This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
With an unbound T, you can use anything for T. That includes classes like File and even Object. What would + mean for two File objects, or for two Object methods?
If you bound T to Number then it would be possible for Integer, Short, etc through auto(un)boxing. But what about BigDecimal or BigInteger? They don't support auto(un)boxing. And what about any custom Number subclass?