The Object Oriented Paradigm in Perspective

Since Alan Key pioneered the object oriented paradigm in the 70s, there has been a clear evolution in how we do programming and technology . Still, the OOP is one of the most misunderstood and badly applied paradigms when it comes to real programing and computing.. Also it has been subject of controversy and multiple different explanations. Does everybody understand polymorphism in the same way?
The purpose of this talk is to understand the object oriented paradigm from another perspective and getting to the essences of what is needed to understand about OOP to properly apply the paradigm when create software.

Transcript

1. The  Object  Oriented  Paradigm  in  perspec2ve  :         From  Plato  and  Aristotle  to  Alan  Kay..  and  beyond   Ruben  Gonzalez  Blanco   Nov  2013  
2. Modeling  and  Understanding  the  World   Philosopher     Computer  Scien2st  
3. The  School  of  Athens  (by  Raphael)  
4. Ideas   Things   Plato   Aristotle  
5. The  Theory  of  Ideas  from  Plato   Sensi4ve  World   Intelligible  World   Horse     Idea     Mammal     Idea     Instan4a4on  
6. First  are  the  Ideas,  then  the  Things  
7. Most  of  our  OOP  soPware  is  “Platonic”   CLASS   Object1   Object2   Object3   Create  Object   INTERFACE   Run4me  World   Design&Coding  World  
8. First  the  Code,  then  the  Objects   Interface    Shape  {                draw(Canvas  c);                move(int  x,  int  y);   }   class  Circle  implements  Shape  {            //  a:ributes              int  cx;              int  cy;              int  radius;                                    //  implementa>on              draw(Canvas  c)  {                            c.drawEllipse(cx,cy,  r);              }              move(int  x,  int  y)  {                            cx  =  cx+x;                            cy  =cy+y              }              setC(int  x,  int  y)  {                              cx=x;                              cy=y;              }              setR(int  radious)  {                              r=radious;          }   }   Main()  {          Circle  s1  =  new  Circle();          s1.setC(100,50)          s1.setR(20);          Canvas  c  =  new  DefaultCanvas();          c.addShape(s1);  //  registered  as  Shape          c.show();     }   $
   
   
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   Design&  Coding  World   Run4me  World  
9. The  Forms  Theory  from  Aristotle   Only  1  World  =  Sensi4ve  World   Concepts  or  Forms  =  Abstrac4ons   Horse     Form   Mammal   Form   Abstrac4on   (extract  the  general  from  the  par2cular)   (in  our  minds)  
10. First  are  the  Things,  then  the  Forms  
11. The  Forms  are  not  separated  from  the  Things   Horse     Form   Horse     Form   Horse     Form  
12. Form  Theory  from  Aristotle   Substance   Form   Physical  Ma`er   Accidents   (Thing)   (Idea)   (thing  proper4es  varia4ons)   •  Forms  can  have  different  levels  of  abstrac2ons   •  Ma`er  can  disappear,  Forms  persist  
