Manually switching system-default version without 3rd party tools:
As detailed in this older answer, on macOS /usr/bin/java is a wrapper tool that will use Java version pointed by JAVA_HOME or if that variable is not set will look for Java installations under /Library/Java/JavaVirtualMachines/ and will use the one with highest version. It determines versions by looking at Contents/Info.plist under each package.
Armed with this knowledge you can:
Info.plist in versions you don't want to use as default (that file is not used by the actual Java runtime itself).$JAVA_HOMEI've just verified this is still true with OpenJDK & Mojave.
On a brand new system, there is no Java version installed:
$ java -version
No Java runtime present, requesting install.
Cancel this, download OpenJDK 11 & 12ea on https://jdk.java.net ; install OpenJDK11:
$ cd /Library/Java/JavaVirtualMachines/
$ sudo tar xzf ~/Downloads/openjdk-11.0.1_osx-x64_bin.tar.gz
System java is now 11:
$ java -version
openjdk version "11.0.1" 2018-10-16
[...]
Install OpenJDK12 (early access at the moment):
$ sudo tar xzf ~/Downloads/openjdk-12-ea+17_osx-x64_bin.tar.gz
System java is now 12:
$ java -version
openjdk version "12-ea" 2019-03-19
[...]
Now let's "hide" OpenJDK 12 from system java wrapper:
$ cd jdk-12.jdk/Contents/
$ sudo mv Info.plist Info.plist.disabled
System java is back to 11:
$ java -version
openjdk version "11.0.1" 2018-10-16
[...]
And you can still use version 12 punctually by manually setting JAVA_HOME:
$ export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-12.jdk/Contents/Home
$ java -version
openjdk version "12-ea" 2019-03-19
[...]