The document provides examples of code snippets and design principles for building banking applications. Some key points discussed include: - Encapsulating state checks and validation logic rather than exposing primitive checks. - Avoiding mutable collections and exposing only immutable views of data. - Modeling domain-specific concepts like transactions, accounts, customers rather than using language primitives. - Implementing single responsibility and telling objects what to do rather than how to do through dependency injection. The examples demonstrate applying object-oriented design principles like encapsulation, single responsibility and dependency injection to model complex domain concepts in banking applications.

1. Simple Programming/Design Tidbits
Banking application
Customer, Accounts, Transactions, ATM,

2. All transactions at an ATM
class AccountService {
List<Transaction> transactionsAt(int accountId, ATM atm) {
List<Transaction> transactionList = new List<Transaction>();
for (Transaction txn : transactions)
if (transaction.wasAtLocation(atm.getLocationId()))
transactionList.add(txn);
return transactionList;
}
}
class Transaction {
ATM atm;
boolean wasAtLocation(int locationId) {
return atm.getLocationId() == locationId;
}
}

3. All transactions at an ATMAll transactions at an ATM
class AccountService {
List<Transaction> transactionsAt(int accountId, ATM atm) {
List<Transaction> transactionList = new List<Transaction>();
for (Transaction txn : transactions)
if (transaction.wasAt(atm))if (transaction.wasAt(atm))
transactionList.add(txn);
return transactionList;
}
}
class Transaction {
ATM atm;
boolean wasAt(ATM atm) {boolean wasAt(ATM atm) {
return this.atm.equals(locationId);return this.atm.equals(locationId);
}
}

4. Pass object not its data
Encasulation can be broken across methods

5. class Customer {
static double eligibilityForHomeLoan = 500000;
static double eligibilityForPersonalLoan = 10000;
double salary;
boolean targetForHomeLoan() {
return salary > eligibilityForHomeLoan;
}
boolean targetForPersonalLoan() {
return salary > eligibilityForPersonalLoan;
}
}
Should target a customer for loan?

6. Should target a customer for loan?Should target a customer for loan?
class Customer {
Salary salary;Salary salary;
boolean targetForHomeLoan() {
return salary.isEligibleForHomeLoan();return salary.isEligibleForHomeLoan();
}
}
class Salary {class Salary {
double amount;double amount;
static double eligibilityForHomeLoan = 500000;static double eligibilityForHomeLoan = 500000;
Salary(double amount) {Salary(double amount) {
this.amount = amount;this.amount = amount;
}}
boolean isEligibleForLoan() {boolean isEligibleForLoan() {
return amount > eligibilityForHomeLoan.amount;return amount > eligibilityForHomeLoan.amount;
}}
}}

7. Avoid primitives in your domain
Primitives not only language but can be domain
primitives e.g. Money/Date/Currency

8. class Customer {
Set<Account> accounts;
Set<Account> getAccounts() {
return accounts;
}
}
class CustomerService {
void addAccount(Customer customer) {
Account account = new Account();
customer.getAccounts().add(account);
}
}
Open new account for a customer

9. class Customer {
Set<Account> accounts;
Account[] getAccounts() {Account[] getAccounts() {
return accounts.toArray();return accounts.toArray();
}}
void addNewAccount() {void addNewAccount() {
Account account = new Account();Account account = new Account();
accounts.add(account);accounts.add(account);
}}
}
class CustomerService {
void addAccount(Customer customer) {
customer.addNewAccount();customer.addNewAccount();
}
}
Open new account for a customerOpen new account for a customer

10. Collections are mutable, donot expose them

11. class Customer {
Set<Account> accounts;
Money balance() {
Money total = Money.Zero();
for (Account account : accounts) {
total += account.getBalance();
}
return total;
}
Money inactiveAccounts() {
for (Account account : accounts) {
if (account.isInactive())
inactiveAccounts.add(account);
}
}
}
Customer's accounts

12. class Customer {
Accounts accounts;Accounts accounts;
}
class Accountsclass Accounts {
Set<Account> accounts;Set<Account> accounts;
Money total = Money.Zero();
Money balance() {
for (Account account : accounts) {
total += account.getBalance();
}
return total;
}
Money inactiveAccounts() {
for (Account account : accounts) {
if (account.isInactive())
inactiveAccounts.add(account);
}
}
}
Customer's accounts

13. Collections are primitives
Create domain specific collections

14. class Account {
InterestPlan interestPlan;
Money bal;
Money applyInterest() {
switch (interestPlan) {
case Simple: interest = new SICalc().calculate(bal);
case Compound: interest = new CICalc().calculate(bal);
......
}
bal += interest;
}
}
enum InterestPlan {
Simple, Compound, InflationAdjusted,
DurationDependent
}
Apply interest

