this is line one
this is line two
...
..
5

Lines of code, descriptive of size/complexity. In this case, pretty simple and easy. I was assuming it would be far more than that in Java.

In case your interested, here it is.

#!/usr/bin/env bash
mkdir thumbs

printf '%s\n' $(ls *png *jpg *gif ) > file
convert @file -resize 300x300 thumbs/thumb%03d.png

rm file

Java or such probably easiest.

5 lines with imagemagick in a bash script.