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.

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.

On Debian based distro you can use two command lines tools to manage java version:

1. update-alternatives

Here is my answer on U&L for the usage of update-alternatives.

2. update-java-alternatives

The galternatives is the graphical setup tool for the update-alternatives cli.

You can do this with sudo update-alternatives :

# Adding a new alternative for "java".
sudo update-alternatives --install /usr/bin/java java /media/mydisk/jdk/bin/java 1

# Setting the new alternative as default for "java".
sudo update-alternatives --config java

You have to do this for other jdk components too. These components are appletviewer, extcheck, idlj, jar, jarsigner, java, javac, javadoc, javah, javap, javaws, jconsole, jdb, jexec, jhat, jinfo, jmap, jps, jrunscript, jsadebugd, jstack, jstat, jstatd, keytool, mozilla-javaplugin.so, native2ascii, orbd, pack200, policytool, rmic, rmid, rmiregistry, schemagen, serialver, servertool, tnameserv, unpack200, wsgen, wsimport, xjc.

For further information, you can see the man pages for update-alternatives : man update-alternatives.

The command update-java-alternatives sets alternatives for java components and it seems to use update-alternatives for this.

EDIT: The answer above targets Java 6.

• For Java 7, you have to add jcmd to the Java 6 list.
• For Java 8, you have to add jcmd, jjs and jdeps to the Java 6 list.
• For Java 9, you have to add jcmd, jjs, jdeps and jshell to the Java 6 list.

This is the correct approach to hand-inserting an alternative in the alternatives system.

Your alternatives tool is a port of the original Debian update-alternatives tool. With that tool, and your port, there is

update-alternatives --display jre_1.8.0_openjdk

which should tell you the "generic name" to use with --install. The vanilla Debian tool also has

update-alternatives --query jre_1.8.0_openjdk

However: On RHEL, Java installations are installed with the --slave option to --install so that the JRE is switched in lockstep with the java program. In fact, there are between 20 and 50 --slave options that the install scripts for the packages set up. Witness the likes of this post-install script template for OpenJDK from AdoptOpenJDK (which isn't even setting up a JRE alternative, notice) for example.

Really this isn't something that you are going to be able to construct, error-free, longhand, to match what you already have. Don't download and install from raw TAR archives. Use the packages.

When you install JDK on Linux, what gets installed depends on the type of package, version and distribution. You can refer to the following links for information about the installation location on linux:

JDK Installation for Linux Platforms - Version 8

JDK Installation for Linux Platforms - Version 7

Once you install JDK, the bin folder containing tools might not get added to the environment variable PATH. Commands typed on the terminal needs to be from the locations specified in the PATH variable. In cases when JDK\bin does not get added, the user would need to configure it manually as mentioned in Installing the JDK Software

alternatives command is being used to create a symbolic link. Here, it is being directed to use the command to add the tools like javac, javaw to /usr/bin which exists in the PATH variable by default.

If( you could execute java -version outside of JDK/bin && not by specifying the complete path && if the version and bundle prints to be as that of package you installed ){ you need not run the alternatives command.}