15. class Account {
InterestPlan interestPlan;
Money bal;
static Map calculators = {static Map calculators = {
Simple => new SICalc(),Simple => new SICalc(),
Compound => new CICalc();Compound => new CICalc();
..........
}}
Money applyInterest() {
interest = calculators[interestPlan].calculate();interest = calculators[interestPlan].calculate();
}
}
Apply interestApply interest

16. Externalize if conditions
Can use for functions as value in map as well

17. class Account {
State state;
boolean transactionsAllowed() {
return state == State.Approved || state == State.Active;
}
boolean onlineTransactionAllowed() {
return state == State.Active || state == State.Locked;
}
}
enum State {
PendingApproval, Approved, Locked, Active, Closed
}
Account Security

18. class Account {
State state;
List transactionsAllowed = {State.Approved, State.Active};List transactionsAllowed = {State.Approved, State.Active};
List onlineTransactionsAllowed = {State.Locked, State.Active};List onlineTransactionsAllowed = {State.Locked, State.Active};
boolean transactionsAllowed() {
return transactionsAllowed.contains(state)transactionsAllowed.contains(state);
}
boolean onlineTransactionAllowed() {
return onlineTransactionsAllowed.contains(state)onlineTransactionsAllowed.contains(state);
}
}
enum State {
PendingApproval, Approved, Locked, Active, Closed
}
Account SecurityAccount Security

19. void monthlyReport() {
TransactionRepository repo = new TransactionRepository(...);
var list = repo.getBiggerTransactionsNotTransferredToSelf();
......
}
class TransactionRepository {
Transactions getBiggerTransactionsNotTransferredToSelf() {
Transactions transactions = loadTransactions(customerId);
....logic...
}
}
Suspicious Transactions

20. void monthlyReport() {
TransactionRepository repo = new TransactionRepository(...);
var list = repo.suspiciousTransactions()repo.suspiciousTransactions();
......
}
class TransactionRepository {
Transactions suspiciousTransactions()suspiciousTransactions() {
Transactions transactions = loadTransactions(customerId);
....logic...
}
}
Suspicious TransactionsSuspicious Transactions

21. Tell what not how
Keep implementation encapsulation
Seen more when doing TDD

22. class Customer {
String name;
Date dateOfBirth;
Accounts accounts;
Money salaryForLoanCalculations;
boolean targetForHomeLoan() {
return salaryForLoanCalculations.isEligibleForHomeLoan();
}
boolean targetForPersonalLoan() {
return salaryForLoanCalculations.isEligibileForPersLoan();
}
}
Customer

23. class Customer {
String name;
Date dateOfBirth;
Accounts accounts;
Money salary;Money salary;
boolean targetForHomeLoan() {
return salary.isEligibleForHomeLoan();return salary.isEligibleForHomeLoan();
}
boolean targetForPersonalLoan() {
return salary.isEligibileForPersonalLoan();return salary.isEligibileForPersonalLoan();
}
}
CustomerCustomer

24. Store what not what for
Seen more when doing TDD

25. class Customer {
String name;
Accounts accounts;
Money salary;
boolean isLoyal() {
if (salary > 20000 && accounts.balance() > 100000) {
return true;
}
return false;
}
}
Is Loyal Customer?

26. class Customer {
String name;
Accounts accounts;
Money salary;
boolean isLoyal() {
return salary > 20000 && accounts.balance() > 100000);return salary > 20000 && accounts.balance() > 100000);
}
}
Is Loyal Customer?Is Loyal Customer?

27. class Customer {
Accounts accounts;
boolean isLoyal() {
....
}
void issueCard(String accountNumber) {
Account account = accounts.get(accountNumber);
if (isLoyal() == true)
account.issuePlatinum();
else
account.issueOfLastType();
}
}
Issue platinum card?

28. class Customer {
Accounts accounts;
boolean isLoyal() {
....
}
void issueCard(String accountNumber) {
Account account = accounts.get(accountNumber);
if (isLoyal())if (isLoyal())
account.issuePlatinum();
else
account.issueOfLastType();
}
}
Issue platinum card?Issue platinum card?

