Introduction to Computer Science, Shimon Schocken, IDC Herzliya. Lecture Writing Classes

Transcription

1 Introduction to Computer Science, Shimon Schocken, IDC Herzliy Lecture Writing Clsses Writing Clsses, Shimon Schocken IDC Herzliy, slide 1 Clsses Two viewpos on es: Client view: how to use n existing Server view: how to design, implement, nd min es Two min uses of es: Auxiliry es: Mth, Definition es: String, Turtle,, Where Jv es come from: The Jv stndrd librry Clsses tht or people write nd mke vilble Clsses tht I write. Writing Clsses, Shimon Schocken IDC Herzliy, slide 2

2 Outline Modulrity Abstrction Clss ntomy Fields Constructors Methods Relted topics Method overloding Revisiting Prmeter pssing Vrible life cycle Encpsultion Method ntomy Clss vs. instnce methods Types nd vlues Methods prmeters Accessors nd muttors Writing Clsses, Shimon Schocken IDC Herzliy, slide 3 Modulrity When building complex rtifct, it lwys helps to divide chllenge o smll, mngeble modules Ech module must hve Clerly defined function Contrct s to how it ercts with or modules Self-contined design tht enbles unit-testing nd locl mennce In prticulr, when we write, we hve to think how to divide things tht is supposed to do o methods Exmple: let s look t two versions of squreroot from Exercise 3 Un-modulr version Modulr version. Writing Clsses, Shimon Schocken IDC Herzliy, slide 4

3 Un-modulr version import import jv.util.scnner; SqureRoot SqureRoot finl finl EPSILON EPSILON 0.1; 0.1; min(string rgs[]) rgs[]) Scnner Scnner scn scn Scnner(System.in); Get Get user's user's input input System.out.pr("Enter number: number: "); "); x scn.nextint(); Problems with unmodulr code The I/O processing nd sqrt computtion re glued toger It s hrd to tell which prt of progrm is responsible for run-time bugs If we ll wnt to chnge I/O only, we hve to chnge entire The sqrt services re inccessible to or clients The design is not elegnt Solution: divide nd conquer. Compute Compute squre squre root root flot flot root root x / 2; 2; while while (Mth.bs((root root) root) - x) x) > EPSILON) EPSILON) improve improve guess guess root root (root (root (x (x/ root)) root)) / 2; 2; Pr Pr result result System.out.prln( The squre squre root root is: is: " sqrt(x)); sqrt(x)); Writing Clsses, Shimon Schocken IDC Herzliy, slide 5 Modulr version import import jv.util.scnner; SqureRootDemo min(string rgs[]) rgs[]) I/O Scnner Scnner scn scn Scnner(System.in); Get Get number number from from user user nd nd squre squre it it System.out.pr("Enter number: number: "); "); x scn.nextint(); System.out.prln( The squre squre root root is: is: " sqrt(x)); sqrt(x)); Wht hve we gined? Redbility Elegnce Unit testing Code re-use Prllel development Computes Computes squre squre root root function function processing sqrt( x) x) precision precision 0.1; 0.1; root root x / 2; 2; while while (Mth.bs((root root) root) - x) x) > precision) precision) root root (root (root (x (x/ root)) root)) / 2; 2; root; root; Using methods is just one exmple of modulr design Modulrity is key design objective tht comes up in numerous res in CS Modulr design begins t bstrction level. Writing Clsses, Shimon Schocken IDC Herzliy, slide 6

4 Outline Modulrity Abstrction Clss ntomy Fields Constructors Methods Relted topics Method overloding Revisiting Prmeter pssing Vrible life cycle Encpsultion Method ntomy Clss vs. instnce methods Types nd vlues Methods prmeters Accessors nd muttors Writing Clsses, Shimon Schocken IDC Herzliy, slide 7 Abstrction OO progrm design begins with bstrctions Abstrctions re used to describe objects like cr, bnk ccount, turtle, friend, etc. To describe n object bstrction, we think bout it terms of: Wht re its ttributes? ( dt-oriented view) Wht re its behviors? ( functionl view) Writing Clsses, Shimon Schocken IDC Herzliy, slide 8

