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.');
        }
    }
}

Exactly Module 22's LibraryException and Module 23's InventoryException pattern, applied here — one dedicated exception type, and every validation rule from Lesson 1's design stated as a clear, individually-testable condition.

@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'));
        }
    }
}

This is Module 31's "Testing Exceptions" lesson, applied to real project code the moment it's written — the try/System.assert(false)/catch pattern, one test method per invalid-input scenario.

@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
}

Exactly Module 31's "confirm it does NOT throw" habit — a valid set of inputs deserves its own explicit test, not just an assumption that "the rejection tests passing" implies the acceptance path works too.