13. OOP  and  Form  Theory   Substance   Form   Physical  Ma`er   Accidents   (Thing)   (Idea)   Object   Class   “Electronic”  Ma`er   (Object  in  the  Computer   Memory,  processed     by  the  CPU)   State   (Object  aKributes=status  varia4ons)   Interface   (thing  proper4es  varia4ons)   code   code   (Abstrac4on)   (Abstrac4on)   (Abstrac4on)   Sensible  World   Run4me  World  
14. “Aristotelic”  view  of  OOP:  What  it  really  ma`ers   are  the  Objects   Interface Shape { draw(Canvas c); move(int x, int y); } class Circle implements Shape { // a$ributes int cx; int cy; int radius; // implementa0on draw(Canvas c) { c.drawEllipse(cx,cy, r); } move(int x, int y) { cx = cx+x; cy =cy+y } setC(int x, int y) { cx=x; cy=y; } setR(int radious) { r=radious; } } draw() move() _Line_draw() _Line_move() X1=100 Y1=100 X2=100 Y2=200 draw() move() _Circle_draw() _Circle_move() Cx=80 Cy=90 R=50 show() addShape() removeShape() _DefaultCanvas_show() … shapes=[s1] c1:DefaultCanvas onMousePressed() onMouseMoved() onMouseReleased() _CircleTool_onMouseReleased() … canvas=c1 .. :CircleTool draw() move() _Circle_draw() _Circle_move() Cx=80 Cy=90 R=50 draw() move() _Circle_draw() _Circle_move() Cx=80 Cy=90 R=50 onMouseReleased(MouseEvent mevent) { //..get radious from mevent posi3on r = …. Circle circle = new Circle(); circle.setC(x,y); circle.setR(r); // registered as Shape canvas.addShape(circle); } App=Run4me  World   Design  &  Coding  World  
15. The  OOP  concept  revisited  40  years  later   Dr.  Alan  Kay    
16. OOP  by  Alan  Kay   "OOP  to  me  means  only  messaging,  local  reten4on  and  protec4on  and  hiding   of  state-­‐process,  and  extreme  late-­‐binding  of  all  things.  It  can  be  done  in   Smalltalk  and  in  LISP.  There  are  possibly  other  systems  in  which  this  is  possible,   but  I'm  not  aware  of  them."  -­‐  Dr.  Alan  Kay  
17. Objects  in  OOP  are  “like”  Cells   I  thought  of  objects  being  like  biological  cells  and/or  individual  computers   on  a  network,  only  able  to  communicate  with  messages  (so  messaging   came  at  the  very  beginning  -­‐-­‐  it  took  a  while  to  see  how  to  do  messaging  in  a   programming  language  efficiently  enough  to  be  useful).     h`p://www.purl.org/stefan_ram/pub/doc_kay_oop_en      
18. A  “primi2ve”  form  of  object   h`p://www.smalltalk.org/smalltalk/TheEarlyHistoryOfSmalltalk_Abstract.html    
19. Dr.  Alan  Kay  OOP  realiza2on  :  Smalltalk   Smalltalk  is  a  founda2onal  programming  language  that  is  based  on  pervasive   message  passing,  pervasive  dynamic  and  strong  typing,  pervasive  reflec4on  and   pervasive  object  orienta4on.   h`p://www.smalltalk.org/ar2cles/ar2cle_20100320_a3_Gekng_The_Message.html     Smalltalk  is  one  of  the  first  object-­‐oriented  programming  languages.  Its  design  was  influenced  by  Lisp,  Logo,   Sketchpad,  Flex  and  Simula.  Smalltalk  was  developed  as  a  research  project  at  Xerox  PARC  in  the  1970s  by  a   team  whose  members  included  Dr.  Alan  Kay,  Dan  Ingalls,  Adele  Goldberg,  Ted  Kaehler,  [Dianna  Merry-­‐ Shipiro],  Sco`  Wallace  and  others.  