5 Exmple: bstrction A is n object tht keeps nd reports time using 24- hour system Attributes: Hours Minutes Seconds Behviors: Crete clock Advnce clock 1 second Query clock time Etc. implementtion (structure only) The The clock clock stte stte (dt) (dt) hours, hours, constructor ( ( h, h, m, m, s); s); Advnce Advnce clock clock one one second second forwrd forwrd secondelpsed(); Query Query methods methods gethours(); getminutes(); getseconds(); Writing Clsses, Shimon Schocken IDC Herzliy, slide 9 Using (exmple) server The The clock clock stte stte hours, hours, constructor constructor ( ( h, h, m, m, s); s); Advnce Advnce clock clock one one second second forwrd forwrd secondelpsed(); secondelpsed(); client Using (n exmple) Some Some Query Query methods methods gethours(); gethours(); Crete Crete nd nd initilize initilize clock clock object object getminutes(); getminutes(); nytime nytime (23,59,59); (23,59,59); getseconds(); getseconds(); Advnce Advnce clock clock seconds seconds for for ( ( j j j j < < 5 5 j) j) nytime.secondelpsed(); nytime.secondelpsed(); Pr Pr clock s clock s current current time time System.out.pr( The System.out.pr( The time time in in New New York York is: is: ); ); System.out.prln(nyTime.getHours() System.out.prln(nyTime.getHours() nytime.gethours() nytime.gethours() : : nytime.getminutes() nytime.getminutes() : : nytime.getseconds()); nytime.getseconds()); Writing Clsses, Shimon Schocken IDC Herzliy, slide 10

6 Outline Modulrity Abstrction Clss ntomy Fields Constructors Methods Relted topics Method overloding Revisiting Prmeter pssing Vrible life cycle Encpsultion Method ntomy Clss vs. instnce methods Types nd vlues Methods prmeters Accessors nd muttors Writing Clsses, Shimon Schocken IDC Herzliy, slide 11 Clss members structure Clss members Fields Constructors The The clock clock stte stte hours, hours, constructor ( ( h, h, m, m, s); s); Advnce Advnce clock clock one one second second forwrd forwrd secondelpsed(); Methods Query Query methods methods gethours(); getminutes(); getseconds(); Visibility modifiers: Privte members: visible only to code; typiclly used for fields Public members: visible to code of or es; typiclly used for methods. Writing Clsses, Shimon Schocken IDC Herzliy, slide 12

7 Fields The object's stte (dt) is represented by vribles, AKA instnce vribles or fields Fields re vribles: y cn be of eir primitive or types Ech object hs nd seprte set of fields The fields re typiclly initilized by constructors. structure The The clock clock stte stte hours, hours, constructor ( ( h, h, m, m, s); s); Advnce Advnce clock clock one one second second forwrd forwrd secondelpsed(); Query Query methods methods gethours(); getminutes(); getseconds(); Writing Clsses, Shimon Schocken IDC Herzliy, slide 13 Constructor server When creting object, we normlly wish to specify its initil stte The The clock clock stte stte This is done using hours, hours, constructor: specil method which is Cretes Cretes object object nd nd initilizes invoked by commnd its its fields fields using using supplied supplied vlues vlues c ( ); ( ( h, h, m, m, s) s) hours hours h; h; minutes minutes m; m; client seconds seconds s; s; Public Public SomeClss SomeClss Or Or methods methods follow follow c1; c1; c1 c1 (12,0,0); c2 c2 (10,60,45); This client code cuses server code to crete two objects. Writing Clsses, Shimon Schocken IDC Herzliy, slide 14

