This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I'm not familiar with that particular class, but MD5 is not an encoding - it is cryptographic hash which cannot be reversed. If you want to compare it to a newly entered password, then you need to run that password through the Md5PasswordEncoder as well, and compare the results.