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.

Probably you have made a change in your system that prevent update-alternatives to work.

Expected behavior

As I know, in a typical installation, the java command is a symbolic link in /usr/bin/java. You must exclude the "real" java path from the $PATH variable. If your computer use a java command in a different directory, the update-alternatives will not work.

-- location of the java command
$ type -p java
/usr/bin/java

-- real location of the java command
$ readlink -f /usr/bin/java
/usr/lib/jvm/java-11-openjdk-amd64/bin/java 

If you get a different result for the type -p java command, you must check if the location of the java command is determined by some folder in the $PATH and remove that part.

NOTE: You may define the JAVA_HOME variable processing the /usr/bin/java symbolic link.

-- real location of the JAVA_PATH variable
$ dirname /usr/lib/jvm/java-11-openjdk-amd64/bin/java 
/usr/lib/jvm/java-11-openjdk-amd64/bin/

$ dirname /usr/lib/jvm/java-11-openjdk-amd64/bin/
/usr/lib/jvm/java-11-openjdk-amd64/

-- setting the JAVA_HOME variable
$ export JAVA_HOME=`type -p java|xargs readlink -f|xargs dirname|xargs dirname`

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

For Oracle Java 6u30, once you've installed their RPMs you can configure alternatives:

Copy/usr/sbin/alternatives --install "/usr/bin/java" "java" "/usr/java/default/bin/java" 2 \
--slave /usr/bin/javac javac /usr/java/default/bin/javac \
--slave /usr/bin/javadoc javadoc /usr/java/default/bin/javadoc \
--slave /usr/bin/jar jar /usr/java/default/bin/jar \
--slave /usr/bin/keytool keytool /usr/java/default/bin/keytool \
--slave /usr/bin/orbd orbd /usr/java/default/bin/orbd \
--slave /usr/bin/pack200 pack200 /usr/java/default/bin/pack200 \
--slave /usr/bin/rmid rmid /usr/java/default/bin/rmid \
--slave /usr/bin/rmiregistry rmiregistry /usr/java/default/bin/rmiregistry \
--slave /usr/bin/servertool servertool /usr/java/default/bin/servertool \
--slave /usr/bin/tnameserv tnameserv /usr/java/default/bin/tnameserv \
--slave /usr/bin/unpack200 unpack200 /usr/java/default/bin/unpack200 \
--slave /usr/share/man/man1/java.1.gz java.1.gz /usr/java/default/man/man1/java.1.gz \
--slave /usr/share/man/man1/keytool.1.gz keytool.1.gz /usr/java/default/man/man1/keytool.1.gz \
--slave /usr/share/man/man1/orbd.1.gz orbd.1.gz /usr/java/default/man/man1/orbd.1.gz \
--slave /usr/share/man/man1/pack200.1.gz pack200.1.gz /usr/java/default/man/man1/pack200.1.gz \
--slave /usr/share/man/man1/rmid.1.gz rmid.1.gz /usr/java/default/man/man1/rmid.1.gz \
--slave /usr/share/man/man1/rmiregistry.1.gz rmiregistry.1.gz /usr/java/default/man/man1/rmiregistry.1.gz \
--slave /usr/share/man/man1/servertool.1.gz servertool.1.gz /usr/java/default/man/man1/servertool.1.gz \
--slave /usr/share/man/man1/tnameserv.1.gz tnameserv.1.gz /usr/java/default/man/man1/tnameserv.1.gz \
--slave /usr/share/man/man1/unpack200.1.gz unpack200.1.gz /usr/java/default/man/man1/unpack200.1.gz

Then activate the configuration:

Copy/usr/sbin/alternatives --config java

And select /usr/java/default/bin/java from the menu.

Plus you must gzip the man pages

Copygzip /usr/java/default/man/man1/*.1

Also, the Oracle java RPMs might have clobbered your alternatives symlink so force it to be normal.

