Objects, Classes, and Inheritance

Transcription

1 Object, Clae, and Inheritance CS111 Computer Programming Department of Computer Science Welleley College The Big Picture Some paradigm of programming: o Imperative Programming Program pecified by conditional, loop, procedure that change variable and mutable data tructure over time. o Functional Programming Program pecified a compoition of function that create new immutable data tructure. Higher-order function are important building block (e.g, map, filter o Object Oriented Programming (OOP Program pecified in term of interacting tateful object whoe behavior i decribed by method. Motivation: Python ha lot of object-oriented feature that we ve ued, but it would be nice to define our own object. Today: Study object with imple example Next time: Ue object in animation Three paradigm of programming we cover in CS111. Object and Clae 2 Object An object i a data value that ha tate and behavior. Example: Python builtin type: number, tring, lit, tuple, dictionarie, c1graphic: circle, canvae, point, Object behavior are defined by method that can be invoked on an object. A method i a function that ha direct acce to the tate of an object. For example, method of a tring object include: "Computer Science".lower( "Computer Science".plit( "Computer Science".index('c' Different kind of object can handle the ame method in different way. For example,.cale( Everything in Python i an object (even function. cale the radiu of a c1graphic le object by cale the and of a c1graphic angle object by Clae A cla define the characteritic of a et of object or intance, including: Repreentation of State: how each object' tate i tored a intance variable in that object Implementation of Behavior: how object behavior i implemented a code for each object' method A cla i ued a a template for making object of one kind. An object made from a cla i called an intance of the cla. Example of clae include: tr lit int dict Canva Polygon le Layer Object are known a intance of a cla. Type help(tr or help(canvato ee all the method of thee clae. Object and Clae 3 Object and Clae 4

2 Cla Example: Geometric Figure State i repreented by intance variable, behavior through method. Define abtract geometric figure clae for circle, rectangle, etc. (Different from thoe in c1graphic! Thee figure do not have poition and we will not draw them! State: o The tate of a circle i it radiu and o The tate of a rectangle i it,, and o The tate of a quare i it ide length and Object Memory Diagram Object memory diagram reemble thoe of r = (2, 1, "red" dictionarie. We call our clae and to dik = (50, "yellow" ditinguih them from the c1graphic = (,, le and angle clae. Object a cloud named by cla name containing intance variable (alo known a attribute, Object drawn member, a boxe or named field by cla name containing intance variable (alo known a attribute, member, or field r 2 1 dik radiu 50 "red" "yellow" Behavior: o Method that return the tate element o Method that change the tate element o Method to compute the perimeter & area of each figure Object and Clae 5 le dik radiu 50 Ditinct intance of one cla have the ame "yellow" method and intance variable name, but they each hold ditinct intance variable content. Object and Clae 6 Big idea #1: Abtraction for Object Client Ue object method to inpect or manipulate abtract tate. hape.area(.getcolor(.etcolor(c hape.area( Contract / API: Method.area(.getColor(.etColor(c Implementation Concrete tate (intance variable and behavior (method code hape Client code ue object and their method without awarene of implementation. radiu "yellow" cla : return ( math.pi *.radiu**2 Object and Clae 7 Method Contructor function invocation = (,, Method invocation.getwidth(.getheight(.getcolor(.perimeter( 54.area( 0 Some method change the tate of an object rather than returning omething.etwidth(30.etheight(25 To Notice - The contructor name tart in uppercae; it ha the name of the cla. - Some method only return a value, they are known a getter method. - Some method change value, are known a etter method. Object and Clae 8

3 Defining a Cla Keyword cla init method uually declare intance variable Definition of intance method cla name init i pecial method automatically invoked each time a new object i created cla : '''A rectangle, pecified by,, and ''' def init (, w, h, c:. = c Firt argument for any intance method i the receiver object that the method i invoked upon def getwidth(: """Return the rectangle' """... A cla a a collection of function definition. They are all bound to the cla body. Object and Clae 9 def init (, w, h, c:. = c How to invoke the contructor: (,, cla name To Notice - The pecial method init ha two undercore at the tart and end. - The contructor function implicitly call init. - Although the init definition ha four parameter, during the invocation we provide only three argument. The pecial variable doen t receive a value explicitly. Object and Clae (,, The memory diagram (tep-by-tep for creating an object during the contructor invocation. (,, 1. Invoke init method with bound to new empty object for the cla and other parameter bound to contructor function argument 2. Execute tatement in body of init method. Thee may add new intance variable to object. init. = c w h init. = c w h c c Object and Clae 11 Object and Clae 12

4 (,, 2. Execute tatement in body of init method. Thee may add new intance variable to object. (,, 2. Execute tatement in body of init method. Thee may add new intance variable to object. init. = c w h init. = c w h c c Important: in thi example, parameter name are different from intance variable name in the object (w v.. Object and Clae 13 You could name them the ame, ( intead of w, but the functionality would remain the ame. Object and Clae 14 Invoking an Intance Method: getwidth (,, 3. Object i returned a the reult of contructor invocation = (,, def getwidth(:.getwidth( 1. Invoke getwidth method with bound to the receiver object argument. getwidth Why? Behavior i aociated with an object. Invoking a method end a meage (getwidth to the receiver object. The receiver object repond by running it getwidth method on it. A method i jut a function, but need an argument indicating the receiver object. The receiver i a pecial argument that alo determine what getwidth method i ued. Object and Clae The receiver argument i written differently to ditinguih it from other argument.

