Create Test Data for Apex Tests

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TestDataFactory.apxc

@isTest
public class TestDataFactory {
    public static List<Account> createAccountsWithOpps(Integer numAccts, Integer numOppsPerAcct) {
        List<Account> accts = new List<Account>();
        
        for(Integer i=0;i<numAccts;i++) {
            Account a = new Account(Name='TestAccount' + i);
            accts.add(a);
        }
        insert accts;
        
        List<Opportunity> opps = new List<Opportunity>();
        for (Integer j=0;j<numAccts;j++) {
            Account acct = accts[j];
            // For each account just inserted, add opportunities
            for (Integer k=0;k<numOppsPerAcct;k++) {
                opps.add(new Opportunity(Name=acct.Name + ' Opportunity ' + k,
                                         StageName='Prospecting',
                                         CloseDate=System.today().addMonths(1),
                                         AccountId=acct.Id));
            }
        }
        // Insert all opportunities for all accounts.
        insert opps;
        
        return accts;
    }
}

TestAccountDeletion.apxc

@isTest
private class TestAccountDeletion {
    @isTest static void TestDeleteAccountWithOneOpportunity() {
        // Test data setup
        // Create one account with one opportunity by calling a utility method
        Account[] accts = TestDataFactory.createAccountsWithOpps(1,1);
        
        // Perform test
        Test.startTest();
        Database.DeleteResult result = Database.delete(accts[0], false);
        Test.stopTest();
        // Verify that the deletion should have been stopped by the trigger,
        // so check that we got back an error.
        System.assert(!result.isSuccess());
        System.assert(result.getErrors().size() > 0);
        System.assertEquals('Cannot delete account with related opportunities.',
                            result.getErrors()[0].getMessage());
    }
    
    @isTest static void TestDeleteAccountWithNoOpportunities() {
        // Test data setup
        // Create one account with no opportunities by calling a utility method
        Account[] accts = TestDataFactory.createAccountsWithOpps(1,0);
        
        // Perform test
        Test.startTest();
        Database.DeleteResult result = Database.delete(accts[0], false);
        Test.stopTest();
        // Verify that the deletion was successful
        System.assert(result.isSuccess());
    }
    
    @isTest static void TestDeleteBulkAccountsWithOneOpportunity() {
        // Test data setup
        // Create accounts with one opportunity each by calling a utility method
        Account[] accts = TestDataFactory.createAccountsWithOpps(200,1);
        
        // Perform test
        Test.startTest();
        Database.DeleteResult[] results = Database.delete(accts, false);
        Test.stopTest();
        // Verify for each record.
        // In this case the deletion should have been stopped by the trigger,
        // so check that we got back an error.
        for(Database.DeleteResult dr : results) {
            System.assert(!dr.isSuccess());
            System.assert(dr.getErrors().size() > 0);
            System.assertEquals('Cannot delete account with related opportunities.',
                                dr.getErrors()[0].getMessage());
        }
    }
    
    @isTest static void TestDeleteBulkAccountsWithNoOpportunities() {
        // Test data setup
        // Create accounts with no opportunities by calling a utility method
        Account[] accts = TestDataFactory.createAccountsWithOpps(200,0);
        
        // Perform test
        Test.startTest();
        Database.DeleteResult[] results = Database.delete(accts, false);
        Test.stopTest();
        // For each record, verify that the deletion was successful
        for(Database.DeleteResult dr : results) {
            System.assert(dr.isSuccess());
        }
    }
}

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Create a contact test factory

public class RandomContactFactory {
    public static List<Contact> generateRandomContacts(Integer iContacts, String sLastName){
        List<Contact> lstRndContacts = new List<Contact>();
        for(Integer i=0; i<iContacts; i++){
            Contact oContact = new Contact(LastName = sLastName, FirstName = 'Test '+i);
            lstRndContacts.add(oContact);
        }
        return lstRndContacts;
    }
}