The document discusses SOLID principles of object-oriented design. It provides examples of code that demonstrate poor adherence to SOLID and ways the code can be refactored to better follow SOLID. Specifically, it shows how to apply the single responsibility principle, open/closed principle, Liskov substitution principle, interface segregation principle and dependency inversion principle to structure code for flexibility, reusability and maintainability.

1. SOLID Principles
Chris Weldon
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4. What is OOD?

5. Abstraction
What is OOD?

6. Abstraction Inheritance
What is OOD?

7. Abstraction Inheritance
What is OOD?
Encapsulation

8. Abstraction Inheritance
What is OOD?
Encapsulation Polymorphism

9. What is SOLID?

10. ?

11. SOLID is not a law

13. An Order

14. public class Order
{
private int _id;
private int _userId;
private Product[] _products;
public int getId() {
return this._id;
}
public void setId(int id) {
this._id = id;
}
public int getUserId() {
return this._userId;
}
public void setUserId(int userId) {
this._userId = userId;
}

15. public Product[] getProducts() {
return this._products;
}
public void addProduct(Product product) {
this._products.add(product);
}
public void setProducts(Product[] products) {
this._products = products;
}

16. private DBConnection _db;
public Order()
{
// Read from configuration to get connection string.
string connectionString = string.Empty;
Handle fileHandle = fopen("/etc/database/app.config", "r");
while (string row = readLine(fileHandle)) {
if (row.StartsWith("dbo")) {
connectionString = row;
}
}
fclose(fileHandle);
this._db = new DBConnection(connectionString);
}

17. public static Order CreateOrder(int userId, Product[] products)
{
Order order = new Order();
order.setUserId(userId);
order.setProducts(products);
return order;
}
public static Order getOrderById(int orderId)
{
if (orderId == null || orderId < 0)
{
DBCommand command = this._db.exec("SELECT * FROM orders");
Order[] results = this._db.getResults(command);
// Get the first result.
return results[0];
}
DBCommand command = this._db.exec("SELECT * FROM orders WHERE id
= ?", orderId);
Order order = this._db.getSingle($command);
return order;
}

18. public void deliverOrder()
{
try {
WebServiceConnection networkConnection =
new WebServiceConnection("http://shpt1/shipctrl.svc");
networkConnection->open();
ShippingManager shippingManager =
new ShippingManager(networkConnection);
shippingManager.setOrder(this);
shippingManager.setDeliveryType(DeliveryType.EXPRESS);
shippingManager.shipViaWebService();
this.notifyCustomer();
} catch (Exception e) {
Handle logFile = fopen("/tmp/logfile.log", "a");
fwrite("An error occurred while delivering the order.", logFile);
fclose(logFile);
}
}

19. public void notifyCustomer()
{
try {
mailer = new Mailer();
mailer.setFrom(“orders@chrisweldon.net”, “Grumpy Baby Orders”);
mailer.setSubject(“Order #” + this.getId() + “ out for
Delivery”);
mailer.setBodyText(“Your order is being shipped!”);
mailer.send();
} catch (Exception $e) {
Handle logFile = fopen("/tmp/logfile.log", "a");
fwrite("An error occurred while emailing the notice.", logFile);
fclose(logFile);
}
}
}

21. OMG

23. Next Sample

24. public void ActivateDrillBit(string customerOption) {
if (customerOption == "small") {
SmallBit drillBit = new SmallBit();
drillBit.activate();
} else if (customerOption == "medium") {
MediumBit drillBit = new MediumBit();
drillBit.activate("120hz");
} else if (customerOption == "large") {
LargeBit drillBit = new LargeBit();
drillBit.activate("240hz", Options.Water);
}
}

25. Requirements Change!
Customer Needs to Specify Options

26. public void ActivateDrillBit(string customerOption, string freq,
Options options) {
if (freq == "") freq = "240hz";
if (options == null) options = Options.NoWater;
if (customerOption == "small") {
SmallBit drillBit = new SmallBit();
drillBit.activate(freq, options);
} else if (customerOption == "medium") {
MediumBit drillBit = new MediumBit();
drillBit.activate(freq, options);
} else if (customerOption == "large") {
LargeBit drillBit = new LargeBit();
drillBit.activate(freq, options);
}
}

