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Programming II.

Programming II. Inheritance Interfaces Exceptions Generics Event handling Recursion. Programming II. Inheritance Interfaces Exceptions Generics Event handling Recursion. Topics of the practice. Inheritance syntax OO principles : encapsulation , data hiding Type polymorphism

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Programming II.

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  1. Programming II. Inheritance Interfaces Exceptions Generics Eventhandling Recursion Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  2. Programming II. Inheritance Interfaces ExceptionsGenerics Eventhandling Recursion Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  3. Topics of thepractice • Inheritancesyntax • OO principles: encapsulation, datahiding • Typepolymorphism • Methodpolymorphism • Virtual and non-virtualmethods • Abstractclasses and methods Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  4. Inheritancein C# • Baseclassescan be specifiedusingthecharacter „:” (onlysingleinheritancein c#) • Thiswaywecanextendthebaseclasswithnewfields/methods classAnimal { intage; publicAnimal( ) { ... } publicvoidEat( ) { ... } } classMammal: Animal { publicMammalGiveBirth( ) { ... } } class Cat: Mammal { stringname; public voidHiss( ) { ... } } Age : int Animal() Eat() Animal GiveBirth() : Mammal Mammal Name : String Hiss() Cat Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  5. Inheritance of constructors • Constructrorsarenotinherited • Wecan (should) callthebaseclass’ constructorusingthe „base” keyword (theproperconstructorwill be calledaccordingtothegivenparameters) • Connectingtheconstructors • Bydefault, theparameter-lessconstructor of thebase is called • Ifit is notpresent, thenwehavetomanuallyconnectone of thebaseconstructorswiththesubclass’ constructor class A { publicA( ) { ... } } class B : A { publicB( ) { ... } } classC : A { publicC(intx) { ... } } class E : C { publicE(inty) : base (y) { ... } } class F : C { publicF( ) : base (5) { ... } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  6. Virtual and non-virtualmethods • Non-virtualmethods • Earlybinding • In C#, this is thedefault (nonexistantin Java) • Virtualmethods • Latebinding • Specialsyntax • Inthefirstclasswheretheyappear, they must be markedwiththekeyword „virtual” • Inthedescendantclassesthesamemethods must be markedwiththe „override” keyword (wecan, butweshouldnothidemethods) • The basemethodcanstill be accessed class A { public virtual int Method( ) { ... } } class B : A { public override int Method( ) { ... } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  7. Hidingmethods • Ifwedeclare a non-virtualmethodwiththesamename • Compilerwarning, unlesswe mark themethodwiththe „new” keyword • The basemethodcanstill be accessed • Hidingvirtualmethods • „newvirtual”  newvirtualmethodchain, overcomplicated(?) • Usuallymethodhiding is notconnectedtovirtualmethods, itcanhappeniftwomethodshappentohavethesamename „byaccident” class A { public intMethodX( ) { ... } public virtual intMethodY( ) { ... } } class B : A { public new intMethodX( ) { ... } public new virtual intMethodY( ) { ... } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  8. Typeconversion (typecasting) • Treat an objectasitwas of anothertype • Implicit: automatictypeconversion (e.g. intfloat, no extra syntax) • Explicit: conversionusingthe standard (newtype) operator • Animal x; Cat y = (Cat)x; ((Cat)x).Nyávog(); • „is” operator • Usage: „VARIABLE is TYPE” , returnswithboolean • Returnswithtrueifthegivenvariablecan be usedasthegiventype (= sametypeorderivedtype) • „as” operator • Usage: „VARIABLE as TYPE” • Conversion (onlyusablewithreferencetypes) • No exceptiononfailure! Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  9. Abstractclasses / methods • They must be markedwiththe „abstract” keyword • Usage of classesforpurelyinheritancepurposes • A classwithevenoneabstractmethod must always be abstract • Wecannotcreateinstancesfromabstractclasses • Abstractmethods must be implementedinthederivedclasses abstract classFigure { public abstractdoubleArea( ); public abstractdoubleCircumference( ); } classRectangle: Figure { inta; intb; public override doubleArea( ) { returna * b; } public override doubleCircumference( ) { return2 * (a + b); } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  10. Sealedclass / method • Must be markedwiththe „sealed” keyword • Sealedclassescannot be usedasbaseclass • Sealedmethodscannot be overridden sealed classRectangle: Figure { inta; intb; publicoverridedoubleArea( ) { returna * b; } publicoverridedoubleCircumference( ) { return2 * (a + b);} } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  11. Objectbaseclass • The rootclass of theclasstree, thebaseclassforeverything • Ifwedonotspecifyourownbaseclass, thenSystem.Objectwill be usedas a base • Somemethods: • public Type GetType()Returnsthetypeobject • public virtual bool Equals(object obj) Equalitybasedonthecontent • public virtual int GetHashCode()Calculates a hashbasedonthecontent • public virtual string ToString()Convertstheinstanceinto a string • public static bool ReferenceEquals(object objA, object objB) Equalitybasedonthereferences • public static bool Equals(object objA, object objB) Equalitybasedonthecontents Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  12. Inheritanceexample • Shape Circle, Shape  Rectangle  Square • Solvethefollowing: • Store 50 shapesin an array • Determinetheonewiththebiggestarea • List thosewiththebiggestcircumference • List thosewherethearea isbiggerthanthecircumference • List thosewherethearea is a primenumber • Selecttheonewherethecircumferencenumber has thebiggestnumber of divisors Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  13. Programming II. Inheritance Interfaces ExceptionsGenerics Eventhandling Recursion Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  14. Interfacesin C# • We must usethe „interface” keyword • Interfacescancontainmethod/property/eventdeclarations • Whencreating a classthatusesinterface(s), we must usethesame „:” character (extendsvsimplements) • Multipleinterfacescan be implementedinoneclass • Wecannotcreateinterfaceinstances, butitcan be usedas a type interfaceIReceiveMessages { boolAvailable{ get; set; } voidSend(stringmsg); } classChatPartner : Person, IReceiveMessages { public boolAvailable{ get {...} set {…} } public voidSend(stringmsg) { ... } } IÜzenetFogadó x =newChatPartner(); <<interface>> IReceiveMessages Available: Boolean Send(String) IReceiveMessages ChatPartner Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  15. Abstractclassvsinterfaces • Abstractclasscanalsoimplementinterfaces • Optionallywecanimplementtheinterfacemethodsor mark themasabstractmethods descendants must implementthose • Theycanlookthesame, buttheyservedifferentpurposes • Both canmakeotherclassesimplementsomemethods, butabstractclassesareusedininheritance and theycancontainactualmethods/fieldsaswell interfaceICanBeSold { boolPrice { get; set; } voidSell( ); } abstract classProduct: ICanBeSold { public abstract boolPrice { get; set; } public abstract voidSell( ); } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  16. Extendinginterfaces • Similartoobjectinheritance (usingthe „:” letter) • An interfacecanextendmultipleinterfaces • A classimplementingainterface must implementallmethodsdefinedbythegiveninterface and allotherinterfacesthatgiveninterfaceextends interfaceICanBeSold { boolPrice{ get; set; } voidSell( ); } interfaceICanBeOnSale:ICanBeSold { voidExecuteSpecialSale(doublediscount); } public classProduct: ICanBeOnSale { public boolPrice { get; set; } public voidSell ( ) { ... } public voidExecuteSpecialSale(doublediscount) { ... } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  17. Implicitinterfaceimplementation • If more thanoneinterfacecontainsthesamemethod (withthesamesignature), thentheclasscan be createdsothatitfulfilstheinterfacemethodusingoneactualmethod • Inthiscaseitdoesnotmatterwhatreferenceweusefortheobjectinstance, thatonemethodwill