You simply can not, and most likely never will be able to. The reason being that the Wii runs a proprietary operating system, and as such programs must be written specifically for that operating system. It is not likely Java will be re-written to run on the Wii without being commissioned to do so. The Opera web browser only came to the Wii because they were commissioned to write a version of it to run on Wii. The only way we will see Java (and better Flash support for that matter) for Wii is if the Opera web browser is updated, which seems unlikely, though there have been rumors of updates of Opera.