Exceptions. Practical Tasks

Make WrongPasswordException a subclass of WrongLoginException, and the line catch (WrongLoginException | WrongPasswordException e) stops compiling: Alternatives in a multi-catch statement cannot be related by subclassing. The fix is to extend both classes from Exception directly — and details like this show up in your own solutions on this page, not just in the theory.

These Java exceptions exercises practice the module «Exceptions»: you will write custom exception classes, throw them with the throw statement, catch them in one multi-catch block, and then use the login and password check for user registration in the Online Shop project (part 7). Both tasks have hints, and the section after the tasks lists the bugs that most often break a working-looking solution.

What to Review Before You Start

Both tasks need three skills: declaring your own exception class, throwing it with throw, and catching it in try-catch. If any of them feels shaky, open the matching lesson section from the table first — each rule is explained there with a runnable example.

Login and Password Validation

1. Create a static method that takes three parameters: login, password and confirmPassword.
2. login must contain only Latin letters, digits and the underscore.
3. The length of login must be less than 20 characters (1 to 19; an empty string is invalid). If login does not meet these requirements, throw WrongLoginException.
4. password must contain only Latin letters, digits and the underscore, and its length must be less than 20 characters. password and confirmPassword must also be equal. If password does not meet these requirements, throw WrongPasswordException.
5. WrongPasswordException and WrongLoginException are custom exception classes with two constructors: a default one, and one that takes the exception message and passes it to the constructor of Exception.
6. Handle the exceptions inside the method.
7. Use a multi-catch block.
8. The method returns true if the values are valid, and false otherwise.

Registration and Login Error Handling

Online Store · part 7 of 7 · ← part 6

Implement authentication and registration of an online shop user:

1. Store registered users in a Map of login–password pairs.
2. If the user enters a login and password that are in the map, the user is authenticated.
3. If the login is in the map but the password does not match, report a login error. Do not change the existing entry.
4. If the login is not in the map, ask for the password once more (confirmPassword) and validate the data with the method from the task «Login and Password Validation».
5. If the validation passes, add the login and password to the map — the user is registered.

Where Solutions Break

Most solutions to these tasks fail not on exceptions but on string checks: find() instead of matches() lets invalid characters through, \w rejects letters such as é, and == compares string references instead of contents. Here is a short snippet with its real output:

Pattern p = Pattern.compile("[A-Za-z0-9_]+");
System.out.println(p.matcher("admin!").find());    // true  - found the substring "admin"
System.out.println(p.matcher("admin!").matches()); // false - the whole string is checked
System.out.println("admin_1".matches("\\w+"));     // true
System.out.println("josé".matches("\\w+"));        // false - \w is only [a-zA-Z_0-9] by default
System.out.println("".matches("[A-Za-z0-9_]+"));   // false - the + quantifier needs at least one character
System.out.println(new String("qwerty") == "qwerty"); // false - two different objects