Inside a Debian 10 VM I have first installed OpenJDK 11, and it was all OK.
Then I have installed Oracle JDK 13 according to instructions posted in Computing for geeks, that means wget-ting the .deb file and install it with dpkg -i.
I have got no errors, but when running update-alternatives --config java I get this answer:
There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java-11-openjdk-amd64/bin/java
Why? How can I switch from one JDK to the other?