28. You
Broke
My
App
!@#!@

30. public class DrillBitActivator {
public void ActivateDrillBit(string customerOption) {
// ...
}
}
public class DrillBitConfigurableActivator : DrillBitActivator {
public DrillBitConfigurableActivator(string freqency, Options options) {
// Configurable!
}
public override function ActivateDrillBit(string customerOption) {
// ...
}
}

31. public function ActivateDrillBit(string customerOption)
IDrillBit .drillBit = DrillBitFactory::CreateDrillBit(customerOption);
drillBit.activate(this._freq, this._options);
}

32. Next:
A Familiar Example

33. public interface IManageCustomers {
void TakeSpecifications(Specs specs);
void ReviewSpecifications(Specs specs);
void GiveToEngineers(Specs specs);
void DealWithTheGoshDarnCustomers();
bool IsPeoplePerson();
}

34. public class GoodManager : IManageCustomers {
public void TakeSpecifications(Specs specs) {
this.ReviewSpecifications(specs);
}
public void ReviewSpecifications(Specs specs) {
// If specs seem good.
this.GiveToEngineers(specs);
}
public void GiveToEngineers(Specs specs) {
// E-mails specs to engineers.
}
public void DealWithTheGoshDarnCustomers() {
// You better believe he does.
}
public bool IsPeoplePerson() {
return true; // Absolutely!
}
}

36. public class Tom : IManageCustomers {
public void TakeSpecifications(Specs specs) {
throw new DelegateToSecretary();
}
public void ReviewSpecifications(Specs specs) {
throw new DelegateToSecretary();
}
public void GiveToEngineers(Specs specs) {
throw new DelegateToSecretary();
}
public void DealWithTheGoshDarnCustomers() {
// Your gosh darn right I do!
}
public bool IsPeoplePerson() {
return true; // I AM a people person, dammit!
}
}

38. public interface ITakeSpecifications {
function TakeSpecifications(Specs specs);
}
public interface IReviewSpecifications {
function ReviewSpecifications(Specs specs);
}
public interface IGiveToEngineers {
function GiveToEngineers(Specs specs);
}
public interface IManageCustomers {
function DealWithTheGoshDarnCustomers();
function IsPeoplePerson();
}

39. public class GoodManager :
IManageCustomers, ITakeSpecifications, IReviewSpecifications, IGiveToEngineers
{
public void TakeSpecifications(Specs specs) {
this.ReviewSpecifications(specs);
}
public void ReviewSpecifications(Specs specs) {
// If specs seem good.
this.GiveToEngineers(specs);
}
public void GiveToEngineers(Specs $specs) {
// E-mails specs to engineers.
}
public void DealWithTheGoshDarnCustomers() {
// You better believe he does.
}
public bool IsPeoplePerson() {
return true; // Absolutely!
}
}

40. public class Tom : IManageCustomers {
public void DealWithTheGoshDarnCustomers() {
// Your gosh darn right I do!
}
public bool IsPeoplePerson() {
return true; // I AM a people person, dammit!
}
}

41. Next Up

42. public interface IDataResource
{
void Load();
void Save();
}

43. public class AppSettings : IDataResource {
public void Load() {
// Load application settings.
}
public void Save() {
// Save application settings.
}
}

44. public class UserSettings : IDataResource {
public void Load() {
// Load user settings.
}
public void Save() {
// Save user settings.
}
}

45. static IDataResource[] LoadAll() {
IDataResource[] resources = new IDataResource[2] {
new AppSettings(),
new UserSettings()
};
foreach (IDataResource resource in resources) {
resource.Load();
}
return resources;
}
static void SaveAll(IDataResource[] resources) {
foreach (IDataResource resource in resources) {
resource.Save();
}
}