20. Understanding  OOP  
21. An  Object  is  a  run4me  en4ty  that  processes   messages  using  dynamic  binding   Message   Dynamic  Binding  =  Late  Binding   Binding     mechanism   Message   Object   Message  processor  Implementa2on   Data  A`ributes=State   Opera2ons   Realiza2on   A`ributes   Interface   Opera2ons   Specifica2on   Class   Data  
22. Dynamic  Binding   Message   At  run-­‐2me   1  
23. Dynamic  Binding   Message   Binding     mechanism   At  run-­‐2me   1   2  
24. Dynamic  Binding   Message   Binding     mechanism   Binding     mechanism   Message  processor  Implementa4on   Data  A`ributes=State   Data   At  run-­‐2me   1   2   3  
25. Dynamic  Binding  =  Late  Binding   Binding     mechanism   Message  processor  Implementa4on   Data  A`ributes=State   Data   Not  Bound   1  
26. Dynamic  Binding  =  Late  Binding   Binding     mechanism   Message  processor  Implementa4on   Data  A`ributes=State   Data   Binding     mechanism   Message  processor  Implementa4on   Data  A`ributes=State   Data   Message   Not  Bound   Linked  when  the  message  is  received   1   2   •  Implementa2on  is  bound  in  the  last  moment   •  (when  a  message  is  received)  
27. The  same  object  can  change  its  implementa2on   between  messages  recep2on   Binding     mechanism   Object2   Message  processor  Implementa2on   Data  A`ributes=State   Data   Object1   1  
28. The  same  object  can  change  its  implementa2on   between  messages  recep2on   Binding     mechanism   Object2   Message  processor  Implementa2on   Data  A`ributes=State   Data   Binding     mechanism   Object2   Message  processor  Implementa2on   Data  A`ributes=State   Data   Object1   Object1   1   2  
29. An  object  can  interact  with  different   Implementa2ons  of  the  same  interface   Binding     mechanism   Object2   Message  processor  Implementa2on   Data  A`ributes=State   Binding     mechanism   Object3   Message  processor  Implementa2on   Data  A`ributes=State   Data   Data   Object1   •  Same  “Interface”  and  different  implementa2on   •  Implementa2on  can  vary  at  run2me     •  Flexibility  =  easily  recombine  exis2ng  objects  into  new  configura2ons   •  Extensibility  =  easily  add  new    behaviours  
30. Polymorphism  and  Encapsula2on   Binding     mechanism   Message  processor  Implementa4on   Data  A`ributes=State   Data   INTERFACE  (specifica2on)   Mul2ple  muta2ons  of  the  same  “Thing”     Same  object  instance  with  varia2ons  in  its  implementa2on     Same  specifica2on  (interface)  with  different  instances  realiza2ons       IMPLEMENTATION1  (realiza2on)   Binding     mechanism   Message  processor  Implementa4on   Data  A`ributes=State   Data   IMPLEMENTATION2  (realiza2on)   Hidden  
31. The  “P”  in  OOP  is  misleading       OOP  is    a  Computa4onal  Model  Not  a  Programming  Model   Can  be  implemented  in  different  programming  languages  
32. An  object  is  like  a  “virtual”  server   OOP  should  be  equals  to    Object  Oriented  Processing  
33. Most  of  the  “modern”  so  called  OO  languages  (C++,  Java,  …)  are   more  focused  on  the  Programming  part  than  the  Computa2onal   part  (the  run2me  objects)     Interface     Class     Inheritance   Messages     Object     Late  Binding   Design  &  Coding  Time   Run4me  Processing   Plato   Aristotle  
34. How  SoPware  Objects  Would  be  like  in  our   physical  World?       (  just  for  adding  clarifica>on  )    
35. How  SoPware  Objects  Would  be  like  in  our   physical  World?    The  Post-­‐World   a  PostBox  Object  
