Generating and opening file with Automator
Add open "${1%.*}".jad on a new line at the end of the script. If you want to open it in a specific application (not the default), change it to open -a "Some Application" "${1%.*}".jad

The $1 is the first argument passed to a script, which in this case is the file path. the ${1%.*} strips the extension (i.e. /Users/you/MyClass.class becomes /Users/you/MyClass), and then .jad is appended, so you get the full path of the new file.