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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
The Instructions say "You have to write a comparator function" and the default (Java) solution clearly says
implements Comparator
This link should help: http://lmgtfy.com/?q=Java+Comparator
Which language you using:
Java
?