What you describe has nothing to do with dates. It's simply string manipulation.
This will do it for the case where it's always XX/YY/ZZZZ. You'll have to tweak the regex if there's flexibility in, say, the number of digits.
String out = in.replaceAll("(\\d{2})/(\\d{2})/\\d{2}(\\d{2})", "$3$2$1");
[url=http://java.sun.com/docs/books/tutorial/extra/regex/index.html]Sun's Regular Expression Tutorial for Java[/url]
[url=http://www.regular-expressions.info/]Regular-Expressions.info[/url]