OO)terminology) Every)(Java))object)is)an)instance)of)some)class)

Transcription

1 OOterminology variablesandmethods+ Theclassofanobjectspecifies (methods oneormoreconstructors:codethatisexecutedwhenthe Every(Javaobjectisaninstanceofsomeclass CIS120 3

2 ObjectsinJava public class Counter { classname classdeclara@on private int r; instancevariable public Counter ( { r = 0; public int inc ( { r = r + 1; return r; constructor methods public class Main { objectcrea@onanduse public int dec ( { r = r - 1; return r; public static void main (String[] args { Counter c = new Counter(; constructor invoca@on System.out.println( c.inc( ; CIS120 methodcall 4

3 DeclareavariabletoholdaCounterobject Typeoftheobjectisthename+oftheclassthatcreatesit InvoketheconstructorforCounter tocreateacounter instancewithkeyword"new"andstoreitinthevariable Counter c = new Counter(; Invokethemethodsofanobjectinstanceusing"dot" c.inc(;

4 Whatisthevalueofansattheendofthisprogram? Counter x = new Counter(; x.inc(; int ans = x.inc(; public class Counter { private int r; public Counter ( { 3. 3 r = 0; 4. NullPointerException public int inc ( { r = r + 1; return r; Answer:2

5 Whatisthevalueofansattheendofthisprogram? Counter x; x.inc(; int ans = x.inc(; public class Counter { private int r; public Counter ( { 3. 3 r = 0; 4. NullPointerException public int inc ( { r = r + 1; return r; Answer:NPE

6 Whatisthevalueofansattheendofthisprogram? Counter x = new Counter(; x.inc(; Counter y = x; y.inc(; int ans = x.inc(; NullPointerException Answer:3xandyarealiases+ public class Counter { private int r; public Counter ( { r = 0; public int inc ( { r = r + 1; return r;

7 ConstructorswithParameters public class Counter { private int r; public Counter (int r0 { r = r0; Constructormethodscantake parameters Constructormusthavethesame nameastheclass public int inc ( { r = r + 1; return r; public int dec ( { r = r - 1; return r; public class Main { public static void main (String[] args { objectcrea@onanduse Counter c = new Counter(3; constructor invoca@on System.out.println( c.inc( ;

8 Mutability EveryJavavariableismutable Counter c = new Counter(2; c = new Counter(4; AJavavariableofreference+typecanalsocontainsthespecial value null Counter c = null; Single=forassignment Double==forreferenceequalitytes@ng

9 Null either null orapointerintotheheap i.e.,ajavavariableofreferencetype"t"islikeanocamlvariableof Thedereferencingofthepointerandthecheckfor null areimplicitly let f (co : counter option ref : int = begin match co.contents with None -> failwith "NullPointerException" Some c -> c.inc( end class Foo { public int f (Counter c { return c.inc(; Ifnullvalueisusedasanobject(i.e.withamethodcallthen anullpointerexcep@onoccurs

10 public class Counter { risprivate+ private int r; public Counter ( { r = 0; public int inc ( { r = r + 1; return r; public int dec ( { r = r - 1; return r; constructorand methodscan refertor otherpartsofthe public class Main { programcanonlyaccess public static void publicmembers main (String[] args { Counter c = new Counter(; System.out.println( c.inc( ; CIS120 methodcall 14

11 Visibilitymodifiersmakethestatelocalby controllingaccess Basically: public:accessiblefromanywhereintheprogram private:onlyaccessibleinsidetheclass Designpaeern firstcut: Makeall+fieldsprivate Makeconstructorsandnonihelpermethodspublic protectedand packageprotected.thedetailsarenotimportantatthispoint. CIS120 15

