Your First Test Class

@isTest
private class CalculatorTest {
    @isTest
    static void calculateAndFormatHandlesAddition() {
        Calculator calc = new Calculator();
        String result = calc.calculateAndFormat(12, 5, '+');
        System.assertEquals('12 + 5 = 17.00', result);
    }
}

1. @isTest — on the class, marks the whole class as test-only.
2. private class CalculatorTest { — private class is the conventional (and typically required) access level for a test class; CalculatorTest matching Calculator is a strong, widely-followed naming convention, not a compiler requirement.
3. @isTest — on the method, marks an individual test method.
4. static void calculateAndFormatHandlesAddition() { — every test method must be static, since it doesn't operate on any instance of the test class itself.
5. Calculator calc = new Calculator(); — sets up the scenario.
6. String result = calc.calculateAndFormat(12, 5, '+'); — runs the code being tested.
7. System.assertEquals('12 + 5 = 17.00', result); — asserts the result.
8. } — closes the test method.
9. } — closes the class.

@isTest
private class CalculatorTest {
    @isTest
    static void additionReturnsCorrectSum() {
        Calculator calc = new Calculator();
        System.assertEquals('12 + 5 = 17.00', calc.calculateAndFormat(12, 5, '+'));
    }

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

1. @isTest — marks the class.
2. private class CalculatorTest { — the class.
3. @isTest — marks the first test method.
4. static void additionReturnsCorrectSum() { — one scenario: addition.
5. Calculator calc = new Calculator(); — sets up.
6. System.assertEquals('12 + 5 = 17.00', calc.calculateAndFormat(12, 5, '+')); — runs and asserts in one line.
7. } — closes this test method.
8. (blank line)
9. @isTest — marks a second test method.
10. static void divisionByZeroReturnsErrorMessage() { — a different scenario: division by zero.
11. Calculator calc = new Calculator(); — sets up.
12. System.assertEquals('Error: cannot divide by zero.', calc.calculateAndFormat(10, 0, '/')); — runs and asserts.
13. } — closes this test method.
14. } — closes the class.

Each @isTest static void method verifies exactly one scenario — this mirrors Module 26's "Design the Requirement" habit of naming each edge case explicitly, now with one dedicated test method per case instead of one long manual script trying to cover everything.

Running CalculatorTest (via the Developer Console, VS Code, or CI) reports each test method as pass or fail individually — additionReturnsCorrectSum passing and divisionByZeroReturnsErrorMessage failing tells you exactly which scenario broke, immediately, rather than needing to guess from one long manual script.