Build the Calculation Engine

public class LoanCalculationService {
    public Decimal calculateInterestRate(Decimal creditScore) {
        if (creditScore >= 700) {
            return 6.5;
        } else if (creditScore >= 600) {
            return 9.0;
        } else {
            return 14.0;
        }
    }

    public Decimal calculateMonthlyPayment(Decimal amount, Decimal annualRatePercent, Integer months) {
        Decimal monthlyRate = (annualRatePercent / 100) / 12;
        return (amount * monthlyRate) / (1 - Math.pow(1 + monthlyRate, -months));
    }
}

1. public class LoanCalculationService { — the class.
2. public Decimal calculateInterestRate(Decimal creditScore) { — Module 6's if/else if/else chain.
3. if (creditScore >= 700) { — the best tier.
4. return 6.5; — its rate.
5. } else if (creditScore >= 600) { — the middle tier.
6. return 9.0; — its rate.
7. } else { — the lowest tier.
8. return 14.0; — its rate.
9. } — closes the if/else if/else.
10. } — closes the method.
11. (blank line)
12. public Decimal calculateMonthlyPayment(Decimal amount, Decimal annualRatePercent, Integer months) { — implements the standard amortized-loan-payment formula.
13. Decimal monthlyRate = (annualRatePercent / 100) / 12; — converts the annual percentage to a monthly decimal rate.
14. return (amount * monthlyRate) / (1 - Math.pow(1 + monthlyRate, -months)); — the principal, annual rate, and loan term in months, combined into the standard formula.
15. } — closes the method.
16. } — closes the class.

@isTest
private class LoanCalculationServiceTest {
    @isTest
    static void highCreditScoreGetsLowestRate() {
        LoanCalculationService service = new LoanCalculationService();
        System.assertEquals(6.5, service.calculateInterestRate(750));
    }

    @isTest
    static void midCreditScoreGetsMiddleRate() {
        LoanCalculationService service = new LoanCalculationService();
        System.assertEquals(9.0, service.calculateInterestRate(650));
    }

    @isTest
    static void lowCreditScoreGetsHighestRate() {
        LoanCalculationService service = new LoanCalculationService();
        System.assertEquals(14.0, service.calculateInterestRate(550));
    }
}

1. @isTest — marks the class.
2. private class LoanCalculationServiceTest { — three test methods, one per branch of the if/else if/else chain.
3. @isTest — marks the first test.
4. static void highCreditScoreGetsLowestRate() { — this is Module 31's Lesson 2 habit ("one test method, one scenario") applied the moment the logic is written.
5. LoanCalculationService service = new LoanCalculationService(); — creates the service.
6. System.assertEquals(6.5, service.calculateInterestRate(750)); — verifies the top tier.
7. } — closes this test.
8. (blank line)
9. @isTest — marks the second test.
10. static void midCreditScoreGetsMiddleRate() { — a different scenario.
11. LoanCalculationService service = new LoanCalculationService(); — creates the service.
12. System.assertEquals(9.0, service.calculateInterestRate(650)); — verifies the middle tier.
13. } — closes this test.
14. (blank line)
15. @isTest — marks the third test.
16. static void lowCreditScoreGetsHighestRate() { — another scenario.
17. LoanCalculationService service = new LoanCalculationService(); — creates the service.
18. System.assertEquals(14.0, service.calculateInterestRate(550)); — verifies the lowest tier.
19. } — closes this test.
20. } — closes the class.

@isTest
static void creditScoreOf700IsTheLowestQualifyingForBestRate() {
    LoanCalculationService service = new LoanCalculationService();
    System.assertEquals(6.5, service.calculateInterestRate(700));
}

@isTest
static void creditScoreOf699JustMissesTheBestRate() {
    LoanCalculationService service = new LoanCalculationService();
    System.assertEquals(9.0, service.calculateInterestRate(699));
}

1. @isTest — marks the first test.
2. static void creditScoreOf700IsTheLowestQualifyingForBestRate() { — testing exactly 700 — the boundary between two branches.
3. LoanCalculationService service = new LoanCalculationService(); — creates the service.
4. System.assertEquals(6.5, service.calculateInterestRate(700)); — 700 should qualify for the best rate.
5. } — closes this test.
6. (blank line)
7. @isTest — marks the second test.
8. static void creditScoreOf699JustMissesTheBestRate() { — the value one below the boundary.
9. LoanCalculationService service = new LoanCalculationService(); — creates the service.
10. System.assertEquals(9.0, service.calculateInterestRate(699)); — 699 should just miss it — this is where off-by-one bugs (> instead of >=, from Module 4) actually hide.
11. } — closes this test.

This is Module 31's "assertions that actually tell you something" lesson, applied specifically to boundary conditions.