When to use Exceptions?

Question

Should i use Exceptions in my classes like this?
try {
auth.loginUser(userName, userPassword);
System.out.println("User logged in.");
catch (InvalidUserException e1) {
System.out.println("User does not exists.");
catch (PasswordDoesNotMatchException e2) {
System.out.println("Password is incorrect.");
catch (SystemException e3) {
System.err.println("System error.");
// logging to file, etc.
System.out.println("Please, try login later.");
..or maybe i should use Exceptions only for "real" Java errors? (file not found, division by zero, etc.)
Thanks!

Answer 1

Well, the word "exception" says a lot here; you throw
an exception when something exceptional happens, in
other words: something you normally don't expect to
happen. And you offer control returned value? Bad solution, because I can
want not only true/false answer, but and reason why it happen. In your offer I must use String answer = "user OK"; or "answer 200"
This quote is from Thinking in Java:
In C and other earlier languages, there could be several of these
formalities, and they were generally established by convention and not as
part of the programming language. Typically, you returned a special value
or set a flag, and the recipient was supposed to look at the value or the flag
and determine that something was amiss. However, as the years passed, it
was discovered that programmers who use a library tend to think of
themselves as invincible�as in, �Yes, errors might happen to others, but
not in my code.� So, not too surprisingly, they wouldn�t check for the error
conditions (and sometimes the error conditions were too silly to check
for1). If you were thorough enough to check for an error every time you
called a method, your code could turn into an unreadable nightmare.
Because programmers could still coax systems out of these languages they
were resistant to admitting the truth: This approach to handling errors
was a major limitation to creating large, robust, maintainable programs.
We can analize it how Login use case.
Flow: If use enter good login and passwd, when ....
Exception: bad passwd, bad login, ban IP and other.
ciau,
kafka