12 JavaCoreLanguage differencesbetweenocamlandjava

13 Expressionsvs.Statements OCamlisanexpression+language++ Everyprogramphraseisanexpression(andreturnsavalue Thespecialvalue(oftypeunitisusedastheresultofexpressions thatareevaluatedonlyfortheirsideeffects Semicolonisanoperatorthatcombinestwoexpressions(wherethe lecihandonereturnstypeunit Javaisastatement+language++ Twoisortsofprogramphrases:expressions(whichcomputevalues andstatements(whichdon t Statementsareterminated+bysemicolons Anyexpressioncanbeusedasastatement(butnotviceiversa

14 Types AsinOCaml,everyJavaexpression+hasatype Thetypedescribesthevaluethatanexpressioncomputes Expression$form$ Example$ Type$ Variablereference x Declaredtypeofvariable Objectcrea@on newcounter( Classoftheobject Methodcall c.inc( Returntypeofmethod Equalitytest x==y boolean Assignment x=5 don t+use+as+an+expression!!+

15 primi;ve+types+ (valuesstored directly inthe stack structuredtypes (a.k.a.reference+ types values storedintheheap OCaml$ int,float,char,bool, tuples,datatypes,records, (objects+encoded+as+records+ of+func;ons Java$ generics+ alist List<A> int,float,double,char,boolean, objects,arrays (records,+tuples,+datatypes,+ strings,+firstuclass+func;ons+are+a+ special+case+of+objects abstract+types+ moduletypes(signatures interfaces public/privatemodifiers

16 OCaml Java =,== == equalitytest <>,!=!= inequality >,>=,<,<= >,>=,<,<= comparisons + + addi@on(andstringconcatena@on i i subtrac@on(andunaryminus * * mul@plica@on / / division mod % remainder(modulus not! logical not && && logical and (shorticircui@ng logical or (shorticircui@ng

17 New:OperatorOverloading Themeaningofanoperatorisdeterminedbythetypes+of thevaluesitoperateson Integerdivision 4/3$$$$ $$1 4.0/3.0$ $ /3.0$$ $ OverloadingisageneralmechanisminJava we llseemoreofitlater

18 Equality equality: pointerequality o1==o2 deepequality o1.equals(o2 everyobjectprovidesan equals methodthat doestherightthing dependingontheclassofthe object tocompareobjects

19 Strings String isabuilt+in+javaclass Stringsaresequencesofcharacters "" "Mount Fuji" "3 Stooges"" " "3" + " " + "Stooges" "3 Stooges" TextinaStringisimmutable(likeOCaml butvariablesthatstorestringsarenot String x = "OCaml"; String y = x; Can'tdoanythingtox sothatychanges The.equals$method$returns$true$when$two$strings$ contain$the$same$sequence$of$characters$

20 Whatisthevalueofansattheendofthisprogram? String x = "CIS 120"; String z = "CIS 120"; boolean ans = x.equals(z; 1. true 2. false 3. NullPointerException Answer:true Thisisthepreferredmethodofcomparingstrings.

21 Whatisthevalueofansattheendofthisprogram? String x1 = "CIS "; String x2 = "120"; String x = x1 + x2; String z = "CIS 120"; boolean ans = (x == z; 1. true 2. false 3. NullPointerException Answer:false Eventhoughxandzbothcontainthecharacters CIS120, theyarestoredintwodifferentloca@onsintheheap.

22 Whatisthevalueofansattheendofthisprogram? String x = "CIS 120"; String z = "CIS 120"; boolean ans = (x == z; 1. true 2. false 3. NullPointerException Answer:true(! Why?Becausestringsareimmutable,twoiden@cal stringsthatareknownwhentheprogramiscompiledcanbe aliased.

23 Moral Alwaysuses1.equals(s2to comparestrings! Youalmostalwayswanttocompare stringswithrespecttotheircontent,not wheretheyareallocatedinmemory (Butbewarned:s1mightbenull!

24 Kind$ PartCofC speech$ Example$ class noun RacingCar (mutablefield, variable (immutablefield, variable noun noun method verb shicgear MILES_PER_GALLON startwithadigit Thelargerthescope,themoreinforma;ve+thenameshouldbe havethesamename