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2. Collaborating objects
How to structure?
Object rules
Collaborating objects
Object structure
Follow a paradigm
OOP
Object behavior 1 2 3 4
1. Abstraction
2. Encapsulation
3. Inheritance
4. ……………..

3. Advanced OO concepts

4. I don’t teach SE differently…
Male vs Female
ISE vs CS vs CEG
2nd, 3rd, 4th year
2nd, 3rd, 4th year
2nd, 3rd, 4th year
2nd, 3rd, 4th year
… but you learn SE differently

5. I want to register Students
Example 1
Gaaah….!
I want to register Students
UGStudent
PGStudent
NGStudent
Here you go!
Admin

6. Example 1 foreach Student s: s.register(); … Can? Cannot! UGStudent
Admin
NGStudent
PGStudent

7. Example 2
Storage s; …
s.load();
Logic
Storage

8. Example 2
TestDriver
Logic
Storage
StorageStub

9. Example 2
TestDriver
Logic
Storage
StorageStub
Dependency Injection

10. Dependency Injection Example 2 Storage s; … s.load(); TestDriver Logic
StorageStub
setStorage(Storage)
Storage s; …
void setStorage(Storage s){
this.s = s; }
Dependency Injection

11. Dependency Injection Example 2 Storage s; … s.load(); TestDriver Logic
StorageStub
setStorage(Storage)
Storage s; …
void setStorage(Storage s){
this.s = s; }
Dependency Injection

12. Example 2 TestDriver Logic Storage setStorage(Storage) StorageStub…
void setStorage(Storage s){
this.s = s; }

13. Treat one type as another and still get the behavior of the actual object
Example 2
Example 1
TestDriver
Logic
Storage
foreach Student s:
s.register(); …
setStorage(Storage)
StorageStub
UGStudent
Storage s; …
void setStorage(Storage s){
this.s = s; }
Admin
NGStudent
PGStudent

14. Polymorphism Example 2 Example 1
Treat one type as another and still get the behavior of the actual object
Example 2
Example 1
TestDriver
Logic
Storage
foreach Student s:
s.register(); …
setStorage(Storage)
StorageStub
UGStudent
Storage s; …
void setStorage(Storage s){
this.s = s; }
Admin
NGStudent
PGStudent

15. Polymorphism

16. Polymorphism

17. Polymorphism Different game, different behavior
Same hardware/ software

18. Polymorphism Different game, different behavior
= ability to take many forms

19. Polymorphism ?

20. Polymorphism
Ability to treat different types as same type and yet get different behavior
Ability to be accepted as a different type but maintain own behavior

21. Inheritence * foreach Student s: s.register(); … :UGStudent UGStudent
Admin
Student
NGStudent
register()
PGStudent

22. * foreach Student s: s.register(); … :UGStudent UGStudent Admin
NGStudent
register()
PGStudent

23. * foreach Student s: s.register(); … :UGStudent UGStudent Admin
NGStudent
register()
PGStudent

24. Substitutability If expecting super class, can accept sub class *
foreach Student s:
s.register(); …
:UGStudent
UGStudent *
Admin
Student
NGStudent
Substitutability
PGStudent

25. Example 2 TestDriver StorageStub Logic setStorage(Storage) Storage
Logic logic; …
logic .setStorage(new StorageStub() );
Storage s;
void setStorage(Storage s){
this.s = s; }
load()
Example 2

26. Storage s; …
void setStorage(Storage s){
this.s = s; }
FileStorage
Storage
TestDriver
Logic
DBStorage
setStorage(Storage)
load()
StorageStub
Logic logic; …
logic .setStorage(new StorageStub() );
All ‘load’ the same way?

