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

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

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.

The rpm itself has to be created to use alternatives and the rpm from Oracle doesn't use alternatives. See the second answer here for a discussion on how to set up alternatives for Oracle's rpm.

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`

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)

I was trying to set the java path while installation of Glassfish on Ubuntu 14.04.

The command goes like in CentOS as

" alternatives --install /usr/bin/java java /opt/java/jdk/bin/java 1"

“alternatives --config java”

Well, the problem is that in Ubuntu the “alternative” command does not work.

So, what can be done regarding this ?

See 'air-dex' example on this post: https://askubuntu.com/questions/159575/how-do-i-make-java-default-to-a-manually-installed-jre-jdk . Note: sometimes people install Java JDK's into '/usr/lib/jvm', and as you can see in the output on my blog post, the system came with default JVMs in this location, but personally I don't recommend that for a JDK. Maybe I would choose '/usr/lib/jvm' if it was a JRE I was installing. If you wanted an alternative location for JDKs and JREs, I would choose either:

   1. /usr/java/
   2. /opt/java/

If I were comparing to windows, I might say:

JDKs: /usr/java = C:\Java
JREs: /usr/lib/jvm = C:\Program Files\Java
Alt:  /opt/java = E:\Java

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.