Code Coverage vs Meaningful Testing

@isTest
static void testCalculateAndFormat() {
    Calculator calc = new Calculator();
    calc.calculateAndFormat(12, 5, '+'); // runs every line... but asserts nothing at all!
}

1. @isTest — marks the test method.
2. static void testCalculateAndFormat() { — this test genuinely executes every line inside calculateAndFormat, contributing real percentage points to the org's code coverage.
3. Calculator calc = new Calculator(); — creates the calculator.
4. calc.calculateAndFormat(12, 5, '+'); — runs it, but asserts nothing at all — it would pass even if the method were completely broken, because it never checks the result. This is exactly Lesson 1's warning, made concrete: coverage counts lines executed, not correctness verified.
5. } — closes the test method.

@isTest
static void additionReturnsCorrectlyFormattedSum() {
    Calculator calc = new Calculator();
    String result = calc.calculateAndFormat(12, 5, '+');
    System.assertEquals('12 + 5 = 17.00', result); // Lesson 5's specific-value assertion
}

@isTest
static void divisionByZeroReturnsErrorMessage() {
    Calculator calc = new Calculator();
    String result = calc.calculateAndFormat(10, 0, '/');
    System.assertEquals('Error: cannot divide by zero.', result);
}

1. @isTest — marks the first test method.
2. static void additionReturnsCorrectlyFormattedSum() { — one scenario per test method (Lesson 2).
3. Calculator calc = new Calculator(); — creates the calculator.
4. String result = calc.calculateAndFormat(12, 5, '+'); — runs the same lines as the weak test above.
5. System.assertEquals('12 + 5 = 17.00', result); — Lesson 5's specific-value assertion — now this scenario actually verifies a specific, correct outcome.
6. } — closes this test method.
7. (blank line)
8. @isTest — marks a second test method.
9. static void divisionByZeroReturnsErrorMessage() { — exception paths tested explicitly (Lesson 6).
10. Calculator calc = new Calculator(); — creates the calculator.
11. String result = calc.calculateAndFormat(10, 0, '/'); — the division-by-zero scenario.
12. System.assertEquals('Error: cannot divide by zero.', result); — asserts the specific expected error message.
13. } — closes this test method.

Same lines of Calculator covered as the weak test — but now every scenario actually verifies a specific, correct outcome. This is the whole module's toolkit converging.

• Does every test method assert a specific expected value, not just "no exception" or "not null"? (Lesson 5)
• Are both the happy path and the exception/edge-case paths covered? (Lesson 6)
• Is there at least one bulk test (200 records) for every trigger? (Lesson 7)
• Is repeated setup extracted into a factory, keeping each test focused on what it's actually verifying? (Lesson 8)

Coverage percentage tells you how much code ran during testing; this checklist is what actually tells you whether the tests are worth trusting.