Sorting in Java

Hello for this question: http://usaco.org/current/data/sol_prob3_silver_dec20.html can someone give a link or explain how that person sorted the arraylists using that thing? (like the comparator and the arrow)

https://usaco.guide/silver/sorting-custom - Click here

This does not have the way the person in the solution sorted the lists. I know he used the -> syntax for one thing, and that is not used in the module. (java by the way)