The command you are looking for is update-alternatives. So you should be running sudo update-alternatives --config java.

I was recommended to run update-alternatives --auto php or update-alternatives --config php which did not work for me. What worked however was this:

1. I manually deleted the faulty symlink under /etc/alternatives/php
2. I ran sudo update-alternatives --install /usr/bin/php php /usr/bin/php8.0 1 which, in contrast to the other commands, created a new and working symlink to the corresponding php version.

This restored normal functionality of update-alternatives and made me able to use php from cmd again.

While this can be a problem for many software, this is a specific problem for Java on Ubuntu. Fortunately there is a specific solution, a program, call:

update-java-alternatives

I have just use it the following way:

sudo update-java-alternatives -s java-1.14.0-openjdk-amd64

It have change the alternatives of most of the Java related tools.

More comment on this subject can be found on askubuntu for a similar question. This question is also related, and an answer link to the same program.

You presumably have /usr/local/bin ahead of /usr/bin in your PATH, and since there’s a php binary in /usr/local/bin, your shell uses that. The php alternative configured by update-alternatives ends up having no effect — at least, on anything based on that PATH.

To fix this, in your case, I recommend removing the PHP installation in /usr/local. A simpler approach would be to re-order your PATH, but that might affect other binaries you rely on in /usr/local/bin.

Assuming one has installed a JDK in /opt/java/jdk1.8.0_144 then:

1. Install the alternative for javac

   $ sudo update-alternatives --install /usr/bin/javac javac /opt/java/jdk1.8.0_144/bin/javac 1
   

2. Check / update the alternatives config:

   $ sudo update-alternatives --config javac
   

If there is only a single alternative for javac you will get a message saying so, otherwise select the option for the new JDK.

To check everything is setup correctly then:

$ which javac
/usr/bin/javac

$ ls -l /usr/bin/javac
lrwxrwxrwx 1 root root 23 Sep  4 17:10 /usr/bin/javac -> /etc/alternatives/javac

$ ls -l /etc/alternatives/javac
lrwxrwxrwx 1 root root 32 Sep  4 17:10 /etc/alternatives/javac -> /opt/java/jdk1.8.0_144/bin/javac

And finally

$ javac -version
javac 1.8.0_144

Repeat for java, keytool, jar, etc as needed.