Aula 10 - Interfaces e Polimorfismo

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Projeto de Software / Programação 3 
Interfaces e Polimorfismo 
Márcio Ribeiro 
marcio@ic.ufal.br 
twitter.com/marciomribeiro 
2 
Parallel	
  development	
  
§  Implement	
  a	
  class	
  to	
  save,	
  remove,	
  and	
  find	
  ShoppingCarts.	
  
A	
  ShoppingCart	
  has	
  an	
  ArrayList	
  of	
  Products	
  
§  Students	
  of	
  Group	
  A	
  
•  FilePersistence	
  
§  Students	
  of	
  Group	
  B	
  
•  JSONPersistence	
  
§  Me	
  
•  Client	
  code	
  
Probably,	
  there	
  is	
  a	
  mess!	
  
Our	
  code	
  do	
  not	
  fit	
  together!	
  
There	
  are	
  conflicts!	
  
In	
  addiFon,	
  it	
  has	
  no	
  flexibility:	
  
how	
  to	
  easily	
  exchange	
  
File	
  for	
  JSON?	
  
5 
FilePersistence	
  and	
  JSONPersistence	
  
§  They	
  are	
  certainly	
  different…	
  
§  For	
  example:	
  different	
  composiFon	
  of	
  objects	
  
•  FileOutputStream	
  versus	
  JsonWriter	
  
•  FileInputStream	
  versus	
  JsonReader	
  
§  So,	
  they	
  do	
  not	
  inherit	
  from	
  a	
  common	
  superclass…	
  
§  Does	
  it	
  mean	
  they	
  do	
  not	
  have	
  similariFes?!	
  
Tamirys
Nota
Elas fazem a mesma coisa praticamente, que são os métodos de save, remove e find, mas de diferentes formas.
So,	
  	
  
save 
remove find 
Tamirys
Nota
Existem diferenças em COMO são salvos, mas as capacidades são as mesmas: remove, save e find.
They	
  do	
  have	
  similariFes!	
  
They	
  share	
  the	
  same	
  capabiliFes!	
  
Interfaces	
  
Tamirys
Nota
AGIR COMO -> CONTRATO
9 
Interfaces	
  
§  No	
  default	
  implementaFon:	
  nothing	
  to	
  inherit!	
  
§  Interfaces	
   are	
   mainly	
   used	
   to	
   factor	
   out	
   responsibiliFes	
  
among	
  different	
  classes	
  whose	
  only	
  commonality	
  are	
  specific	
  
shared	
  capabiliFes	
  
§  Much	
   like	
   two	
   objects	
   are	
   related	
   if	
   they	
   inherit	
   from	
   the	
  
same	
   superclass,	
   objects	
   can	
   be	
   related	
   if	
   they	
   implement	
  
the	
  same	
  interface!	
  
§  Interfaces	
  model	
  “act	
  as”	
  relaFonships	
  
From	
  http://www.cs.brown.edu/courses/cs015/lectures.html	
  
Tamirys
Realce
Tamirys
Nota
Unmarked definida por Tamirys
10 
PersistenceSession	
  interface	
  
§  Interfaces	
  only	
  declare	
  capabiliFes	
  that	
  the	
  object	
  must	
  have	
  
§  There	
  are	
  no	
  definiFons	
  with	
  code!	
  
public interface PersistenceSession {	
	
 void save(ShoppingCart shoppingCart);	
		
 void remove(ShoppingCart shoppingCart);	
		
 ShoppingCart find(int customerID);	
		
}	
Tamirys
Nota
Inteface: Comum a todas as classes que AGEM como alguma coisa (classe).
11 
Contract	
  
§  Interfaces	
  define	
  contracts…	
  
§  …	
  and	
  force	
  implementers	
  to	
  obey	
  it!	
  
§  Minimizes	
  conflicts	
  
Tamirys
Nota
Implements: Colocar o contrato em prática.
12 
Here	
  is	
  one	
  implementaFon	
  of	
  PersistenceSession…	
  
public class FilePersistence implements PersistenceSession {	
	
 @Override	
 public void save(ShoppingCart shoppingCart) {	
 …	
 }	
	
 @Override	
 public void remove(ShoppingCart shoppingCart) {	
 …	
 }	
	
 @Override	
 public ShoppingCart find(int customerID) {	
 …	
 } 		
}	
Tamirys
Nota
Implements: Age como.nullnullFilePersistence AGE COMO PersistenceSession.
13 
…	
  and	
  here	
  is	
  another…	
  
public class JSONPersistence implements PersistenceSession {	
	
 @Override	
 public void save(ShoppingCart shoppingCart) {	
 …	
 }	
	
 @Override	
 public void remove(ShoppingCart shoppingCart) {	
 …	
 }	
	
 @Override	
 public ShoppingCart find(int customerID) {	
 …	
 } 		
}	
<<interface>>!
PersistenceSession!
+ save() : void!
+ remove() : void!
+ find() : Product!
FilePersistence!
+ save() : void"
+ remove() : void"
+ find() : Product"
JSONPersistence!
+ save() : void"
+ remove() : void"
+ find() : Product"
Tamirys
Nota
LINHA PONTILHADA: AGEM COMO.
Tamirys
Nota
ITÁLICO, ASSIM COMO CLASSE ABSTRATA. MAS, PARA DIFERENCIAR DE CLASSE ABSTRATA É UTILIZADO O ESTERIÓTIPO <<interface>>.
Now,	
  look	
  how	
  great	
  
interfaces	
  are!	
  
