Want to know what JDK versions are installed and available on your machine? There is a command for that.
$ /usr/libexec/java_home -V
Matching Java Virtual Machines (3):
9.0.4, x86_64: "Java SE 9.0.4" /Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents/Home
1.8.0_162, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home
1.8.0_161, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_161.jdk/Contents/Home
/Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents/Home
The listed VMs show what JDK versions you have and the final line shows which is currently the default version.