Depending on your distribution it may be easiest to install from your package manager. On Ubuntu, for example, you can do:

Copysudo aptitude install open-jdk

Otherwise, usually the sun (oracle) version of java comes with a .bin file which you need to make executable and run as root in order to get java properly installed.

You shouldn't need to set any environment variables to get java to work, although some applications will require you to set JAVA_HOME, JDK_HOME and/or JRE_HOME to point to the java install's bin directory. You can see where your java install is by looking at where the symlink from

Copy/etc/alternatives/java

points.

The name of the control panel is different depending on whether you are using the Oracle JDK or OpenJDK.

Search for "control panel" in the dash. If you're using the Oracle JDK, you'll see the Oracle Java 7 Plugin Control Panel. If you're using OpenJDK, you'll see the Iced Tea Web Control Panel.

You know how to set the variable in a shell, but for the record you can write:

export _JAVA_OPTIONS='-Dawt.useSystemAAFontSettings=on'

and all programs you start from this shell session after that will have the variable set.

If you want it to be set for every shell you start afterwards, add that line to ~/.profile as well. In that case it will apply to all future shells you start, but not any that are currently running.

.profile will generally work for the GUI as well, but that can be broken by system configuration and how you start things up. This is per-user configuration only.

If you want it set for every user all the time, you can add an assignment to /etc/environment. The format is a little different there: just KEY=VAL on separate lines, with no required quoting and none of anything else.

_JAVA_OPTIONS=-Dawt.useSystemAAFontSettings=on

This is parsed by the pam_env module. There is a per-user ~/.pam_environment file as well, which has the same effect for just the one user. These both require logging out and back in for the change to take effect. The variables will be set for every future login session, both at the console and in X.

Similarly, you can make a file in /etc/profile.d with an export statement in it and it will be loaded into every future session by any user. There will likely be some pre-existing files there to model it on, but just the export line above will be fine.

Alternatively, you can add the export statement in ~/.xinitrc (if you use startx), ~/.xsession, or ~/.xprofile. KDE also supports a directory ~/.kde/env that can contain as many shell files as you want, which contain export statements as above. I would probably prefer one of the other approaches.

Feel free to use this as a reference to tinkering with Java at runtime.

Choosing your JRE

To choose your JRE, use

sudo update-alternatives --config java

This will give something like the following output.

  Selection    Path                                            Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/java-6-openjdk/jre/bin/java         1061      auto mode
* 1            /usr/lib/jvm/java-6-openjdk/jre/bin/java         1061      manual mode
  2            /usr/lib/jvm/java-6-sun/jre/bin/java             63        manual mode
  3            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1051      manual mode

You can then select which Java runtime you want through the number selection.

Choosing your JVM

Sun/Oracle have two JVM choices, -server and -client. If you select the OpenJDK as your Java runtime environment you have additional options.

When you type java into the terminal with no other parameters, the help lists several optional VMs. I'm not sure which ones come with OpenJDK but 3 popular ones are JamVM, Zero and Cacao

To use these, simply type

java -jamvm 'your other parameters here'
java -cacao 'your other parameters here'
java -zero 'your other parameters here'
java -server 'your other parameters here

The -server VM is normally the default. You can also specify -client but in 64-bit IcedTea6 it appears to run the same version as -server. There are most likely others but I find the default option to be the most responsive.

Setting your Memory

Finally, how to set the memory of Java (just because)

java -Xmx1024m -Xms128m 'your other parameters here'

This limits the memory allowed for the Java program to a maximum of 1024 MB, and sets its initial memory size to 128 MB. This is a great way of defining minimum system requirements. The Java 6 man page for the java command describes these options and others.

That's all. If anyone has additional Java tweaks for Ubuntu then leave them in the comments and I'll add them.

1. find /usr/lib/jvm/java-1.x.x-openjdk

2. vim /etc/profile

   Prepend sudo if logged in as not-privileged user, ie. sudo vim

3. Press 'i' to get in insert mode

4. add:

   export JAVA_HOME="path that you found"
   
   export PATH=$JAVA_HOME/bin:$PATH
   

5. logout and login again, reboot, or use source /etc/profile to apply changes immediately in your current shell

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

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/