8 The this keyword The The clock clock stte stte hours, hours, Constructor: one one style style ( ( h, h, m, m, s) s) hours hours h; h; minutes minutes m; m; seconds seconds s; s; Inside n instnce method, this keyword refers to object on which method is currently operting this cn be thought of s generic object vrible, poing to current object. Constructor: Constructor: nor nor style style ( ( hours, hours,,, seconds) seconds) ) this.hours hours hours hours; hours; this.minutes minutes minutes; minutes; this.seconds seconds seconds Constructor Or Or methods methods follow follow Writing Clsses, Shimon Schocken IDC Herzliy, slide 15 Constructor definition, properly documented using Jvdoc hours, hours, / / Cretes Cretes clock clock nd nd set set it it to to specified specified time. time. If If one one of of prmeters prmeters is is not not in in llowed llowed rnge, rnge, constructor constructor hs hs no no effect effect on on clock s clock s hours hours The The hours hours to to be be set set minutes minutes The The minutes minutes to to be be set set seconds seconds The The seconds seconds to to be be set set (0-59) (0-59) / / Resulting API, following jvdoc.jv ( ( hours, hours, seconds) seconds) if if ((seconds ((seconds > > 0) 0) && && (seconds (seconds < < 60) 60) && && (minutes (minutes > > 0) 0) && && (minutes (minutes < < 60) 60) && && (hours (hours > > 0) 0) && && (hours (hours < < 24)) 24)) this.hours this.hours hours; hours; this.minutes this.minutes minutes; minutes; this.seconds this.seconds Writing Clsses, Shimon Schocken IDC Herzliy, slide 16

9 Constructor ntomy, behind scene Creting Cloc object (exmple) SomeClss SomeClss c1, c1, c2, c2, c3; c3; c1 c1 (13,0,0); c2 c2 (22,10,1); When n object vrible is declred, only reference vrible is llocted The object dt is creted if nd when constructor is invoked vi Memory c c1 c3 c1 object c2 object Writing Clsses, Shimon Schocken IDC Herzliy, slide 17 Outline Modulrity Abstrction Clss ntomy Fields Constructors Methods Relted topics Method overloding Revisiting Prmeter pssing Vrible life cycle Encpsultion Method ntomy Clss vs. instnce methods Types nd vlues Methods prmeters Accessors nd muttors Writing Clsses, Shimon Schocken IDC Herzliy, slide 18

10 Methods A method, k subroutine, is stnd-lone piece of code designed to crry out well defined computtion or opertion Clss () methods: ssocited with level. Exmple: sqrt Instnce methods: methods tht re ssocited with objects. Exmple: turnright method ssocited with prticulr Turtle objects Clss methods provide generl-purpose functionlity Instnce methods implement object behviors. Writing Clsses, Shimon Schocken IDC Herzliy, slide 19 Instnce method exmple server hours, hours, Constructor (omitted) (omitted) The desired behviors of object re implemented by methods. Ech method is designed to perform one bstrct opertion Advnce Advnce clock clock by by 1 second second client secondelpsed() Public if if (seconds (seconds < 59) Public someclss someclss 59) else else c (12,0,0); seconds seconds c.secondelpsed(); if if (minutes (minutes < 59) 59) minutes minutes ; else else minutes minutes hours hours hours hours < 24 24? Hours Hours 1 : 1; 1; Semntics: invoke secondelpsed method on c (or methods) ( object tht c refers to). methods) Writing Clsses, Shimon Schocken IDC Herzliy, slide 20

11 Method types nd vlues client SomeClss SomeClss server c (0,0,0); hours, hours, Invoking Invoking method method c.hourelpsed(2); hourelpsed() Invoking Invoking typed typed methods methods hours hours (hours (hours 1) 1) % 24; 24; h c.gethours(); System.out.prln(c.toString); gethours() gethours() hours; hours; String String tostring() tostring() (hours (hours "\t" "\t" minutes minutes "\t" "\t" seconds); seconds); Void method: hs no type nd no vlue Typed method: hs eir primitive type or n object type. Returns vlue tht must conform to method s type. Writing Clsses, Shimon Schocken IDC Herzliy, slide 21 Method prmeters Methods my or my not hve prmeters A prmeter is like locl vrible which is initilized by method s cller. Forml prmeters cllee settime( hours, hours, seconds) seconds) if if ((seconds ((seconds > > 0) 0) && &&(seconds < 60) 60) && && (minutes (minutes > > 0) 0) && &&(minutes < 60) 60) && && (hours (hours > > 0) 0) && &&(hours < 24)) 24)) this.hours this.hours hours; hours; cller this.minutes minutes; minutes; Public this.seconds Public someclss someclss c1 c1 (12, (12, 0, 0, 0); 0); no no effect effect if if input input is is invlid invlid c1.settime(2, 15, 15, 0); 0); Actul prmeters (rguments) Writing Clsses, Shimon Schocken IDC Herzliy, slide 22