36. The  Post-­‐World   aPostboxObject   Ann   aPostBoxObject   Teacher  Postbox   (abstract  or  representa>on  box)   slot  Interface   How  it  is  seen  by  other  postBoxObjects   (only  the  interface  is  seen,  not  the  implementa>on)     As  a  Teacher  implementa>on   Abstract  Form   slot   teachLesson   Dynamic  binding  
37. Teacher   Student   Robert   Marie   Student   Ann   Dynamic  binding  
38. Teacher   Student   Robert   Marie   Student   Robert   Dynamic  binding  
39. Teacher   Student   Robert   Marie   Student   aTapeRecorder   Dynamic  binding  
40. A  C++,Java  like  “Typed”  Post-­‐World  would  be  also  possible,  but   would  be  “less”  OO  (less  polymorphic)    and  more  complicated  to   deal  with   Teacher   answerQues2on   teachLesson   Teacher   aTapeRecorder  exposed/referenced   as  a  Teacher   TapeRecorderTeacher   teachLesson   answerQues2on   play   stop   forward   rewind   aTapeRecorder  exposed/referenced    as   TapeRecoderTeacher   teachLesson   answerQues2on   play   stop   forward   rewind   concrete  slots  are  hidden   Referenced  as  Teacher   Teacher   •  Type=“slots  Interface”  is  hardcoded.   •  Different  slots  are  presented  in  different  situa2ons  (type  cas2ng)   •  To  be  “more”  OO  polymorphic,  objects  always  try  to  present  themselves  as  the  most  abstract  type  possib
41. Analysis  of  the  OOP  idea  and  how  we  have   implemented  it  during  the  past  40  years      
42. Dynamic  Binding  is  present  in  many  places     •  func2on  pointers  in  C  and  C++   •  DLLs  and  Shared  Objects   •  “CGI”  like  mechanism  in  HTTP  servers   •  GUIs     For  origins  of  dynamic  binding:   “Fundamental  Concepts  in  Programming  Languages”.  CHRISTOPHER  STRACHEY  -­‐  1967  
43. Dynamic  Binding  is  present  in  many  places   a  basic  example  in  C     typedef int (*foo)(int i);! int fooImp1(int i ) {! return 2*i;! }! ! int fooImp2(int i ) {! return i/2;! }! ! void doFoo(int i,foo afoo) {! ! int r = afoo(i);! ! printf (“result=%d\n”,r);! !! }! ! int main (int argc, char** argv){! ! foo myfoo = &fooImp1;! ! doFoo (4, myfoo); ! }! //foo  is  polymorphic   2A10F memory   … … _fooImp2(int i) fooImp2()  implementa2on   … afoo 1A001 1A001 _fooImp1(int i) fooImp1()  implementa2on   i 4 //sets  the  concrete  “instance”  
44. OOP  applica2ons  are  executed  as  a  set  of   collabora2ng  objects   :User   object   op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp
45. DrawingTool+ Selec/onTool+ Rectangle+ Line+ Circle+ Canvas+ The  so  typical  OOP  Applica2on  example…  
46. Anatomy  of  an  object  in  C,  C++,  Java…   draw()   move()   _Circle_draw()   _Circle_move()   cx=100   cy=50   r=0   s1:Shape   s1:Circle   Opera>ons     specifica>on   Opera>ons     realiza>on   a:ributes   draw()   move()   s1:Shape   How  it  is  seen  by  other  objects   s1  referenced  as  a  Shape   Dynamic  Binding   s1  is  a  Circle  class  instance     that  implements  the  Shape  interface  
47. Anatomy  of  an  object  in  C,  C++,  Java…   draw()   move()   _Circle_draw()   _Circle_move()   …   cx=100   cy=50   r=20   s1:Shape   s1:Circle   Dynamic  binding   Interface   Implementa>on   data   Opera>ons     specifica>on   Opera>ons     realiza>on   a:ributes   Interface    Shape  {                draw(Canvas  c);                move(int  x,  int  y);   }   class  Circle  implements  Shape  {            //  a:ributes              int  cx;              int  cy;              int  radius;                                    //  implementa>on              draw(Canvas  c)  {                            c.drawEllipse(cx,cy,  r);              }              move(int  x,  int  y)  {                            cx  =  cx+x;                            cy  =cy+y              }              setC(int  