Copyln -sf /etc/alternatives/java /usr/bin/java

if you need to install java in Ubuntu, you can try using following command and you will get oracle-java-installer in your synaptic package manager, click it and install it.

sudo add-apt-repository ppa:webupd8team/java

Else try following steps

 $ wget http://download.oracle.com/otn-pub/java/jdk/6u34-b04/jre-6u34-linux-i586.bin
 $ chmod u+x jre-6u34-linux-i586.bin
 $ ./jre-6u34-linux-i586.bin
 $ sudo mkdir -p /usr/lib/jvm
 $ sudo mv jre1.6.0_34 /usr/lib/jvm/
 $ sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jre1.6.0_34/bin/java" 1
 $ sudo update-alternatives --install "/usr/lib/mozilla/plugins/libjavaplugin.so" "mozilla-javaplugin.so" "/usr/lib/jvm/jre1.6.0_34/lib/i386/libnpjp2.so" 1
 $ sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/jre1.6.0_34/bin/javaws" 1

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.

The way update-alternatives works is described in its manpage. The list of available alternatives for a given command (or file, in general — alternatives aren’t limited to commands) is stored in a file in /var/lib/dpkg/alternatives on Debian and derivatives; thus the alternatives for java are stored in /var/lib/dpkg/alternatives/java. On Fedora, RHEL, and derivatives, the files are stored in /var/lib/alternatives.

When a package wishes to provide an alternative, it installs it using update-alternatives --install (and appropriate parameters); when it wishes to remove an alternative, it does so using update-alternatives --remove. You can use these to provide your own alternatives if necessary.

Note that in Java’s case specifically, on Debian and derivatives, the alternatives handling is a little more complex and you should use update-java-alternatives instead of manipulating all the alternatives manually.

If for some reason an alternative-managed file is not a symlink, update-alternatives will consider that the alternative is broken and will refuse to touch it.

Those errors are harmless because you're using a JRE; the missing commands are provided by JDKs only. After running update-java-alternatives -s jre-8-oracle-x64, java -version will confirm you're using the Oracle JRE by default:

$ java -version
java version "1.8.0_102"
Java(TM) SE Runtime Environment (build 1.8.0_102-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)

sudo update-alternatives --config java

Configures the default for the program "java". That's the Java VM.

sudo update-alternatives --config javac

Configures the default Java compiler.

You can also see that, because the first command lists a lot of "JRE" (Java Runtime Environment) folders and the Program is just called "java".

If I check which version is being used by issuing the command java -version or javac -version, I can see, that each command changes the program being used.

However, using update-java-alternatives with a JDK Version changes both programs for me. Using the first commands, you can use a Java VM and Java Compiler from different JDKs.

update-java-alternatives requires presence of a file with extension .jinfo in directory /usr/lib/jvm. The openjdk package is shipped with a .jinfo file, the jdk of Oracle (formerly Sun) is not. As alternative, you configure alternatives without update-java-alternatives:

For example, to add java from jvm-directory /usr/lib/jvm/jdk-12.0.1 (default directory of Debian package of Oracle) with priority 2082, use the following command:

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk-12.0.1/bin/java 2082

As for switching for different development environments:

Are you talking about starting the IDE itself with different Java versions or using different versions in the IDE for compilation and running your app?

• For 1.: You can specify which JVM to use in the eclipse.ini, as described here. I don't know how to do that for the Arduino IDE.

• For 2.: In Eclipse you can select the JRE/JDK to be used in Window -> Preferences -> Java -> Installed JREs. And under Java -> Compiler you could choose an older Java compliance if you wish.

EDIT: This DigitalOcean page also has a very nice explanation of everything related to Java on Ubuntu.

To fix it simply run the following commands

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk-11.0.1/bin/java 3
sudo update-alternatives --config java

The first correct the errors you made where you had /usr/lib/jvm it should have been /usr/bin/java. The second command gives you the opportunity to pick the default version you want.

Try

sudo update-alternatives --install /usr/bin/java java  path_to_java 1

In my case the path was /usr/lib/jvm/java-8-openjdk-amd64

If you copy-pasted the command from somewhere, you probably got a Unicode character instead of an ASCII one (em-dash instead of -, or non-breaking space instead of space, etc.). If it were an em-dash or similar, however, it wouldn't be seen as an option argument (instead it's seen as an unrecognized option). Since --install /usr/bin/java is presented as one argument in the error, the space in between is probably not an ASCII space, but a Unicode one.