As you know, under linux, you can have multiple application versions  (you can have 2 C++ compilers, or 4 java runtimes ) in the same time on your system.

So how can you choose the right Java runtime if you have multiple installation Java applications?

Under Ubuntu / Debian just run update-alternatives command:

update-alternatives --config java

There are 5 alternatives which provide `java'.

Selection    Alternative
-----------------------------------------------
1    /usr/lib/jvm/java-6-sun/jre/bin/java
+        2    /usr/lib/jvm/java-6-openjdk/jre/bin/java
3    /usr/bin/gij-4.1
4    /usr/lib/jvm/java-gcj/jre/bin/java
5    /usr/bin/gij-4.2

Press enter to keep the default[*], or type selection number:

and now just choose what Java runtime do you want to use.

The same is if you want to choose your C compiler or other application, just replace java with gcc or with the name of your application.