Enforce Validation Rules

public class LoanValidationException extends Exception {}

public class LoanValidationService {
    public void validate(Decimal requestedAmount, Decimal annualIncome, Decimal creditScore) {
        if (requestedAmount == null || requestedAmount <= 0) {
            throw new LoanValidationException('Requested amount must be positive.');
        }
        if (annualIncome == null || annualIncome <= 0) {
            throw new LoanValidationException('Annual income must be positive.');
        }
        if (creditScore == null || creditScore < 300 || creditScore > 850) {
            throw new LoanValidationException('Credit score must be between 300 and 850.');
        }
    }
}

1. public class LoanValidationException extends Exception {} — exactly Module 22's LibraryException and Module 23's InventoryException pattern — one dedicated exception type.
2. (blank line)
3. public class LoanValidationService { — the validation service.
4. public void validate(Decimal requestedAmount, Decimal annualIncome, Decimal creditScore) { — the method.
5. if (requestedAmount == null || requestedAmount <= 0) { — the first rule, stated as a clear, individually-testable condition.
6. throw new LoanValidationException('Requested amount must be positive.'); — rejects with a specific message.
7. } — closes the if.
8. if (annualIncome == null || annualIncome <= 0) { — the second rule.
9. throw new LoanValidationException('Annual income must be positive.'); — rejects with its own message.
10. } — closes the if.
11. if (creditScore == null || creditScore < 300 || creditScore > 850) { — the third rule.
12. throw new LoanValidationException('Credit score must be between 300 and 850.'); — rejects with its own message.
13. } — closes the if.
14. } — closes the method.
15. } — closes the class.

@isTest
private class LoanValidationServiceTest {
    @isTest
    static void negativeAmountThrows() {
        LoanValidationService validator = new LoanValidationService();
        try {
            validator.validate(-1000, 50000, 700);
            System.assert(false, 'Expected a LoanValidationException');
        } catch (LoanValidationException e) {
            System.assert(e.getMessage().contains('amount'));
        }
    }

    @isTest
    static void creditScoreAbove850Throws() {
        LoanValidationService validator = new LoanValidationService();
        try {
            validator.validate(10000, 50000, 900);
            System.assert(false, 'Expected a LoanValidationException');
        } catch (LoanValidationException e) {
            System.assert(e.getMessage().contains('Credit score'));
        }
    }
}

1. @isTest — marks the class.
2. private class LoanValidationServiceTest { — the class.
3. @isTest — marks the first test.
4. static void negativeAmountThrows() { — this is Module 31's "Testing Exceptions" lesson, applied to real project code the moment it's written.
5. LoanValidationService validator = new LoanValidationService(); — creates the validator.
6. try { — wraps the call expected to throw.
7. validator.validate(-1000, 50000, 700); — an invalid, negative amount.
8. System.assert(false, 'Expected a LoanValidationException'); — reached only if it unexpectedly didn't throw.
9. } catch (LoanValidationException e) { — catches the expected failure.
10. System.assert(e.getMessage().contains('amount')); — verifies it's the right error.
11. } — closes the try/catch.
12. } — closes this test.
13. (blank line)
14. @isTest — marks the second test.
15. static void creditScoreAbove850Throws() { — a different invalid-input scenario.
16. LoanValidationService validator = new LoanValidationService(); — creates the validator.
17. try { — wraps the call.
18. validator.validate(10000, 50000, 900); — an out-of-range credit score.
19. System.assert(false, 'Expected a LoanValidationException'); — reached only if it didn't throw.
20. } catch (LoanValidationException e) { — catches it.
21. System.assert(e.getMessage().contains('Credit score')); — verifies the right error.
22. } — closes the try/catch.
23. } — closes this test.
24. } — closes the class.

@isTest
static void validInputsDoNotThrow() {
    LoanValidationService validator = new LoanValidationService();
    validator.validate(10000, 50000, 700); // should complete without throwing
    System.assert(true); // reaching this line confirms no exception was thrown
}

1. @isTest — marks the test method.
2. static void validInputsDoNotThrow() { — exactly Module 31's "confirm it does NOT throw" habit — a valid set of inputs deserves its own explicit test.
3. LoanValidationService validator = new LoanValidationService(); — creates the validator.
4. validator.validate(10000, 50000, 700); — should complete without throwing.
5. System.assert(true); — reaching this line confirms no exception was thrown, rather than just assuming "the rejection tests passing" implies the acceptance path works too.
6. } — closes the test method.