The first thing you'd want to implement is perft: https://www.chessprogramming.org/Perft_Results It actually doubles as a performance test(like it's name) and a debugging tool to verify that you can generate moves correctly by comparing your numbers vs the positions given on the website and a few large suites.

I'd encourage you to get involved with the passionate community at talkchess: http://talkchess.com/forum3/index.php

And join a small group on IRC at Freenode ##chessprogramming where we discuss chess AIs, libraries and often other games like Ataxx and Go.

I myself am an author of a chess library: https://github.com/Mk-Chan/libchess ! Great to see development for Java as well