Three Step Process:
First:
open Terminal->$ whereis java
it would give output like this:
java: /usr/bin/java /usr/share/java /usr/share/man/man1/java.1.gz
Second:
ls -l /usr/bin/java
It would give output like this:
lrwxrwxrwx 1 root root 22 Feb 9 10:59 /usr/bin/java -> /etc/alternatives/java
Third:
ls -l /etc/alternatives/java
output is the JDK path:
lrwxrwxrwx 1 root root 46 Feb 9 10:59 /etc/alternatives/java -> /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java