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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.