I would use
String.valueOf(...)
You can use the same code for all types, but without the hideous and pointless string concatenation.
Note that it also says exactly what you want – the string value corresponding to the given primitive value. Compare that with the "" + x approach, where you’re applying string concatenation even though you have no intention of concatenating anything, and the empty string is irrelevant to you. (It’s probably more expensive, but it’s the readability hit that I mind more than performance.)