12 By wy Current Current time time represented by by clock clock hours, hours, Now tht we hve settime method, it mkes sense to re-define constructor. Constructor ( ( hours, hours, seconds) seconds) settime(hours, seconds); seconds); Sets Sets time time to to given given vlues vlues settime( hours, hours, seconds) seconds) if if ((seconds ((seconds > > 0) 0) && &&(seconds < 60) 60) && && (minutes (minutes > > 0) 0) && &&(minutes < 60) 60) && && (hours (hours > > 0) 0) && &&(hours < 24)) 24)) this.hours this.hours hours; hours; this.minutes minutes; minutes; this.seconds no no effect effect if if input input is is invlid invlid Writing Clsses, Shimon Schocken IDC Herzliy, slide 23 Accessor methods server hours, hours, Accessor methods: used to query nd informtion bout vribles Enble controlled ccess to vribles from outside. gethours() gethours() hours; hours; getminutes() minutes; minutes; getseconds() client someclss someclss c (12,0,0); h c.gethours(); System.out.pr(c.getSeconds) Writing Clsses, Shimon Schocken IDC Herzliy, slide 24

13 Muttor methods server hours, hours, sethours( hours) hours) if if ((h ((h > 0) 0) && &&(h (h < < 24)) 24)) this.hours this.hours hours; hours; setminutes( minutes) minutes) if if ((m ((m > > 0) 0) && &&(m (m < 60)) 60)) this.minutes minutes; minutes; setseconds( seconds) seconds) if if ((s ((s > > 0) 0) && &&(s (s < 60)) 60)) this.seconds Muttor methods: used to set vlues of vribles Enble controlled updte of vribles from outside. client someclss someclss dedlinehour 2 2 c (12,0,0); c.sethours(17); c.sethours(dedlinehour-1); c.setminutes(scn.nextint()); Writing Clsses, Shimon Schocken IDC Herzliy, slide 25 Method invoction exmples Demo Demo min(string[] rgs) rgs) c1 c1 (12,59,59); c2 c2 (12,59,59); h gethours(); h c1.gethours()); c1.secondelpsed(); Error! Which clock re we tlking bout? Invoking method System.out.prln(c1.getHours()); 1 System.out.prln(c2.getHours()); x Mth.sqrt(c2.getMinutes()); Sttic method invoction Instnce method invoction Writing Clsses, Shimon Schocken IDC Herzliy, slide 26

14 Outline Modulrity Abstrction Clss ntomy Fields Constructors Methods Relted topics Method overloding Revisiting Prmeter pssing Vrible life cycle Encpsultion Method ntomy Clss vs. instnce methods Types nd vlues Methods prmeters Accessors nd muttors Writing Clsses, Shimon Schocken IDC Herzliy, slide 27 Method overloding cllee MyMth MyMth sum sum ( ( x, x, y)) y)) x x y; y; sum sum ( ( x, x, y, y, z)) z)) x x y y z; z; sum sum ( ( x, x, y)) y)) x x y; y; cller SomeClss SomeClss System.Out.prln(MyMth.sum(5,3)); System.Out.prln(MyMth.sum(5,3)); Will Will prs prs 8 8 System.Out.prln(MyMth.sum(7,2,8)); System.Out.prln(MyMth.sum(7,2,8)); Will Will prs prs System.Out.prln(MyMth.sum(5.3,4)); System.Out.prln(MyMth.sum(5.3,4)); Will Will pr pr Jv llows defining different methods with sme nme Method signture: method nme, prmeter types, prmeter nmes Jv knows which method to invoke ccording to method signture implied by cller Advntges: Promotes shorter, fewer, nd redble method nmes. Writing Clsses, Shimon Schocken IDC Herzliy, slide 28

