1. Go to
http://slackbuilds.org/repository/14.1/development/jdk/
and download the SlacKBuild for jdk.
2. Once that is saved on your harddrive (assume it is in your home dir), untar it:
tar xvf jdk-7u67-linux-i586.tar.gz
cd jdk-7*gz
3. Bring in the archive file that you have:
mv /path/to/jdk-8u20-linux-i586.tar.gz .
4. Now alter the version that you want to install
sed -i 's/7u67/8u20/g' jdk.SlackBuild
5. And finally, make the SlackBuild executable and execute:
chmod +x jdk.SlackBuild
./jdk.SlackBuild
6. And then install:
su -c "installpkg /tmp/jdk*8u20*t?z"
That is all.