5 Invoking an Intance Method: getwidth Invoking an Intance Method: getwidth = (,, def getwidth(:.getwidth( = (,, def getwidth(:.getwidth( 2. Statement in the body are executed 3. Value i returned a reult of method invocation. getwidth Object and Clae Object and Clae 18 Invoking an Intance Method: etwidth Invoking an Intance Method: etwidth = (,, def etwidth(, newwidth:. = newwidth = (,, def etwidth(, newwidth:. = newwidth 30.etWidth(30.etWidth(30 1. Invoke etwidth method with bound to receiver object and newwidth bound to the argument 2. Statement in the body are executed etwidth etwidth. = newwidth. = 30 newwidth 30 newwidth 30 Object and Clae 19 Object and Clae 20

6 Invoking an Intance Method: etwidth = (,, def etwidth(, newwidth:. = newwidth.etwidth(30 3. Here, nothing i returned a reult of method invocation ince there i no return tatement 30 Object and Clae 21 Define the entire cla cla : def init (, w, h, c:. = c def getwidth(: def getheight(: return. def getcolor(: return. def etwidth(, newwidth:. = newwidth def etheight(, newheight:. = newheight def etcolor(, newcolor:. = newcolor *. def perimeter(: return 2*(.+. def cale(, factor:. *= factor. *= factor To Notice - All method of the cla have the pecial variable a their firt parameter. - Intance variable are acceed through the pecial variable in all method. - We will normally have one line of pace after each function definition, but here it i omitted due to lack of pace. - Additionally, doctring are miing for the ame reaon. Object and Clae 22 repr method o What happen if you print a object? (try thi in the notebook = (,, print o We'd like to control how the object i repreented when diplayed. Thi i done with the repr method o Method name with around them are handled by Python in a pecial way o repr alway take only a an argument and return a tring def repr (: return ' <: {}, : {}, : {}>'.format(.,.,. Object and Clae 23 Now define the cla in the notebook cla : def init (, radiu, : """make a circle with the given radiu and """ def getradiu(: """return radiu of circle""" def getcolor(: """return of circle""" def etradiu(, radiu: """change radiu""" def etcolor(, : """change """ """return area of circle""" def perimeter(: """return perimeter (circumference"" def cale(, factor: """multiply the circle radiu by factor""" def repr (: """tring repreentation of circle""" Object and Clae 24

7 So far in thi lecture: Object are data value with tate (tored in intance variable and behavior (implemented a method. Clae are template for creating object of one kind. Cla definition tart with the keyword cla and all the method included in the body of the cla. To intantiate an object we invoke the contructor, a function with the cla' name, which create a new object intance, invoke init on it, and return the new intance. The firt parameter of a method definition i the pecial variable. Coming up Polymorphim: Code can ue any kind of object with the ame method. Why not dictionarie? Create your own MutableString cla. Inheritance: Clae can inherit method from other clae to hare and reue behavior and code. Object and Clae 25 Big idea #1+2: Abtraction and Modularity for Polymorphim (1 Code that interact with an object by invoking it method can ue any object that provide the right method, regardle of that object' internal. Client hape = [ (2,, (4, 3, "red" ] # totalarea = um([.area( for in hape ] Work for any object with a.area( method that return a number. Contract / API: Method.area( hape radiu 2 Implementation 0 1 cla : return ( math.pi *.radiu**2 4 3 "red" cla : return (. *. Object and Clae 26 Big idea #1+2: Abtraction and Modularity for Polymorphim (2 A well defined abtraction allow multiple implementation. A well defined abtraction make it eay to ue alternative implementation later. Client = ( # print.area( Work for any repreentation of. No change needed in client code when implementation change. Contract / API: Method.area( Diameter-baed diam 4 cla : return ( math.pi * (.diam/2**2 Implementation "red" Radiu-baed radiu 2 "red" cla : return ( math.pi *.radiu**2 Object and Clae 27 Why not Dictionarie? One attempt: what if we wih to wap implementation? def makedict(w, h, c: return {'figtype': '', '': w, '': h, '': c} def makedict(r, c: return {'figtype': '', 'radiu': w, '': c} def etwidth(fig, : # call with etwidth(myfig, 3, v. myfig.etwidth(3 if fig['figtype'] == '': return fig[''] = elif fig['figtype'] == '': raie Exception(' doe not upport etwidth' # Add one branch for every type of hape. def area(fig: # call with area(myfig, v. myfig.area( if fig['figtype'] == '': return fig[''] * fig[''] elif fig['figtype'] == '': return math.pi * fig['radiu']**2 # Add one branch for every type of hape. Object and Clae 28

