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I guess this is for historical reasons, but in my opinion the compile-time type checking and IDE auto-completion are well worth converting this to enums in Vaadin 7. In the rare cases that it is necessary to associate a specific integer with a value (e.g. for bit masks, or for compatibility), this is perfectly possible using enums.
(note that I haven't looked at Vaadin 7 yet, so maybe this is already done)