Aula 03 - Orientação a Objetos

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Projeto de Software / P3 
Orientação a Objetos: Conceitos Básicos 
Márcio Ribeiro 
marcio@ic.ufal.br 
twitter.com/marciomribeiro 
Procedural	
  Programming	
  
3 
Func0on-­‐based	
  
§  Programming	
  unit:	
  func0on	
  
§  We	
  group	
  func0ons	
  to	
  form	
  a	
  program	
  
4 
What	
  is	
  the	
  size	
  of	
  an	
  array?	
  
23 12 25 32 33 53 21 2 … 89 
function doSomething() {!
 $array = array(23,12,25,32,33,53,21,2,...,89);!
 for ($i = 0; $i < count($array); $i++) {!
 ...!
 }!
}!
The array does NOT know! 
5 
Use	
  this	
  idea	
  for	
  all	
  objects	
  in	
  the	
  world…	
  
What? 
You don’t know your age? 
You also don’t know if you 
are romantic? 
6 
The	
  array	
  could	
  “know”	
  its	
  own	
  size!	
  
23 12 25 32 33 53 21 2 … 89 
count(array);!
!
versus 
!
array.size();!
Object-­‐Oriented	
  Programming	
  
8 
Collec0on	
  of	
  interac0ng	
  objects	
  
Object 1 
Object 5 
Object 2 
Object 3 
Object 4 
9 
What	
  can	
  we	
  model	
  with	
  objects?	
  
§  Tangible	
  things	
  
§  Airplane	
  
§  Seat	
  
§  Conceptual	
  things	
  
§  Posi0on	
  job	
  (pilot)	
  
§  Date	
  	
  
§  Processes	
  
§  Sor0ng	
  passengers	
  
§  Finding	
  a	
  passenger	
  
From	
  http://www.cs.brown.edu/courses/cs015/lectures.html	
  
10 
Objects	
  have	
  
§  Proper0es	
  
§  Features	
  that	
  describe	
  them	
  
§  Capabili0es	
  
§  What	
  they	
  can	
  do,	
  how	
  they	
  behave	
  
11 
Proper0es	
  
§  Features	
  that	
  describe	
  the	
  object	
  
§  Constant	
  or	
  variable	
  
§  Proper0es	
  may	
  themselves	
  be	
  objects	
  
engines 
speed 
capacity 
______________ 
______________ 
______________ 
12 
State	
  
Collec0on	
  of	
  all	
  of	
  an	
  object’s	
  
proper0es	
  and	
  their	
  values;	
  it	
  changes	
  
if	
  any	
  property	
  value	
  changes	
  
From	
  http://www.cs.brown.edu/courses/cs015/lectures.html	
  
13 
Capabili0es	
  
§  Allow	
  objects	
  to	
  perform	
  ac0ons	
  
§  Ini0al	
  state	
  
§  Change	
  state	
  
§  Responses	
  based	
  on	
  proper0es	
  
§  Objects	
  “know”	
  how	
  to	
  do	
  things	
  
accelerate 
decelerate 
______________ 
______________ 
______________ 
14 
Modeling	
  objects	
  
§  How	
  to	
  model	
  every	
  object	
  in	
  earth?	
  
§  Commonali0es	
  
§  Variabili0es	
  
§  We	
  need	
  a	
  way	
  to	
  create	
  objects	
  sets	
  taking	
  the	
  advantage	
  
of	
  their	
  commonali0es	
  
15 
Classifica0on	
  
§  Group	
   of	
   objects	
   that	
   has	
   the	
   same	
   set	
   of	
   proper0es	
   and	
  
capabili0es	
  
§  Define	
  an	
  object	
  type:	
  AircraX	
  
Airline company: GOL 
Manufacturer: Boeing 
Model: 737 
N. of engines: 2 
Capacity: 174 
Airline company: Lufthansa 
Manufacturer: Boeing 
Model: 747 
N. of engines: 4 
Capacity: 540 
accelerate 
decelerate 
accelerate 
decelerate 
How	
  to	
  create	
  an	
  object?	
  
17 
Think	
  about	
  it	
  
§  An	
  object	
  has:	
  
§  Proper0es	
  
§  Capabili0es	
  
§  An	
  object	
  know	
  how	
  to	
  do	
  things…	
  
§  Proper0es	
  and	
  capabili0es	
  should	
  be	
  “inside”	
  the	
  object	
  
§  So,	
  we	
  can	
  use	
  a	
  box	
  to	
  throw	
  everything	
  in	
  it	
  
18 
Let’s	
  create	
  a	
  snack	
  object…	
  
§  …	
  using	
  a	
  box	
  
We	
  need	
  a	
  TEMPLATE!	
  
