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.
Your example Java code is exactly right; the devil is in the details, though, so perhaps some detail is wrong in your real code. Maybe it's a package thing? Make sure the package containing the interface is imported when you compile your other code (assuming everything's not in one package.)