using leading zeroes for constants is indicating the octal notation, by which only digits 0..7 are allowed. so if you use leading zeroes for making number pretty print, that is a bad idea as the value it represents is different.
See http://docs.oracle.com/javase/specs/jls/se5.0/html/lexical.html#3.10.1 and watch out for "OctalNumeral".
