Why Tests Matter

Every project since Module 26 has ended with an Execute Anonymous script confirming the feature worked at that moment. But nothing about that verification survives — if someone changes AccountRollupService next month, nothing re-runs those checks automatically. A test class is Apex code that verifies other Apex code, and — critically — it runs automatically every time the org is deployed to, catching a regression the instant it's introduced.

@isTest
private class CalculatorTest {
    @isTest
    static void additionWorksCorrectly() {
        Integer result = 2 + 2;
        System.assertEquals(4, result, 'Expected 2 + 2 to equal 4');
    }
}

@isTest (first previewed in Module 13's Annotations lesson) marks this entire class as test-only code. Inside it, additionWorksCorrectly() sets up a scenario, runs the code being tested, and asserts the actual result matches the expected one — the exact same shape as every manual verification since Module 26, just automated and permanent.

Salesforce requires at least 75% of an org's Apex code to be covered by tests before it can be deployed to production — but hitting exactly 75% coverage says nothing about whether the tests actually verify anything meaningful. This module's final lesson, "Code Coverage vs Meaningful Testing," returns to this distinction directly — for now, the key idea is: coverage is a deployment gate, not evidence the code actually works correctly.