27. Overriding FileStorage Storage load() TestDriver Logic DBStorage
setStorage(Storage)
load()
load()
Overriding
StorageStub
load()

28. s.load(); load() At compile time… Storage s; … FileStorage Storage
TestDriver
Logic
DBStorage
setStorage(Storage)
load()
load()
StorageStub
load()

29. s.load(); load() :Logic s:StorageStub At run-time… Storage s; …
void setStorage(Storage s){
this.s = s; }
Storage s; …
s.load();
FileStorage
Storage
load()
TestDriver
Logic
DBStorage
setStorage(Storage)
load()
load()
Logic logic; …
logic .setStorage(new StorageStub() );
StorageStub
load()

30. s.load(); :Logic s:StorageStub load() At run-time… Storage s; …
FileStorage
Storage
load()
TestDriver
Logic
DBStorage
setStorage(Storage)
load()
load()
Logic logic; …
logic .setStorage(new StorageStub() );
StorageStub
load()

31. Dynamic binding s.load(); :Logic s:??????? load() At run-time…
Storage s; …
s.load();
FileStorage
Storage
load()
TestDriver
Logic
DBStorage
setStorage(Storage)
load()
load()
StorageStub
load()

32. s.load(); :Logic s:??????? load() At run-time… Storage s; … Cannot!
FileStorage
Storage
load()
TestDriver
Logic
DBStorage
setStorage(Storage)
load()
load()
void load (){
//do nothing }
StorageStub
Can remove?
load()

33. load() FileStorage Storage load() TestDriver Logic DBStorage
setStorage(Storage)
load()
load()
All Storage child classes should support a load method, but implementation is up to the child classes
StorageStub
load()

34. abstract void load () ; load() FileStorage Storage load() TestDriver
Logic
DBStorage
setStorage(Storage)
load()
load()
StorageStub
abstract void load () ;
load()

35. load() {abstract} Incomplete! Cannot create objects! FileStorage
TestDriver
Logic
DBStorage
setStorage(Storage)
load() {abstract}
load()
StorageStub
load()

36. Abstract method/class
FileStorage
{abstract}
Storage
load()
TestDriver
Logic
DBStorage
setStorage(Storage)
load() {abstract}
load()
StorageStub
load()

37. Abstract method/class
FileStorage
{abstract}
Storage
load()
TestDriver
Logic
{abstract}
DBStorage
setStorage(Storage)
load() {abstract}
init()
store()
StorageStub
A sub class must implement all inherited abstract methods, or it will become an abstract class itself
load()

38. subs {a|b|c} e.g. subs b feed2103 blah blah…
Which of the subclass is not substitutable?
BankSystem
Account
Throws exception if not 0 < i < 100
getAccount(int i)
i is a parameter from BankSystem to Account
AccountTypeA
AccountTypeB
getAccount(int i)
AccountTypeC
… if not 0 < i < 1000
… if not 0 < i < 100
… if not 0 < i < 10
subs {a|b|c}
e.g. subs b
feed2103 blah blah…
OR tinyurl.com/answerpost

39. Throws exception if not 0 < i < 100
BankSystem
Account
Throws exception if not 0 < i < 100
getAccount(int i)
i is a parameter from BankSystem to Account
AccountTypeA
AccountTypeB
getAccount(int i)
AccountTypeC
… if not 0 < i < 1000
… if not 0 < i < 100
… if not 0 < i < 10
To preserve substitutability, sub classes should not impose conditions that are more restrictive than the super class.

40. Substitutability
Based on Liscov Substitution Principle by Barbara Liskov
To preserve substitutability, sub classes should not impose conditions that are more restrictive than the super class.

41. Command cmd = createCommand(commandString); cmd.execute();
history.add(cmd); …
cmd.undo();
Command
execute()
undo()
Edit
Delete
Add
History
add(Command)

42. Command cmd = createCommand(commandString); cmd.execute();
history.add(cmd); …
cmd.undo();
Add
{abstract}
Command
execute()
undo() *
History
execute() {abstract}
undo() {abstract}
Delete
add(Command)
execute()
undo()
Edit
execute()
undo()

43. <<interface>>
Command cmd = createCommand(commandString);
cmd.execute();
history.add(cmd); …
cmd.undo();
Add
<<interface>>
Command
execute()
undo() *
History
execute()
undo()
Delete
add(Command)
execute()
undo()
Edit
execute()
undo()

44. Collaborating objects
How to structure?
Object rules
Collaborating objects
Object structure
Follow a paradigm
OOP
Object behavior 1 2 3 4
1. Abstraction
2. Encapsulation
3. Inheritance
4. Polymorphism