This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
No, of course you can’t use || to separate two Strings; it only operates on booleans. (Booleans are un-boxed if you try to use them with ||.)
You would have to use s.startsWith("x") || s.startsWith("y") or similar.
I think you are going to want more “starting with” letters, so you might do well to learn about regular expressions; you could use a regex which permits starting with any of a large selection of characters.