16 
Parallel	
  development	
  revisited:	
  two	
  developers	
  
Okay! I call it and 
then I manipulate the 
ShoppingCart data 
I will provide a 
createPersistenceSession 
method 
§  They	
   are	
   talking	
   about	
   how	
   they	
   will	
   implement	
   the	
  
ShoppingCart	
  persistence	
  
17 
Calling	
  createPersistenceSession	
  
ShoppingCart shoppingCart4 = new ShoppingCart(4);	
	
PersistenceSession persistence = createPersistenceSession();	
	
persistence.save(shoppingCart4);	
ShoppingCart sc = persistence.find(4);	
persistence.remove(shoppingCart4);	
I’m gonna call 
createPersistenceSession 
as we defined… 
18 
ImplemenFng	
  createPersistenceSession	
  
public PersistenceSession createPersistenceSession() {	
 return new FilePersistence();	
}	
I will use the File 
default implementation 
19 
Change	
  request:	
  JSON	
  
public PersistenceSession createPersistenceSession() {	
 return new JSONPersistence();	
}	
Are you nuts? 
We need to use JSON! 
No problem! 
I hate 
this guy! 
Tamirys
Nota
Vai funcionar porque JSONPersistence AGE COMO PersistenceSession.
Change	
  request:	
  JSON	
  
public PersistenceSession createPersistenceSession() {	
 return new JSONPersistence();	
}	
Does my change break 
your code?! 
ShoppingCart shoppingCart4 = new ShoppingCart(4);	
	
PersistenceSession persistence = createPersistenceSession();	
	
persistence.save(shoppingCart4);	
ShoppingCart sc = persistence.find(4);	
persistence.remove(shoppingCart4);	
Of course not! 
I am using the 
PersistenceSession 
interface 
Now,	
  look	
  the	
  difference...	
  
<<interface>>!
PersistenceSession!
+ save() : void!
+ remove() : void!
+ find() : Product!
FilePersistence!
+ save() : void"
+ remove() : void"
+ find() : Product"
JSONPersistence!
+ save() : void"
+ remove() : void"
+ find() : Product"
DBPersistence!
+ save() : void"
+ remove() : void"
+ find() : Product"
+ begin() : void"
+ commit() : void"
+ rollback() : void"
Here	
  is	
  a	
  problemaFc	
  scenario…	
  
ShoppingCart shoppingCart4 = new ShoppingCart(4);	
	
DBPersistenceSession persistence =	
 (DBPersistenceSession) createPersistenceSession();	
	
persistence.begin();	
persistence.save(shoppingCart4);	
ShoppingCart sc = persistence.find(4);	
persistence.remove(shoppingCart4);	
persistence.commit();	
public PersistenceSession createPersistenceSession() {	
 return new JSONPersistence();	
}	
Okay! 
@#$%*!!! 
JSON! 
I coupled my code 
to one specific 
implementation 
Tamirys
Nota
Errado: Implementa utilizando uma classe, utilizando os métodos dessa classe. Ao implementar JSONPersistence, o erro será: Cast Exception.nullnullErrado: O código está acoplado para uma implementação e não é flexível, não é viável.nullnullCerto: Implementar para interface, tornando tudo mais genérico e mais geral.
Tamirys
Nota
Usar If persistence is InstanceOf DBPersistenceSession ...
“Program	
  to	
  an	
  interface,	
  
not	
  to	
  an	
  implementaFon”	
  
From	
  Gamma	
  et.	
  Al,	
  Design	
  Patterns:	
  Elements	
  of	
  Reusable	
  OO	
  Software	
  
Polymorphism	
  
26 
Polymorphism	
  
§  Key	
  OOP	
  concept	
  
§  Replacement	
  capacity	
  
	
  
§  MulFple	
  forms	
  of	
  response	
  to	
  the	
  same	
  message	
  
ShoppingCart shoppingCart4 = new ShoppingCart(4);	
	
PersistenceSession persistence = new createPersistenceSession();	
	
persistence.save(shoppingCart4);	
Tamirys
Nota
Polimorfismo: Este objeto é capaz de receber múltiplas formas, por conta da programação para a interface. As formas podem ser: JSON, File e DB.
27 
Dynamic	
  binding	
  
§  The	
   appropriate	
  method	
   cannot	
   be	
   determined	
   at	
   compile	
  
Fme	
  
ShoppingCart shoppingCart4 = new ShoppingCart(4);	
	
PersistenceSessionpersistence;	
	
if (userInput.equals("file")) {	
 persistence = new FilePersistence();	
} else {	
 persistence = new JSONPersistence();	
}	
	
persistence.save(shoppingCart4);	
Which save method?! 
Now	
  imagine	
  an	
  UFlity	
  class...	
  
29 
UFlity	
  class	
  
Utility!
+ getCheapest(ArrayList) : Product"
+ getMostExpensive(ArrayList) : Product"
+ getAveragePrice(ArrayList): double"
§  Implement	
  it!	
  
30 
Change	
  request	
  
§  LinkedList	
  seems	
  be[er…	
  
§  Now,	
  list	
  is	
  a	
  LinkedList	
  
	
  
§  So,	
  please,	
  implement	
  the	
  change	
  request!	
  
Product cheapest = getCheapest(list);	
…	
Product mostExpensive = getMostExpensive(list);	
…	
double average = getAveragePrice(list);	
Keep	
  this	
  in	
  mind...	
  
You	
  changed	
  MANY	
  places!	
  
Again,	
  program	
  to	
  an	
  interface,	
  
not	
  to	
  an	
  implementaFon	
  
33 
Use	
  the	
  List	
  interface!	
  
Utility!
+ getCheapest(List) : Product"
+ getMostExpensive(List) : Product"
+ getAveragePrice(List): double"
Product cheapest = getCheapest(list);	
…	
Product mostExpensive = getMostExpensive(list);	
…	
double average = getAveragePrice(list);	
List list = new ArrayList();	
List list = new LinkedList();