Can I Use an iPhone 4 USB to Plug Adapter for any USB Device?

I have an MP3 player. It charges via USB when connected to a computer. This is a nuisance because I don't usually have access to a computer on the go. There's an official outlet adapter for this MP3 player, but it is overpriced. I have a few iPhone USB to Outlet adapters and I want to know if I can safely connect my MP3 to them in order to charge it.

Yes, you can. The voltage is the same.