Where can I find the Java SDK in Linux after installing it?

This depends a bit from your package system ... if the java command works, you can type readlink -f $(which java) to find the location of the java command. On the OpenSUSE system I'm on now it returns /usr/lib64/jvm/java-1.6.0-openjdk-1.6.0/jre/bin/java (but this is not a system which uses apt-get).

On Ubuntu, it looks like it is in /usr/lib/jvm/java-6-openjdk/ for OpenJDK, and in some other subdirectory of /usr/lib/jvm/ for Suns JDK (and other implementations as well, I think).

Debian is the same.

For any given package you can determine what files it installs and where it installs them by querying dpkg. For example for the package 'openjdk-6-jdk': dpkg -L openjdk-6-jdk

update-java-alternatives -l

will tell you which java implementation is the default for your system and where in the filesystem it is installed. Check the manual for more options.

$ which java 

should give you something like

/usr/bin/java

This question will get moved but you can do the following

which javac

or

cd /
find . -name 'javac'