preinst is in libgcj-common 1:4.9.3-9ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | #!/bin/sh
set -e
case "$1" in
    install|upgrade)
	if [ -L /usr/share/doc/libgcj-common ]; then
	    rm -f /usr/share/doc/libgcj-common
	fi
	;;
    abort-upgrade)
	;;
    *)
        echo "preinst called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
exit 0
 |