Write a Full Test Suite

@isTest
private class TransactionTriggerTest {
    @isTest
    static void updatesBalanceForBulkDeposits() {
        Bank_Account__c acc = new Bank_Account__c(Name = 'Test', Balance__c = 0, Status__c = 'Active');
        insert acc;

        List<Transaction__c> deposits = new List<Transaction__c>();
        for (Integer i = 0; i < 200; i++) {
            deposits.add(new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Deposit', Amount__c = 10));
        }

        Test.startTest();
        insert deposits;
        Test.stopTest();

        Assert.areEqual(2000, [SELECT Balance__c FROM Bank_Account__c WHERE Id = :acc.Id].Balance__c);
    }
}

1. @isTest — marks the test class.
2. private class TransactionTriggerTest { — the test class.
3. @isTest — marks the test method.
4. static void updatesBalanceForBulkDeposits() { — states what's verified.
5. Bank_Account__c acc = new Bank_Account__c(Name = 'Test', Balance__c = 0, Status__c = 'Active'); — test data, starting at zero.
6. insert acc; — saves it.
7. (blank line)
8. List<Transaction__c> deposits = new List<Transaction__c>(); — a list to build in bulk.
9. for (Integer i = 0; i < 200; i++) { — Module 31's "always test at bulk scale" rule — the maximum trigger batch size.
10. deposits.add(new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Deposit', Amount__c = 10)); — 200 identical deposits.
11. } — closes the for loop.
12. (blank line)
13. Test.startTest(); — the governor-limit boundary.
14. insert deposits; — one bulk DML statement, firing the trigger once with all 200 records.
15. Test.stopTest(); — closes the boundary.
16. (blank line)
17. Assert.areEqual(2000, [SELECT Balance__c FROM Bank_Account__c WHERE Id = :acc.Id].Balance__c); — a specific expected total — 200 deposits of 10 each.
18. } — closes the test method.
19. } — closes the test class.

200 deposits of 10 each should net to exactly 2000 — this is Module 31's "always test at bulk scale" rule, catching any hidden per-record assumption Lesson 2's trigger might have.

@isTest
static void rejectsOverdraftWithdrawal() {
    Bank_Account__c acc = new Bank_Account__c(Name = 'Test', Balance__c = 100, Status__c = 'Active');
    insert acc;

    Transaction__c overdraft = new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Withdrawal', Amount__c = 500);

    Test.startTest();
    try {
        insert overdraft;
        Assert.fail('Expected an overdraft DmlException');
    } catch (DmlException e) {
        Assert.isTrue(e.getMessage().contains('Insufficient funds'));
    }
    Test.stopTest();
}

1. @isTest — marks the test method.
2. static void rejectsOverdraftWithdrawal() { — states the exact scenario under test.
3. Bank_Account__c acc = new Bank_Account__c(Name = 'Test', Balance__c = 100, Status__c = 'Active'); — a modest starting balance.
4. insert acc; — saves it.
5. (blank line)
6. Transaction__c overdraft = new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Withdrawal', Amount__c = 500); — a withdrawal larger than the balance.
7. (blank line)
8. Test.startTest(); — the boundary.
9. try { — Module 31's exception-testing shape.
10. insert overdraft; — expected to throw, given the validation trigger.
11. Assert.fail('Expected an overdraft DmlException'); — fails the test if no exception was thrown.
12. } catch (DmlException e) { — addError() surfaces as a DmlException to the calling code.
13. Assert.isTrue(e.getMessage().contains('Insufficient funds')); — confirms the specific rejection reason.
14. } — closes the try/catch.
15. Test.stopTest(); — closes the boundary.
16. } — closes the test method.

This is Module 31's negative-path testing principle — confirming the rejection itself works correctly is just as important as confirming a valid deposit updates the balance, since an untested rejection path could silently let an overdraft through.

@isTest
static void reportsToLedgerAsynchronously() {
    Test.setMock(HttpCalloutMock.class, new LedgerMockSuccess());
    Bank_Account__c acc = new Bank_Account__c(Name = 'Test', Balance__c = 0, Status__c = 'Active');
    insert acc;

    Test.startTest();
    insert new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Deposit', Amount__c = 50);
    Test.stopTest(); // forces the enqueued Queueable job to run synchronously here

    // assert on a field the mock's callout would have set, or that no exception occurred
}

1. @isTest — marks the test method.
2. static void reportsToLedgerAsynchronously() { — states what's verified.
3. Test.setMock(HttpCalloutMock.class, new LedgerMockSuccess()); — Module 36's mocking, swapping in a fabricated response.
4. Bank_Account__c acc = new Bank_Account__c(Name = 'Test', Balance__c = 0, Status__c = 'Active'); — test data.
5. insert acc; — saves it.
6. (blank line)
7. Test.startTest(); — the boundary.
8. insert new Transaction__c(Bank_Account__c = acc.Id, Type__c = 'Deposit', Amount__c = 50); — fires both triggers, including the enqueue of the ledger job.
9. Test.stopTest(); // forces the enqueued Queueable job to run synchronously here — Module 31's boundary, forcing the async job to completion.
10. (blank line)
11. // assert on a field the mock's callout would have set, or that no exception occurred — a placeholder for the real assertion.
12. } — closes the test method.

Test.setMock and Test.stopTest() forcing the enqueued job to run are Module 31's and Module 36's exact async-callout testing patterns, combined here for the ledger-integration job from Lesson 4.