Instead of comparing two files byte by byte, you will compare their checksum values. You will calculate a checksum for each file using an algorithm of your choice.
Your program will sort the duplicate lists by the number of files in each list. For example, if four duplicates of a file is detected, then this four files will be printed before a duplicate list of size two.
Testing
java -jar target/Duplicate-File-Finder-1.0.jar abc/
it will print out
b.txt a.txt