Should this method throw an exception

Question

Hi,
I have a method in a Database interface implementation class.
updateField( String fieldname, String value ) throws InvalidFieldException { 
   if ( ! is_Valid ( fieldname, value  ) {
     throw  new InvalidFieldException();
}This will check the value does not exceed a certain length and it's format is correct, else it won't apply the update and should let the user know to correct the format of the value string.
is it good practice to have the method throw an InvalidFieldException() to return control to the calling method or how would you deal with an invalid field.
I know/ have heard that exceptions are for "Unexpected" things and a user entering data in the wrong format would not be classed as "unexpected" so by this definition using Exceptions is not the way to go.
Any ideas?

Answer 1

This will check the value does not exceed a certain
length and it's format is correct, else it won't
apply the update and should let the user know to
correct the format of the value string.
is it good practice to have the method throw an
InvalidFieldException() to return control to the
calling method or how would you deal with an invalid
field.There are two strategies, both work. Just don't mix them:
- using contracts: clearly state that your method requires a certain range of values as an argument, and that everything outside this range will probably cause a problem. It's putting the responsibility of checking the argument's correctness to the caller, who anyway should know best how to provide the values and how to deal with wrong ones.
- defensive programming: take everything they give you and check yourself - throwing an IllegalArgumentException. Might be more secure, but far more work. I advise only to use this way on public methods, especially if you're going to distribute your classes as a library for others to use.
I know/ have heard that exceptions are for
"Unexpected" things and a user entering data in the
wrong format would not be classed as "unexpected" so
by this definition using Exceptions is not the way to
go.You're right, but there's a limit to what your method can do to fix the problem. If it can't do that by itself you need an escalation mechanism. Hence you can use an exception.