Well, NumberFormatException means the string's not a number. Anything can have a string representation so it's not clear what you mean. You can have an empty string or a null string or a non-string in terms of a class other than string. Certainly there is NullPointerException (I guess
Java does have pointers) and ClassCastException and IllegalArgumentException (of which NumberFormatException is a family member) but that's about it.