8 Python String are Immutable In [11]: = 'eed' In [12]: [2] = 'n' TypeError: 'tr' object doe not upport item aignment Let Define a Mutable String Cla Create a MutableString cla that upport the following behavior: getting a character at a given index changing a character at a given index to a new value getting the length of the mutable tring revering the mutable tring returning a tring repreentation of the mutable tring We can create new tring that are baed on exiting tring: In [13]: 2 = [:2] + 'n' + [3:] In [14]: 2 Out[14]: 'end' But we cannot modify an exiting tring. Object and Clae 29 = MutableString('deer' print deer print.length( 4 print.getchar(3 r.revere( print reed.etchar(0, '' print eed.etchar(2, 'n' print end Object and Clae 30 Mutable String Cla cla MutableString: def init (, data: # make a MutableString def length(: # return number of char def getchar(, index: # return char at index def etchar(, index, c: # change char at index to c def revere(: # revere char of object def repr (: # tring rep of object Big idea #1+2: Abtraction and Modularity upport alternative implementation A well defined abtraction allow multiple implementation. A well defined abtraction make it eay to ue alternative implementation later. Client = MutableString( "hello world".etchar(0, 'c' print Work for both implementation of MutableString. Contract / API: Method.area( String-baed MutableString tring "cello world" Implementation Lit-baed MutableString char 'c' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' Object and Clae 31 Object and Clae 32

9 Special Syntax & Method Name for Collection Method Python tranlate pecial yntax for familiar collection operator into method name that involve. For example: len(coll coll. len ( coll[index] coll. getitem (index coll[index] = newval coll. etitem (index, newval coll1 + coll2 coll1. add (coll2 elt in coll coll. contain (elt So in MutableString,if we rename length to len, getchar to getitem, and etchar to etitem, we can ue the pecial yntax: = MutableString('deer' print len( [3] = 'p' print print [3] Object and Clae 33 A hierarchy of figure o Notice that and have: o Method with the ame name but different implementation area, perimeter, cale o Method with the ame name AND the ame implementation getcolor, etcolor o Method that are unique to each cla in name a well a implementation getradiu, getradiu, getwidth, etwidth, getheight, etheight o Thi will be the cae for other figure clae we might define too (Square, Triangle, etc. o How can we follow the DRY (Don't Repeat Your principle? Object and Clae 34 A hierarchy of figure Inheritance Fig Triang Squ Inheritance i a mechanim for abtracting over data and behavior common to different clae When cla B inherit from cla A, cla B acquire all of the attribute (intance variable and method from cla A. New attribute (intance variable and method can be added to B. B i called the child cla or the ubcla. A i called the parent cla or the upercla. Each cla "inherit" all the method of it parent Object and Clae 35 Object and Clae 36

10 Child Clae Squ (quare are a pecific kind of (rectangle cla Squ(: # inherit all method # plu can define extra one def getsidelen(: def etsidelen(, idelen:. = idelen. = idelen Abtraction: Uer of Squ doe not need to know that it ue and A ubcla method with the ame name a an inherited upercla method replace or override the inherited method. def init (, idelen, : # ue the contructor. init (, idelen, idelen, Object and Clae 37 Parent Clae and have hared method name and definition cla : cla : def init (, w, h, c:. = c def getwidth(: def getheight(: return. def getcolor(: return. def etwidth(, w: def etheight(, h: def etcolor(, c:. = c *. def perimeter(: return 2*(.+. def cale(, factor:. *= factor. *= factor def init (, rad, c:.radiu = rad. = c def getradiu(: return.radiu Code in blue how repetition. def getcolor(: return. def etradiu(, rad:.radiu = rad def etcolor(, c:. = c return pi*.radiu**2 def perimeter(: return 2*pi*.radiu def cale(, factor:.radiu *= factor Object and Clae 38 Parent Clae: Define a cla Fig to capture common tate and behavior and will inherit from Fig (ee notebook cla Fig: def init (, :. = def getcolor(: return. def etcolor(, :. = pa def perimeter(: pa def cale(, factor: pa and can now inherit from Fig cla (Fig: def init (, radiu, : Fig. init (,.radiu = radiu # other definition the ame, but no need # to define getcolor and etcolor cla (Fig: def init (,,, : Fig. init (, idth eight # other definition the ame, but no need # to define getcolor and etcolor Object and Clae 39 Object and Clae 40

11 Inheritance: c1graphic Clae are often defined in inheritance hierarchie to maximize haring and reue of behavior (method code. Object and Clae 41 Benefit of Object-Oriented Programming Modularity: Common tate and behavior packaged up Polymorphim: Same method can do different thing for different type of object (e.g., perimeter for Fig Encapulation: Hide detail of how we tore the object' information (e.g., different implementation of MutableString Not actually enforced in Python. Java upport private variable and method that can be ued only within method of the cla. Inheritance: Subclae hare tate and behavior of uperclae (avoiding repetition of code Object and Clae 42