Assertions That Actually Tell You Something

System.assertEquals(17.00, result, 'Expected the total to be 17.00');
System.assertNotEquals(null, savedRecord.Id, 'Expected the record to have been saved with an Id');
System.assert(list.size() > 0, 'Expected the list to contain at least one item');

1. System.assertEquals(17.00, result, 'Expected the total to be 17.00'); — the message is the final argument.
2. System.assertNotEquals(null, savedRecord.Id, 'Expected the record to have been saved with an Id'); — same shape.
3. System.assert(list.size() > 0, 'Expected the list to contain at least one item'); — same shape again.

All three accept an optional message as their final argument — always worth including, since it's exactly what shows up in the test failure output, saving whoever's debugging a failed test from having to reverse-engineer what the assertion was even checking.

// WEAK: passes even if the method returns completely wrong data
System.assertNotEquals(null, result);

// STRONG: actually verifies the specific expected value
System.assertEquals('12 + 5 = 17.00', result);

1. // WEAK: passes even if the method returns completely wrong data — the label for the weak version.
2. System.assertNotEquals(null, result); — only confirms something was returned — it would pass even if calculateAndFormat(12, 5, '+') returned 'wrong answer' or '42'.
3. (blank line)
4. // STRONG: actually verifies the specific expected value — the label for the strong version.
5. System.assertEquals('12 + 5 = 17.00', result); — this is exactly the kind of assertion that lets a real bug slip through a "passing" test suite, satisfying the coverage requirement (Lesson 1) while verifying almost nothing — the strong version actually checks it.

@isTest
static void discountAppliesCorrectPercentage() {
    Decimal result = DiscountCalculator.apply(200, 10); // 10% off 200

    // WEAK — would still pass even with a subtle off-by-one bug
    System.assert(result < 200);

    // STRONG — catches the exact expected value, off-by-one bugs included
    System.assertEquals(180, result);
}

1. @isTest — marks the test method.
2. static void discountAppliesCorrectPercentage() { — the test.
3. Decimal result = DiscountCalculator.apply(200, 10); — 10% off 200, so 180 is expected.
4. (blank line)
5. // WEAK — would still pass even with a subtle off-by-one bug — the label.
6. System.assert(result < 200); — if DiscountCalculator.apply had a bug computing 182 instead of 180, this would still pass.
7. (blank line)
8. // STRONG — catches the exact expected value, off-by-one bugs included — the label.
9. System.assertEquals(180, result); — only the strong, specific assertion actually catches the discrepancy.
10. } — closes the test method.

This is the single most important habit this lesson teaches: assert the exact expected value whenever one is knowable, not just a loose approximation of correctness.