How to set default Java version?

Re your first question:

possibly you may be confusing that the webupd8 script is 0.5b. That is the version of the script - it doesnt refer to the java version.

Further to the setting of the javac version.

I suspect you need to explicitly give the path of the javac compiler

i.e.

sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/java-6.31-oracle/bin/javac" 1

followed by:

sudo update-alternatives --config javac

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With regards to setting up the java chrome plugin.

The master question:

How do I install Oracle JDK 6?

includes this information - since your folder structure is slightly different your link command should be:

ln -s /usr/lib/jvm/java-6.31-oracle/jre/lib/i386/libnpjp2.so ~/.mozilla/plugins/

See this; run

sudo  update-java-alternatives --list

to list off all the Java installations on a machine by name and directory, and then run

sudo  update-java-alternatives --set [JDK/JRE name e.g. java-8-oracle]

to choose which JRE/JDK to use.

If you want to use different JDKs/JREs for each Java task, you can run update-alternatives to configure one java executable at a time; you can run

sudo  update-alternatives --config java[Tab]

to see the Java commands that can be configured (java, javac, javah, javaws, etc). And then

sudo  update-alternatives --config [javac|java|javadoc|etc.]

will associate that Java task/command to a particular JDK/JRE.

You may also need to set JAVA_HOME for some applications: from this answer you can use

export JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:bin/java::")

for JREs, or

export JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:jre/bin/java::")

for JDKs.

I think you should take a look at the update-java-alternatives command from the java-common package and the *.jinfo files used by it. With that you should be able to switch completely between java installations (regardless of JDK, JRE, ...).