JDK

To install the oracle jdk, simply download the tarball from the oracle website and unpack it to /opt/jdk/jdkXXXX.

Then create a symbolic to it named e.g. /opt/jdk/current.

(This way you don't have to update the links in /usr/bin everytime you upgrade. All you have to do is upgrading the symlink in /opt.)

Finally create symlinks to the executables java, javac and jar in the $jdk/bin directory. On Debian, you can do this using update-alternatives:

JDK_INSTALL_PATH=/opt/jdk/current
for CMD in {java,javac,jar,javaws}; do update-alternatives --install /usr/bin/$CMD $CMD $JDK_INSTALL_PATH/bin/$CMD 1; done