In general, you can't do this. UTF-8 is capable of encoding any Unicode code point. ISO-8859-1 can handle only a tiny fraction of them. So, transcoding from ISO-8859-1 to UTF-8 is no problem. Going backwards from UTF-8 to ISO-8859-1 will cause "replacement characters" (�) to appear in your text when unsupported characters are found.
To transcode text:
byte[] latin1 = ...
byte[] utf8 = new String(latin1, "ISO-8859-1").getBytes("UTF-8");
or
byte[] utf8 = ...
byte[] latin1 = new String(utf8, "UTF-8").getBytes("ISO-8859-1");
You can exercise more control by using the lower-level Charset
APIs. For example, you can raise an exception when an un-encodable character is found, or use a different character for replacement text.