15 Method overloding SqureRootDemo min(string rgs[]) rgs[]) System.out.prln(sqrt(17) sqrt(17)); System.out.prln(sqrt(17, sqrt(17, sqrt(17, )); )); sqrt( sqrt( x) x) sqrt(x,0.1); sqrt( sqrt( x, x, precision) precision) root root x / 2; 2; while while (Mth.bs((root root) root) - x) x) > precision) precision) root root (root (root (x (x/ root)) root)) / 2; 2; root; root; Writing Clsses, Shimon Schocken IDC Herzliy, slide 29 Constructor overloding hours, hours, server Constructors Constructors ( ( hours, hours, seconds) seconds) settime(hours, settime(hours, seconds); seconds); client ( ( hours) hours) someclss someclss this.hours this.hours hours; hours; this.minutes this.minutes c1 c1 (12,59,59); this.seconds this.seconds c2 c2 (12); c3 c3 (); (); Alterntive Alterntive definition: definition: ( ( hours) hours) this(hours, this(hours, 0 0,0);,0); () () hours hours minutes minutes seconds seconds An overloded constructor provides multiple wys to set up object. Writing Clsses, Shimon Schocken IDC Herzliy, slide 30

16 revisited Account Account numbers numbers re re llocted llocted from from 1 1 onwrd. onwrd. nextaccountnumber nextaccountnumber 1; 1; Fields Fields number; number; Account Account number number String String owner; owner; Account Account owner owner blnce; blnce; Current Current blnce blnce Sets Sets up up ccount ccount (String (String owner, owner, blnce) blnce) this.number this.number nextaccountnumber; nextaccountnumber; this.owner this.owner owner; owner; this.blnce this.blnce blnce; blnce; Returns Returns textul textul object object description description String String tostring tostring () () (number (number "\t" "\t" owner owner "\t" "\t" () () blnce); blnce); Demo Demo server (Only subset of members is shown) client min(string[] min(string[] rgs) rgs) liceaccount liceaccount ("Alice", ("Alice", 0); 0); bobaccount bobaccount ("Bob", ("Bob", 100); 100); System.out.prln(liceAccount.toString()); System.out.prln(liceAccount.toString()); System.out.prln(bobAccount.toString()); System.out.prln(bobAccount.toString()); Writing Clsses, Shimon Schocken IDC Herzliy, slide 31 revisited Only Only subset subset of of members members is is shown shown server bnkdeposits bnkdeposits client Demo Demo Fields Fields number; number; min(string[] min(string[] rgs) rgs) String String owner; owner; liceaccount liceaccount ("Alice", ("Alice", 0); 0); blnce; blnce; bobaccount ("Bob", 100); bobaccount ("Bob", 100); System.out.prln(liceAccount.toString()); Hndles Hndles deposit System.out.prln(liceAccount.toString()); deposit deposit deposit ( ( mount) mount) System.out.prln(bobAccount.toString()); System.out.prln(bobAccount.toString()); chrge chrge commission(mount); commission(mount); blnce blnce blnce blnce mount mount - - chrge; liceaccount.deposit(1000); chrge; liceaccount.deposit(1000); bnkdeposits bnkdeposits (mount (mount chrge); chrge); liceaccount.trnsferto(300,bobaccount); liceaccount.trnsferto(300,bobaccount); System.out.prln(liceAccount.toString()); System.out.prln(liceAccount.toString()); System.out.prln(bobAccount.toString()); Hndles Hndles withdrwl System.out.prln(bobAccount.toString()); withdrwl withdrw withdrw ( ( mount) mount) chrge chrge commission(mount); commission(mount); blnce blnce blnce blnce - - mount mount - - chrge; chrge; bnkdeposits bnkdeposits - - (mount (mount - - chrge) chrge) ; ; Hndles Hndles trnsfer trnsfer Constructor trnsferto( trnsferto( mount, mount, trgetaccount) trgetaccount) trgetaccount.deposit(mount); trgetaccount.deposit(mount); this.withdrw(mount); this.withdrw(mount); Writing Clsses, Shimon Schocken IDC Herzliy, slide 32