x,  int  y)  {                              cx=x;                              cy=y;              }              setR(int  radious)  {                              r=radious;          }   }   s1  Instan>a>on   (new)   Main()  {          Circle  s1  =  new  Circle();          s1.setC(100,50)          s1.setR(20);          Canvas  c  =  new  DefaultCanvas();          c.addShape(s1);  //  registered  as  Shape          c.show();     }   Object  s1  shown  as  “Shape”  interface  while  its   implementa>on  is  class  “Circle”   Instan&a&on  Code   Defini&on  Code  
48. A  basic  implementa2on  in  C  (not  C++)   $
    
    
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    typedef struct tShape Shape;! typedef struct tCanvas Canvas;! ! // SHAPE INTERFACE DECLARATION! struct tShape {! void (*draw) (Shape * this, Canvas * canvas);! void (*move) (Shape * this , int x, int y);! !! };! ! ! // CIRCLE CLASS DECLARATION! ! typedef struct tCircle Circle;! ! struct tCircle {! void (*draw) (Circle * this, Canvas * canvas);! void (*move) (Circle * this, int x, int y);! ! double (*circunference) (Circle * this );! !! int cx;! int cy;! int r;! ! };! ! // CIRCLE CLASS OPERATIONS! ! void _Circle_draw(Circle * this, Canvas* canvas) {! // code for drawing a circle! }! ! double _Circle_circunference(Circle * this) {! return 2*M_PI*this->r;! }! ! // code for allocating a Circle in memory! ! Circle * newCircle (int cx, int cy, int r) {! Circle * circle = malloc(sizeof(Circle));! ! circle->cx = cx;! circle->cy = cy;! circle-> r= r;! ! // filling function pointer members! ! circle->draw = &_Circle_draw;! circle->move = &_Circle_move;! circle->circunference = &_Circle_circunference;! ! return circle;! ! }! "
    
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49. draw()' move()' _Circle_draw()' _Circle_move()' …' cx=100' cy=50' r=20' s1:Shape' s1:Circle' show()' s1,s2'referenced'as'a'Shape.' Implementa9on'is'bound'dynamically'at'run9me.'' Line'and'Circle'implementa9on'is'not'known'by'Canvas'object' show()' _DefaultCanvas_show()' …' shapes= [s1,s2]' c1:DefaultCanvas' GUI' OS' c1:Canvas' draw()' draw()' move()' _Line_draw()' _Line_move()' …' x0=0' y0=0' x1=20' y1=20' s2:Shape' s2:Line' draw()' Dynamic'binding' Dynamic'binding' Dynamic'binding' class%DefaultCanvas()'{' ''''''Shape[]'shapes;' ' '''''addShape(Shape's)'{' ''''''''shapes.add(s);' ''''}' ''''show(){' '''''''for's'in'shapes'{' ''''''''''''s.draw(this);' '''''''}' '''}' }' Dynamic  binding  in  ac2on  
50. Fine…But….   Messages  are  synchronous  (like  “func2on  calls”)     Classes  are  defined  at  coding  2me,  can  not  be   changed  at  run2me.       Too  much  focus  on  coding  the  Hierarchies  (aaarg..)       No  Reflec2on  (yes  in  Java)     Late  binding  limited  to  message  processing       In  Java,  C++  like  languages  ,  inheritance  is  a  coding  mechanism,  for  reusing  both   specifica2on  (interface)  and    realiza2on  (implementa2on)  
51. Making  OOP  “survive”  in  C++,Java  like  languages    
52. Basic  Advise  for  java,c++  like  programmers     •  Programing  to  an  interface  not  an  Implementa4on   •  Composi4on  and  Delega4on  over  inheritance    
