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'));
    }
}

1. @isTest — marks the first test.
2. static void creditScoreOf300IsValid() { — 300 — the exact edge of the valid range.
3. LoanValidationService validator = new LoanValidationService(); — creates the validator.
4. validator.validate(10000, 50000, 300); — should not throw.
5. System.assert(true); — confirms it didn't.
6. } — closes this test.
7. (blank line)
8. @isTest — marks the second test.
9. static void creditScoreOf299IsInvalid() { — the value one below the boundary.
10. LoanValidationService validator = new LoanValidationService(); — creates the validator.
11. try { — wraps the call expected to throw.
12. validator.validate(10000, 50000, 299); — 299 — precisely where a < vs <= mistake in LoanValidationService.validate() would hide.
13. System.assert(false, 'Expected a LoanValidationException'); — reached only if it didn't throw.
14. } catch (LoanValidationException e) { — catches the expected failure.
15. System.assert(e.getMessage().contains('Credit score')); — verifies the right error, exactly like the 599/600 boundary from the previous lesson.
16. } — closes the try/catch.
17. } — closes this test.

@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]);
}

1. @isTest — marks the test method.
2. static void oneInvalidApplicationInABatchBlocksTheWholeInsert() { — the test.
3. List<Loan_Application__c> applications = new List<Loan_Application__c>{ — a batch with one good and one bad record.
4. LoanTestDataFactory.createQualifyingApplication(), — a perfectly valid application.
5. LoanTestDataFactory.createApplication(-5000, 50000, 720) — an invalid one, with a negative amount.
6. }; — closes the list literal.
7. (blank line)
8. try { — wraps the insert expected to fail.
9. insert applications; — because beforeInsert throws an exception (via validate()) for the second application, the entire insert statement fails — not just the bad record.
10. System.assert(false, 'Expected a DmlException from the invalid second record'); — reached only if it unexpectedly succeeded.
11. } catch (DmlException e) { — catches the expected failure.
12. System.assert(e.getMessage().contains('amount')); — verifies the right error.
13. } — closes the try/catch.
14. (blank line)
15. System.assertEquals(0, [SELECT COUNT() FROM Loan_Application__c]); — confirms this explicitly: zero applications were saved, including the perfectly valid first one, exactly the all-or-nothing DML behavior from Module 19.
16. } — closes the test method.