You are really asking, "How do I display glyphs for unicode characters in Linux?" Unicode is already working for you. Strings in Java use 16 bit unicode characters. Any data processing you do with these unicode Strings will be done flawlessly.

To display all unicode characters, you need to be using a font that has glyphs for the characters you wish to display. I don't know what your linux distribution comes with, but you may have to download fonts. If you have the fonts on your system, you have to make sure that Java is using them. Java gets its font information from the font.properties files. Sun has documentation about editing these files.

Some people that I've talked to have had good luck merging the font.ja.properties into font.properties to get japanese fonts working properly.