53. Basic  Advise  for  java,c++  like  programmers  I   Interface!!Shape!{! !!!!!!!draw(Canvas!c);! !!!!!!!move(int!x,!int!y);! }! class!Circle!implements!Shape!{! !!!!!//!a$ributes! !!!!!!int!cx;! !!!!!!int!cy;! !!!!!!int!radius;! !!!!!!!!!!! !!!!!!//!implementa0on! !!!!!!draw(Canvas!c)!{! !!!!!!!!!!!!!c.drawEllipse(cx,cy,!r);! !!!!!!}! !!!!!!move(int!x,!int!y)!{! !!!!!!!!!!!!!cx!=!cx+x;! !!!!!!!!!!!!!cy!=cy+y! !!!!!!}! !!!!!!setC(int!x,!int!y)!{! !!!!!!!!!!!!!!cx=x;! !!!!!!!!!!!!!!cy=y;! !!!!!!}! ! !!!!!setR(int!radious)!{! !!!!!!!!!!!!!!r=radious;! !!!!}! }! draw()! move()! setC()! setR()! _Circle_draw()! _Circle_move()! _Circle_setC()! _Circle_setR()! cx=100! cy=50! r=20! s1:Circle! Shape!instanceShape()!{! !!!!Circle!s1!=!new!Circle();! !!!!s1.setC(100,50)! !!!!s1.setR(20);! !!!!return!s1;! }! draw()! move()! setC()! setR()! _Circle_draw()! _Circle_move()! _Circle_setC()! _Circle_setR()! cx=100! cy=50! r=20! s1:Shape! draw()! move()! s1:Shape! draw()! How!it!is!seen!by!other!objects! s1!referenced!as!a!Shape! s1!referenced!as!a!Circle! Instan0a0on! Object!allocated!and!running!in!memory! Programmed!code! Reference  an  Object  (class  Circle)  as  its  most  abstract  form  possible  (Shape)  
54. Basic  Advise  for  java,c++  like  programmers  II   •  new  operator  is  hardcode   –  Instan2a2on  should  be  encapsulated  and  done  via  scrip2ng  or  declara2ve  like    mechanisms  (xml,   proper2es  file,  json…)     •  Never  instan2ate  a  concrete  object  from  other  object  methods   –  (inject  dependencies  via  abstract  refs)   class CircleDrawingTool extends DrawingTool { DefaultCanvas canvas; CircleDrawingTool(){ this.canvas = new DefaultCanvas(); } …. } class CircleDrawingTool extends DrawingTool { Canvas canvas; CircleDrawingTool(Canvas c){ this.canvas = c; } …. } void instantiation (File configurationFile) { Configuration cfg = parse(configurationFile); Canvas canvas; CircleDrawingTool ctool; if (cfg.property(“canvas.type”)== DEFAULT_CANVAS) { Canvas = new DefaultCanvas(); //… set canvas attributes } // canvas injected through constructor ctool = new CircleDrawingTool(canvas) … } Or  se:er  injec>on  
55. Basic  Advise  for  java,c++  like  programmers  III     op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp collabora2on   alloca2on   op1() op2() op3() … _class_Imp_op1() _class_Imp_op2() … a11=v1 a12=v2 … :Imp concrete  object   referenced  through  abstract  forms   Circle   Shape   Shape   op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp CircleDrawingTool   Canvas   op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp Canvas   op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp op1() op2() … _class_Imp_op1() _class_Imp_op2() … a01=v1 a02=v2 … :Imp CircleDrawingTool   dependency  injec2on   Shape   new   addShape   show   draw   draw   Clearly  Separate  object  Alloca4on  and  Set  up  (aKributes,  rela4onships)  from  objects  Collabora4on  (message  passing)   Instan4a4on   Execu4on  
56. Basic  Advise  for  java,c++  like  programmers  IV   •  A  switch-­‐case  by  type  (if  object  instanceof  )  it  is  a   symptom  of  bad  polymorphic  interface-­‐class  design   void handle(Shape* shape) { if (shape instanceof Circle) { Circle c = (Circle) shape; // handling a circle } else if (shape instanceof Line) { Line l = (Line) shape; // handling a line } else if … } void handle(Shape* shape) { shape->doHandle(); } Or  more  sophis2cated  pa`ern  (i.e  Visitor,  Chain  of  Reponsabili2es)  
57. Basic  Advise  for  java,c++  like  programmers  V   Use  Design  PaKerns   –  Help  you  to  have  polymorphic  designs  (Composite,  Builder,  Prototype,  Visitor,   Chain  of  Responsibili2es,  Decorator,  Observer….)   •  Composi4on  and  Delega4on  over  inheritance:     –  Favor  extending  the  behavior  of  an  object  by  delega2ng  its  message  processing  to  a   collabora2ng  object  that  are  part  of  it  (they  compose  it)     •  Programing  to  an  interface  not  an  Implementa4on:     –  Make  the  applica2on  work  through  a  set  of  objects  that  collaborate  between  them   through  interfaces  or  abstrac2ons  that  hide  the  concrete  implementa2ons  or  concrete   classes.   h`p://www.amazon.com/Design-­‐Pa`erns-­‐Elements-­‐Reusable-­‐Object-­‐Oriented/dp/0201633612/ref=sr_1_1?s=books&ie=UTF8&qid=1383378933&sr=1-­‐1&keywords=design+pa`erns     h`p://en.wikipedia.org/wiki/Design_Pa`erns     Gang  of  Four     Ward  Cunningham  
