Build a Bulk-Safe Trigger

public class LoanApplicationTriggerHandler {
    private static final Integer LOAN_TERM_MONTHS = 60;

    public void beforeInsert(List<Loan_Application__c> newApplications) {
        LoanValidationService validator = new LoanValidationService();
        LoanCalculationService calculator = new LoanCalculationService();

        for (Loan_Application__c app : newApplications) {
            validator.validate(app.Requested_Amount__c, app.Annual_Income__c, app.Credit_Score__c);

            Decimal rate = calculator.calculateInterestRate(app.Credit_Score__c);
            app.Interest_Rate__c = rate;
            app.Monthly_Payment__c = calculator.calculateMonthlyPayment(app.Requested_Amount__c, rate, LOAN_TERM_MONTHS);
            app.Status__c = determineStatus(app.Credit_Score__c, app.Requested_Amount__c, app.Annual_Income__c);
        }
    }

    private String determineStatus(Decimal creditScore, Decimal amount, Decimal income) {
        if (creditScore >= 700 && amount <= income * 0.4) {
            return 'Pre-Approved';
        } else if (creditScore >= 600) {
            return 'Manual Review';
        } else {
            return 'Declined';
        }
    }
}

This is a genuinely realistic trigger handler — validation (Lesson 3), calculation (Lesson 2), and the approval decision (Lesson 1's policy, finally implemented) all working together, entirely in memory, no SOQL or DML inside the loop at all.

@isTest
private class LoanApplicationTriggerHandlerTest {
    @isTest
    static void bulkApplicationsAllProcessCorrectly() {
        List<Loan_Application__c> applications = new List<Loan_Application__c>();
        for (Integer i = 0; i < 200; i++) {
            applications.add(new Loan_Application__c(
                Requested_Amount__c = 10000, Annual_Income__c = 50000, Credit_Score__c = 720
            ));
        }

        Test.startTest();
        insert applications;
        Test.stopTest();

        List<Loan_Application__c> saved = [SELECT Status__c, Interest_Rate__c FROM Loan_Application__c];
        System.assertEquals(200, saved.size());
        for (Loan_Application__c app : saved) {
            System.assertEquals('Pre-Approved', app.Status__c);
            System.assertEquals(6.5, app.Interest_Rate__c);
        }
    }
}

This is Module 31's "Testing Triggers: Bulk and Edge Cases" lesson, applied the moment the trigger exists — 200 records, one insert, and an assertion that every single one processed identically and correctly.