It's often required to run Java application with concrete version
of JDK: 1.4, 1.5 or 1.6, even if JRE is backward compatible.

To achieve this, new executables should be created in /usr/bin:

java1.4
java1.5
java1.6
javac1.4
javac1.4
javac1.6

and so on.

These files should be symlinks to

/etc/alternatives/*

Similar names present Python (python2.4, python2.5), although Python
is installed without alternatives.