29. class Customer {
String name;
Date dateOfBirth;
public void validateForNew() throws ValidationException {
ValidationException exception = new ValidationException();
if (name == null || name.isEmpty())
exception.add(“Name not specified”);
if (dateOfBirth == null)
exception.add(“Date of birth not specified”);
else if (dateOfBirth.isBefore(1990))
exception.add(“below 20 years are not eligible”);
if (exception.hasErrors()) {
throw exception;
}
}
}
New Customer

30. class Customer {
String name;
Date dateOfBirth;
public ValidationError validateForNew() {public ValidationError validateForNew() {
ValidationError error = new ValidationError();ValidationError error = new ValidationError();
if (name == null || name.isEmpty())
error.add(“Name not specified”);
if (dateOfBirth == null)
error.add(“Date of birth not specified”);
else if (dateOfBirth.isBefore(1990))
error.add(“below 20 years are not eligible”);
return error;return error;
}
}
New CustomerNew Customer

31. Error during validation is not exceptional scenario

32. public class Customer {
public void gotMarried() {
title = "Mrs";
}
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Customer customer = (Customer) o;
if (!firstName.equals(customer.firstName)) return false;
if (!lastName.equals(customer.lastName)) return false;
if (!title.equals(customer.title)) return false;
return true;
}
public int hashCode() {
int result = firstName.hashCode();
result = 31 * result + lastName.hashCode();
result = 31 * result + title.hashCode();
return result;
}
}
Customer got married

33. public void test() {
Customer customer = new Customer("foo", "bar", "Miss");
Map map = new HashMap();
map.put(customer, "rich");
assertTrue(map.get(customer) != null);
customer.gotMarried();
assertTrue(map.get(customer) != null); <-- FailsassertTrue(map.get(customer) != null); <-- Fails
}
Customer got married

34. Don't use mutable fields for hashcode

35. Account account = AccountRepository.load(1);
....
cleanUp(account);
public static void cleanUp(Account account) {
if (null != account) {
deleteAccount(account, null);
account = null;
}
}
Get rid of large object from memory

36. Objects references are always passed by value
Thats why C# has explicit pass by ref (not
recommending essentially)

37. Class Customer {
Name name;
Date dateOfBirth;
Accounts accounts;
Name getName() {
if (name == null) name = new Name();
return name;
}
}
Get customer's name

38. Do not change data in getter, it deceives

39. ToString() is object's user interface
Reserve it and use for debugging only

40. public class Customer {
List<Account> accounts = new ArrayList<Account>();
public void isAccountInactive(String accountNumber, int
maxInactivityAllowed) {
int i = accounts.indexOf(new Account(accountNumber));
Account account = accounts.get(i);
inactive(account, maxInactivityAllowed);
}
private static boolean inactive(Account account, int
maxInactivityAllowed) {
return account.getLastActivity().within(maxInactivityAllowed);
}
}
What can we do here?

41. public class Customer {
List<Account> accounts = new ArrayList<Account>();
public void isAccountInactive(String accountNumber, int
maxInactivityAllowed) {
int i = accounts.indexOf(new Account(accountNumber));
Account account = accounts.get(i);
account.inactive(maxInactivityAllowed);account.inactive(maxInactivityAllowed);
}
}
Look for staticsLook for statics

42. Look out for static methods and hints (by IDE) to
make methods static
Use the suggestion by not accepting it

43. class Account {
Transactions transactions;
Customer customer;
boolean suggestLinkingOfAccounts() {
if (transactions != null) {
Transactions customerXAs = transactions.with(customer);
return customerXAs.totalAmount() > 10000;
}
return false;
}
}
Suggest linking of accounts?

44. class Account {
Transactions transactions;
Customer customer;
boolean suggestLinkingOfAccounts() {
if (transactions != null) {
return transactions.withAndInExcessOf(customer, 10000);return transactions.withAndInExcessOf(customer, 10000);
}
return false;
}
}
Suggest linking of accounts?Suggest linking of accounts?

45. Tell don't ask at second level
(unless for performance reasons)

46. class Customer {
Transactions todaysTransactions;
Accounts accounts;
public Customer(Accounts account, Date date) {
this.accounts = accounts;
todaysTransactions = accounts.TransactionDoneOn(date);
}
Transactions todaysTransaction() {
Return todaysTransactions;
}
}
Today's transactions

47. class Customer {
Accounts accounts;
public Customer(Accounts account)public Customer(Accounts account) {
this.accounts = accounts;
}
Transactions allTransactionsOn(Date date) {Transactions allTransactionsOn(Date date) {
Return accounts.TransactionDoneOn(date);Return accounts.TransactionDoneOn(date);
}}
}
Today's transactions

48. Model state based on domain
State shouldn't be based on object's usage,
methods are for that

49. Suggestions might not always apply