/local/opt/openjdk@8/bin:$PATH"' >> ~/.zshrc

For compilers to find openjdk@8 you may need to set:
  export CPPFLAGS="-I/usr/local/opt/openjdk@8/include"