17 revisited Only Only subset subset of of members members is is shown shown bnkdeposits bnkdeposits finl finl COMMISSION_RATE COMMISSION_RATE 0.005; 0.005; finl finl COMMISSION_STEP COMMISSION_STEP number; number; String String owner; owner; blnce; blnce; Hndles Hndles deposit deposit deposit deposit ( ( mount) mount) chrge chrge commission(mount); commission(mount); blnce blnce blnce blnce mount mount - - chrge; chrge; bnkdeposits bnkdeposits (mount (mount chrge); chrge); Returns Returns blnce blnce getblnce getblnce () () blnce; blnce; Returns Returns commission commission tht tht bnk bnk chrges chrges for for mount mount commission commission ( Constructor ( mount) mount) ((mount ((mount > > COMMISSION_STEP) COMMISSION_STEP)?? (mount (mount COMMISSION_RATE COMMISSION_RATE / / 2) 2) : : (mount (mount COMMISSION_RATE)); COMMISSION_RATE)); Writing Clsses, Shimon Schocken IDC Herzliy, slide 33 Cll by vlue / cll by reference cllee Fields Fields number; number; String String owner; owner; blnce; blnce; Hndles Hndles trnsfer trnsfer Cll by vlue Cll by reference trnsferto( trnsferto( mount, mount, trgetaccount) trgetaccount) trgetaccount.deposit(mount); trgetaccount.deposit(mount); this.withdrw(mount); this.withdrw(mount); mount mount just just for for demo demo cller liceacc liceacc ( Alice, ( Alice, 800); 800); bobacc bobacc ( Bob, ( Bob, 100); 100); Cll by vlue: Used when prmeter is of primitive type A vlue is computed nd pssed to method The method cnnot chnge prmeter. mount mount liceacc.trnsfer(mount, liceacc.trnsfer(mount, bobacc); bobacc); System.out.prln(mount); System.out.prln(mount); Will Will pr pr Cll by reference Used when prmeter is of object type A poer to object is pssed to method The method cn chnge referred object. Writing Clsses, Shimon Schocken IDC Herzliy, slide 34

18 Vrible life cycle Sttic vribles Instnce vribles Locl vribles Sttic vribles: creted first time method from is invoked Instnce vribles: creted when object is creted nd recycled when object is destroyed Locl vribles: creted when method is invoked nd recycled when it s Prmeters: sme s locl vribles. Initilized by rguments supplied by cller. lstaccountnumber 1; 1; bnkdeposits number; number; String String owner; owner; blnce; blnce; Sets Sets up up ccount ccount (String (String owner, owner, blnce) blnce) this.number lstaccountnumber; this.owner this.owner owner; owner; this.blnce blnce; blnce; prmeter Hndles Hndles withdrwl withdrwl withdrw withdrw ( ( mount) mount) blncetemp blnce blnce mount mount if if (blncetemp > > 0) 0) blnce blnce blncetemp; bnkdeposits - - mount; mount; else else omitted omitted Writing Clsses, Shimon Schocken IDC Herzliy, slide 35 Outline Modulrity Abstrction Clss ntomy Fields Constructors Methods Relted topics Method overloding Revisiting Prmeter pssing Vrible life cycle Encpsultion Method ntomy Clss vs. instnce methods Types nd vlues Methods prmeters Accessors nd muttors Writing Clsses, Shimon Schocken IDC Herzliy, slide 36

19 Encpsultion bstrction: A is n object tht keeps nd reports time using 24- hour system Required services: Crete clock Advnce clock 1 second Query clock time Set clock time ( (hours, seconds) secondelpsed() getseconds() getminutes() gethours() Encpsultion: Seprting contrctul erfce of n bstrction from its implementtion. setseconds() setminutes() sethours() More More services Writing Clsses, Shimon Schocken IDC Herzliy, slide 37 Encpsultion Clients Methods deposit() withdrw() Dt tildown number x,y Owner direction blnce trnsfer() getblnce() Encpsultion: Etc Hiding, or encpsulting, ernl stte of object from outside Protects egrity of object by preventing users from setting its ernl dt o n invlid or inconsistent stte A criticlly importnt OO design objective. Writing Clsses, Shimon Schocken IDC Herzliy, slide 38

20 Encpsultion nd visibility modifiers Vribles Violte encpsultion Enforce encpsultion Methods Provide services to clients Support or methods in sme Writing Clsses, Shimon Schocken IDC Herzliy, slide 39