Test Edge Cases and Exceptions

@isTest
static void creditScoreOf300IsValid() {
    LoanValidationService validator = new LoanValidationService();
    validator.validate(10000, 50000, 300); // should not throw
    System.assert(true);
}

@isTest
static void creditScoreOf299IsInvalid() {
    LoanValidationService validator = new LoanValidationService();
    try {
        validator.validate(10000, 50000, 299);
        System.assert(false, 'Expected a LoanValidationException');
    } catch (LoanValidationException e) {
        System.assert(e.getMessage().contains('Credit score'));
    }
}

300 and 299 — the exact edge of the valid range — is precisely where a < vs <= mistake in LoanValidationService.validate() would hide, exactly like the 599/600 boundary from the previous lesson.

@isTest
static void oneInvalidApplicationInABatchBlocksTheWholeInsert() {
    List<Loan_Application__c> applications = new List<Loan_Application__c>{
        LoanTestDataFactory.createQualifyingApplication(),
        LoanTestDataFactory.createApplication(-5000, 50000, 720) // invalid: negative amount
    };

    try {
        insert applications;
        System.assert(false, 'Expected a DmlException from the invalid second record');
    } catch (DmlException e) {
        System.assert(e.getMessage().contains('amount'));
    }

    System.assertEquals(0, [SELECT COUNT() FROM Loan_Application__c]);
}

Because beforeInsert throws an exception (via validate()) for the second application, the entire insert statement fails — not just the bad record. The final assertion confirms this explicitly: zero applications were saved, including the perfectly valid first one, exactly the all-or-nothing DML behavior from Module 19.