You can do this with sudo update-alternatives :
# Adding a new alternative for "java".
sudo update-alternatives --install /usr/bin/java java /media/mydisk/jdk/bin/java 1
# Setting the new alternative as default for "java".
sudo update-alternatives --config java
You have to do this for other jdk components too. These components are appletviewer, extcheck, idlj, jar, jarsigner, java, javac, javadoc, javah, javap, javaws, jconsole, jdb, jexec, jhat, jinfo, jmap, jps, jrunscript, jsadebugd, jstack, jstat, jstatd, keytool, mozilla-javaplugin.so, native2ascii, orbd, pack200, policytool, rmic, rmid, rmiregistry, schemagen, serialver, servertool, tnameserv, unpack200, wsgen, wsimport, xjc.
For further information, you can see the man pages for update-alternatives : man update-alternatives.
The command update-java-alternatives sets alternatives for java components and it seems to use update-alternatives for this.
EDIT: The answer above targets Java 6.
- For Java 7, you have to add
jcmd to the Java 6 list.
- For Java 8, you have to add
jcmd, jjs and jdeps to the Java 6 list.
- For Java 9, you have to add
jcmd, jjs, jdeps and jshell to the Java 6 list.