58. Basic  Advise  for  java,c++  like  programmers  VI   Follow  SOLID  principles  in  your  Design   •  Single  Responsibility  Principle     –  A  class  should  have  one,  and  only  one,  reason  to  change   •  Open  Closed  Principle   –  You  should  be  able  to  extend  a  classes  behavior,  without  modifying  it.  Classes  should  be  open  for   extension,  but  closed  for  modifica2on.   •  Liskov  Subs2tu2on  Principle   –  Derived  classes  must  be  subs4tutable  for  their  base  classes  and  vice  versa  as  long  as  they  are  referenced   by  the  same  abstract  interface.  The  base  abstrac2ons  must  be  really  polymorphic.     •  Interface  Segrega2on  Principle   –  Make  fine  grained  interfaces  that  are  client  specific.  Many  client  specific  interfaces  are  be`er  than  one   general  purpose  interface   •  Dependency  Inversion  Principle   –  Depend  on  abstrac4ons,  not  on  concre4ons.  Never  have  references  or  directly  instan2ate  concrete  classes   from  the  class  that  collaborates  with  them,  just  have  a  references  to  abstrac2ons.  Let  other  part  of  the   program  do  the  concrete  instan2a2on  and  control  the  injec2on  of  the  dependency  object  referenced  as  an   abstrac2on.   h`p://butunclebob.com/Ar2cleS.UncleBob.PrinciplesOfOod   “Uncle  Bob”  Robert  Cecil  Mar2n  
59. But  …  (c++,  java  like  languages)   •  S2ll  separa2on  between  Coding&Design  Time  vs   Run2me   –  Should  be  the  same  :  interac2ve  programming   •  Once  classes  and  interfaces  are  “wired”  they  can  not   be  (easily)  altered  at  run2me   •  “Cumbersome”  and  complex  mechanisms  for   separa2ng  instan2a2on  part  from  execu2on  part     –  like  spring  framework,  xml  coding,  more  than  2100   classes!!  
60. Being  Closer  to  Alan  Kay  OOP  idea  
61. Smalltalk  and  its  dialects   A  dialect  of  Smalltalk  based  on  the  concept   of  Prototypes   Opensource  Smalltalk   …..Only  in  academic  environments   | myButton |! ! myButton := Button new.! myButton label: 'press me'.! myButton action: [ myButton destroy ].! myButton open.!
62. JavaScript  object  model   h`p://www.ecma-­‐interna2onal.org/publica2ons/files/ECMA-­‐ST/Ecma-­‐262.pdf     prototype-­‐based  language   objects  are  Associa2ve  Arrays  than  can  vary  at  run2me  
63. Being  closer  to  OOP  Alan  Kay   •  Objec2ve  C,  Java,  C#  provide  dynamic  mechanisms,   most  of  them  inherited  from  smalltalk  language  and   its  tools   –  java.lang.reflect,  garbage  collector,  refactoring…   @implementation GSTimeSliderCell! ! - (void)stopTracking:(NSPoint)lastPoint at:(NSPoint)stopPoint inView:(NSView *)controlView mouseIsUp:(BOOL)flag! {! if (flag) {! [[NSNotificationCenter defaultCenter] postNotificationName:GSMouseUpNotification object:self];! }! [super stopTracking:lastPoint at:stopPoint inView:controlView mouseIsUp:flag];! }! ! @end!
64. Being  closer  to  OOP  Alan  Kay   •  Actors  model   –  Proposed  by  by  Carl  HewiK  in  1973   –  Actors  are  ac2ve  objects  which  communicate  by  message  passing   •  Erlang,  Scala    
65. Conclusions   •  We  have  been  too  platonic  and  having  imperfect   realiza2ons  of  the  OOP  idea   •  We  should  try  to  be  more  aristotlelic,  focusing  on   objects  as  message  processors  with  late  binding   •  Be  aware  of  the  Compu2ng  Science  History   –  Most  of  what  we  think  is  modern  was  invented  in  the  70s    
66. END