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);
    }
}

• @isTest on the class marks the whole class as test-only.
• private class is the conventional (and typically required) access level for a test class.
• @isTest on the method, plus static void, marks an individual test method — every test method must be static, since it doesn't operate on any instance of the test class itself.
• TestClassName matching ClassNameTest (here, CalculatorTest testing Calculator) is a strong, widely-followed naming convention, not a compiler requirement.

@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, '/'));
    }
}

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.