20 
Template	
  to	
  organize	
  our	
  snack	
  
21 
Class	
  
§  Defines	
  a	
  template	
  for	
  making	
  objects	
  instances	
  
§  Defines	
  proper0es	
  as	
  a\ributes	
  
	
  
§  Defines	
  capabili0es	
  as	
  methods	
  
From	
  http://www.cs.brown.edu/courses/cs015/lectures.html	
  
22 
Objects	
  instances:	
  individual	
  objects	
  
§  Made	
  from	
  class	
  
§  Many	
  instances	
  of	
  the	
  same	
  class	
  
§  Same	
  a\ributes,	
  poten0ally	
  with	
  different	
  values	
  
From	
  http://www.cs.brown.edu/courses/cs015/lectures.html	
  
23 
Implemen0ng	
  a	
  Class	
  in	
  Java	
  
public class Aircraft {	
	
 String manufacturer;	
 int speed;	
 int numberOfEngines;	
	
 public Aircraft(String manufacturer,	
 int speed,	
 int numberOfEngines) {	
	
 this.manufacturer = manufacturer;	
 this.speed = speed;	
 this.numberOfEngines = numberOfEngines;	
 }	
	
 public void accelerate(int speed) {	
 …	
 }	
		
}!
Properties 
(attributes) 
Capabilities 
(methods) 
24 
No0ce	
  that…	
  
§  There	
  is	
  a	
  special	
  method	
  named	
  “constructor”	
  
§  Code	
  conven0ons!	
  
§  numberOfEngines	
  
§  AircraX	
  (class);	
  AircraX	
  (constructor)	
  
25 
How	
  to	
  instan0ate	
  an	
  object	
  of	
  AircraX?!	
  
§  New	
  operator:	
  
§  Used	
  to	
  create	
  instances	
  
§  We	
  call	
  the	
  constructor	
  capability	
  
public class Main {	
	
 public static void main (String[] args) {	
 Aircraft boeing737 = new Aircraft(“Boeing”, 0, 2);	
 	
 boeing737.accelerate(840);	
 }	
		
}!
26 
Changing	
  the	
  object	
  state	
  
§  Changing	
  the	
  speed	
  
§  We	
  do	
  it	
  by	
  calling	
  accelerate…	
  
§  …	
  or	
  by	
  assigning	
  a	
  new	
  value	
  to	
  speed	
  directly	
  
§  Can	
  you	
  see	
  any	
  problem?!	
  
public class Main {	
	
 public static void main (String[] args) {	
 Aircraft boeing737 = new Aircraft(“Boeing”, 0, 2);	
 	
 boeing737.accelerate(840);	
	
 boeing737.speed = -840;	
 }	
	
}!
27 
Encapsula0on	
  
§  Access	
  to	
  a\ributes	
  only	
  through	
  the	
  methods!	
  
§  Protects	
  the	
  object	
  state	
  
public class Aircraft {	
	
 private String manufacturer;	
 private int speed;	
 private int numberOfEngines;	
	
 public void accelerate(int speed) {	
 if ((speed >= 0) && (speed > this.speed)) {	
 this.speed = speed;	
 }	
 }	
		
}!
Protect 
your 
attributes! 
28 
Encapsula0on	
  
Private 
Properties 
Public 
Capacity 
Public 
Capacity 
Public 
Capacity 
Public 
Capacity 
Public 
Capacity 
Public 
Capacity 
Public 
Capacity 
Public 
Capacity 
From	
  http://www.cs.brown.edu/courses/cs015/lectures.html	
  
29 
Now…	
  
§  …	
  nega0ve	
  speeds	
  are	
  not	
  allowed!	
  
public class Main {	
	
 public static void main (String[] args) {	
 Aircraft boeing737 = new Aircraft(“Boeing”, 0, 2);	
 	
 boeing737.accelerate(-840);	
	
 boeing737.speed = -840;	
 }	
	
}!
30 
Abstrac0on	
  
§  We	
  don’t	
  care	
  about	
  how	
  the	
  methods	
  are	
  implemented!	
  
§  How	
  does	
  “accelerate”	
  work?	
  WE	
  DON’T	
  CARE!	
  
§  How	
  does	
  “decelerate”	
  work?	
  Again,	
  WE	
  DON’T	
  CARE!	
  
§  So,	
  we	
  hide	
  the	
  strategy	
  used	
  in	
  the	
  implementa0on	
  
§  Focus	
  on	
  “what”	
  
§  Forget	
  about	
  “how”	
  
How What 
Procedural	
  Programming	
  
versus	
  
Object-­‐Oriented	
  Programming	
  
32 
Array	
  size	
  example	
  
function doSomething() {!
 $array = array(23,12,25,32,33,53,21,2,...,89);!
 for ($i = 0; $i < count($array); $i++) {!
 ...!
 }!
}!
public class Vector {	
	
 private Object[] elementData;	
	
 public int size() {	
 …	
 }	
		
}!
Exercises	
  
34 
Exercises	
  
§  Think	
  about	
  an	
  object	
  
§  Define	
  its	
  proper0es	
  
§  Define	
  its	
  capabili0es	
  
§  Are	
  the	
  proper0es	
  other	
  objects?	
  
§  If	
  so,	
  define	
  its	
  proper0es	
  and	
  capabili0es	
  as	
  well