If you run which java, you will see that it's linked to /usr/bin/java, which is a symlink to /etc/alternatives/java. To change the default java version, you need to run the update-alternatives command. You will see output similar to this:
$ update-alternatives --config java There are 2 choices for the alternative java (providing /usr/bin/java). Selection Path Priority Status ------------------------------------------------------------ * 0 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 auto mode 1 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 manual mode 2 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode Press <enter> to keep the current choice[*], or type selection number:
Now, since you installed via a tarball and not a PPA (as Oracle Java is not part of the Mint repos), this method for Oracle Java won't work since the java installation is not registered with the system. If you plan on switching between Java versions, I would recommend finding a PPA and installing that way.