46. Our “Duck”

47. public class UnsaveableSettings : AppSettings {
public override void Load() {
// Loads settings.
}
public override void Save() {
throw new CannotSaveException();
}
}

48. static IDataResource[] LoadAll() {
IDataResource[] resources = new IDataResource[3] {
new AppSettings(),
new UserSettings(),
new UnsaveableSettings()
};
foreach (IDataResource resource in resources) {
resource.Load();
}
return resources;
}
static void SaveAll(IDataResource[] resources) {
foreach (IDataResource resource in resources) {
if (resource is UnsaveableSettings) continue;
resource.Save();
}
}

50. public interface IDataResource
{
void Load();
}
public interface ISaveResource
{
void Save();
}

51. public class AppSettingsLoaderBase : IDataResource {
public function Load() {
// Load application settings.
}
}

52. public class AppSettings extends AppSettingsLoaderBase
implements ISaveResource {
public function Save() {
// Save application settings.
}
}

53. public class UnsaveableSettings extends AppSettingsLoaderBase {
public override void Load() {
// Loads settings.
}
}

54. static IDataResource[] LoadAll() {
IDataResource[] resources = new IDataResource[3] {
new AppSettings(),
new UserSettings(),
new UnsaveableSettings()
};
foreach (IDataResource resource in resources) {
resource.Load();
}
return resources;
}
static void SaveAll(ISaveResource[] resources) {
foreach (ISaveResource resource in resources) {
resource.Save();
}
}

55. Final Problem

56. public class Authenticator {
private DataAccessLayer _repository;
public DataAccessLayer() {
this._repository = new DataAccessLayer();
}
public bool authenticate(string username, string password) {
string hashedPassword = md5(password);
User user = this._repository.findByUsernameAndPassword(
username, hashedPassword);
return user == null;
}
}

57. Problems?

58. Problems?
Authenticator
authenticate() : bool
DataAccessLayer
findByUsernameAndPassword : array

59. Problems?
Authenticator
authenticate() : bool
DataAccessLayer
findByUsernameAndPassword : array
Strongly coupled to DataAccessLayer

61. class Authenticator {
private DataAccessLayer _repository;
public Authenticator(DataAccessLayer repository) {
this._repository = repository;
}
public bool authenticate(string username, string password) {
string hashedPassword = md5(password);
User user = this._repository.findByUsernameAndPassword(
username, hashedPassword);
return user == null;
}
}

62. Dependency Inversion
• “High-level modules should not depend upon low level modules. They
should depend upon abstractions.
• “Abstractions should not depend upon details. Details should depend
upon abstractions.”
Robert Martin

63. Combine Principles!

64. public interface IUserRepository {
User findByUsernameAndPassword(string username, string password);
}
public class DataAccessLayer : IUserRepository {
public User findByUsernameAndPassword(string username, string password) {
// Do some database stuff.
}
}

65. class Authenticator {
private IUserRepository _repository;
public Authenticator(IUserRepository repository) {
this._repository = repository;
}
public bool authenticate(string username, string password) {
string hashedPassword = md5(password);
User user = this._repository.findByUsernameAndPassword(
username, hashedPassword);
return user == null;
}
}

66. Comparison

67. Comparison
Authenticator
authenticate() : bool
DataAccessLayer
findByUsernameAndPassword : array

68. Comparison
Authenticator
authenticate() : bool
DataAccessLayer
findByUsernameAndPassword : array

69. Comparison
Authenticator
authenticate() : bool
Authenticator
authenticate() : bool
IUserRepository
findByUsernameAndPassword : array
DataAccessLayer
findByUsernameAndPassword : array
DataAccessLayer
findByUsernameAndPassword : array

70. Benefit: Flexibility
public class WebServiceUserRepository : IUserRepository {
public User findByUsernameAndPassword(string username, string password) {
// Fetch our user through JSON or SOAP
}
}
public class OAuthRepository : IUserRepository {
public User findByUsernameAndPassword(string username, string password) {
// Connect to your favorite OAuth provider
}
}

71. Thank You!
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