be executedwhencalled interfaceIManageFiles { stringDelete( ); } interfaceIManageDocuments { stringDelete( ); } public classTextFile: IManageFiles,IManageDocuments { publicstringDelete( ){ … } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  18. Explicitinterfaceimplementation • Ifwechoosetoimplement a separatemethodforseparateinterfaces • The executedmethoddependsonthevariable/referencetype interfaceIManageFiles { stringDelete(); } interfaceIManageDocuments { stringDelete(); } publicclassTextFile : IManageFiles, IManageDocuments { stringIManageFiles.Delete() { return"FILES"; } stringIManageDocuments.Delete() { return"DOCUMENTS"; } publicstringDelete() { return"TEXTFILE"; } } classProgram { staticvoid Main(string[] args) { TextFile a = newTextFile(); a.Delete(); (a asIManageFiles).Delete(); (a asIManageDocuments).Delete(); Console.ReadLine(); } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  19. Exercise #1 • Create a classthatcanstorestudents (random name + random credit number), make a sortablearrayfromtheclassinstances The class must implementtheIComparableinterface • Generic and non-genericversionsfromthatinterface, weusethenon-genericnow • int CompareTo(Object obj)<0 – thecurrentinstance is beforeobj0 – thecurrentinstance and theobjareatthesame spot>0 – thecurrentinstance is afterobj • UseIComparable + Array.Sort(Array) Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  20. Exercise #2 • Wewanttocreate a carrace • Car Name, protectedSpeed, protectedRnd, publicPosition+ ctor + ToString • ICanAccelerate, ICanBreakDown • BMW : Car, ICanAccelerate (everythirdtime) • Porsche : Car, ICanBreakDown (1:X chance) • Jaguar : Car, ICanAccelerate, ICanBreakDown (1:5 chanceforboth) • Program class: Init() + Main() + IsOver() Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  21. Programming II. Inheritance Interfaces ExceptionsGenerics Eventhandling Recursion Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  22. Multi-layerarchitectures • Three-layerarchitecture (notnecessarilythree-tier) • Typically • Everylayerperformsonedistincttask, and servesdatatothelayerabove • Eveniftheyaredeployedas a one-tierapplication, thelogicalseparation is highlyadvised • Easiertomange and changeandscalethecode • Reusablecode!!! View Controller(ViewModel, Presentation) Model(Resource) Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  23. Topics of thepractice • EventsvsExceptions • Steps of creatingexceptions • Exceptionhierarchy • DivideByZeroException, FormatException, OverflowException • Exceptionbaseclass • NullReferenceException, IndexOutOfRangeException • ArgumentException, ArgumentNullException, ArgumentOutOfRangeException Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  24. Programming II. Inheritance Interfaces ExceptionsGenerics Eventhandling Recursion Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  25. Aim • Aim: create a setthatcanstorepeople, usingtheleastpossibleperson-dependentlines (e.g., ZERO) • Whattypeshould be usedas a storage? • Person – The wholething has to be rewrittenfor a set of dogs • CommonancestorforPerson and Dog – notpossibleinmanycircumstances... Nothing is common... • Object (globalancestor) – possiblesolution ArrayListin .NET1, typecastingforever • Definethetypeas a paremeter, and definetheactualtypelater Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  26. Generics • Templateclassesin C++ • Duringthedefinition, wespecifygenerictype(s) thatwill be definedlater • Duringthedefinition, weonlyreference a typeassimply „T”, and this „T” typewill be actuallydefinedlater, whentheinstance is created • Definition:class MyGenericClass<T> { } • Declaration and instancecreationMyGenericClass<int>mgcINT = new MyGenericClass<int>(); • MyGenericClass<Person>mgcP= new MyGenericClass<Person>(); • Typicalusage: built-incollections! Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  27. Collections • ArrayList storesobjects, typecasting is needed, deprecated • List<T>: Add(), Insert(), Remove(), RemoveAt(), Reverse(), [],Clear(), Contains(), Count • Queue<T>: Enqueue(), Dequeue(), Peek(),Clear(), Contains(), Count • Stack<T>: Push(), Pop(), Peek(),Clear(), Contains(), Count • Multiplegenerictypeparameters: classsomething<T,U> { } • Dictionary<T, U>: Add(), Remove(), TryGetValue(), Keys, Values,Clear(), ContainsKey(), ContainsValue(), Count Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  28. Usinggenericclasses Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  29. Initializinggenerictypes • objectreference=null; intvalue=0; stringimmutable=””; • T something = ???? • No idea abouttheexacttype, no idea abouttheexactwaytoinitialize • defaultexpression • Returnswithnullorbitwisezero T something =default(T); Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  30. Constraints / Restrictions • Onething is sure: type T descendsfromSystem.Object • Thus, onlytheusualmethodsareaccessible (ToString(), ....) • Wecanspecifyconstraintstoenforcesomerules: thedevelopercannotuseanythingas T, onlythosetypes... • Descendant of...(where T : Person) • Implements... (where T:IComparable) • Valuetype (where T:struct) • Referencetype (where T:class) • Withconstructor(where T:new()) Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  31. Interfacerestriction: foreach class MyClass2<T> { public void Iterate(T data) { foreach (object item in data) { Console.WriteLine(item); } } } Can I useforeachontype„T”?(The foreachloopcanonlyworkon a typeifitimplementsIenumerable, thiswill be detailedlater) class MyClass2<T>where T : IEnumerable<T>{ public void Iterate(T data) { foreach (objectitem in data) { Console.WriteLine(item); } } } Sameway: wecanuseInterfaces / Baseclassestoenforcetheexistance of somemethod/property OK Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  32. Operator restriction class Arithmetic<T>{ public T Cubed(T number){ return number * number * number; } } Canwemultiply „T” ? class Arithmetic<T>where T : System.Int32{ public T Cubed(T number){ return number * number * number; } } Primitive / valuetypescannot be usedasconstraints! Whenusinggenerics, wecannotuse most of thebasic operators, wealwayshavetousemethods! Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  33. class Program2 { public T Min<T>(T a, T b)where T :IComparable<T> { if (a<b) return a; else return b; } } classProgram { public T Min<T>(T a, T b) where T :IComparable<T> { if (a.CompareTo(b)<0) return a; else returnb; }… }  No standard operators Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  34. Genericmethod • Independent fromgenericclasses: genericclassescancontainnon-genericmethods; non-genericclassescancontaingenericmethods • A method is genericifit has generictypeparameters Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  35. Callinggenericmethods • Callinggenericmethods: theparametertypesspecify „T” :inti=Min(2, 3); //integersfloat f=Min(2.0f, 3.0f);//floats • !IMPORTANT! wecan’t havetwomethods w thesamesignature • Whatif T=int? • Whenthere is a conflict, alwaysthenon-generic version is called • IftherearetwomethodswithgenericparametersMethod(T arg), Method(U arg), and T and U happento be thesame, thenthemethodcallwillbeunavailable („ambiguouscall”) Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  36. Staticfields class MyGenericClass<T> { private static int counter = 0; public MyGenericClass() { counter++; } public static void Count() { Console.WriteLine(counter); } } Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  37. Accessingstaticfields/methods • Toaccessstaticmembers, an actualtype must be used:MyGenericClass<int>.Count(); • Differentstaticfieldsbelongtodifferenttypes • The staticconstructor is automaticallycalled Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  38. Generics and inheritance • Genericclassescan be derivedfrombothgeneric and non-genericclass • Genericclassescan be theancestor of bothgeneric and non-genericclasses • However , thetypeparameter must always be accessibleorset! • IfDerived<T> : BaseGeneric<T> , and newDerived<int>()  type T will be int inbothclasses. • IfDerived : BaseGeneric<T>, and newDerived()  type T inthebaseclass is notdefined, NOT ALLOWED • IfDerived<T> : BaseGeneric<U>, and newDerived<int>()  type U inthebaseclass is notdefined, NOT ALLOWED Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  39. Generics and inheritance Melyik eset lehetséges? Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  40. Generics and inheritance • The restrictions must be repeatedinthedescendantclasses, thedescendantclasscanintroducenewconstraints • GenericsareaccessibleinallmanagedCLR-basedlanguage • Genericsarestoredinthe MSIL level, theactualtypereplacement is doneduringthe JIT compile Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  41. Programming II. Inheritance Interfaces ExceptionsGenerics Eventhandling Recursion Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  42. Multi-layerarchitectures • Three-layerarchitecture (notnecessarilythree-tier) • Typically • Everylayerperformsonedistincttask, and servesdatatothelayerabove • Eveniftheyaredeployedas a one-tierapplication, thelogicalseparation is highlyadvised • Easiertomange and changeandscalethecode • Reusablecode!!! View Controller(ViewModel, Presentation) Model(Resource) Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  43. Topics of thepractice • EventsvsExceptions • Steps of creatingeventsusinginterfaces • Steps of creatingeventsusingdelegates Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  44. Intheexamplestherewill be twoclasses, theeventcreator and theeventhandlerclass. This is usuallythecase, butevents (justlikeexceptions) can be created and handledwithinonesingleclass Aim: signalevents (information) toothermodules/classes Possibilities: Function/methodpointers (DOS), delegates (C#), interfaces (JAVA) Eventhandling Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  45. Wehavetocreate an interfaceISomethingthatdescribesthesignature and thename of themethodthatwillhandleourevent The eventcreator/senderclass must maintain a privatelist of ISomethinginstances Usingpublicaccessors, anotherclass (theeventhandlerclass) can add orremoveitself (orotherclasses) tothislist (subscribe, unsubscribe) ElementsinthelisthavethetypeISomething theyallcontainthemethoddescribedintheinterface, and thismethodwillhandletheevent The eventcreatorclasssimplyloopsthroughthesubscribedclassesfoundinthelist, and callsthesamemethodforeveryinstance Usinginterfacesforevents Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  46. A typethatallowsusto „store” methodsinavariable Inthedeclarationwe must specifythesignature of themethodsthatthegivendelegatecanstore:delegatedoubleMyDelegate(int param1, string param2); Name: MyDelegate Returntype: double Parameters: int+string Important: this is only a typedeclaration Notusablewithout a variable/field of thistype Referencetype Likeclass/enum: cannot be declaredin a method; it must be declaredin a namespaceorinsideaclass Delegate Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  47. delegatedoubleMyDelegate(int param1, string param2); doublefunct(intfirst, stringsecond) { returnfirst + second.Length;} Unicastdelegate:MyDelegate del;del = newMyDelegate(funct);Console.WriteLine(del(5, "hello")); Usingdelegates Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  48. delegatedoubleMyDelegate(int param1, string param2); doublefunct(intfirst, stringsecond) { returnfirst + second.Length;} Declaring a delegatefield: MyDelegate del; Multicastdelegate: del += funct;Console.WriteLine(del(5, "hello")); Aslongasthedelegate is empty, itsvaluewill be NULL  we must alwayscheckitbeforecallingthedelegate Multiplemethodscan be putintothesamedelegate Usingdelegates Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  49. delegateboolMyComparer(objectleft, object right); classSimpleExchangeSorter { publicstaticvoid Sort(object[] Arr, MyComparerbigger) { for (int i = 0; i < Arr.Length; i++) for (int j = i + 1; j < Arr.Length; j++) if (bigger(Arr[j], Arr[i])) { objecttemp = Arr[i]; Arr[i] = Arr[j]; Arr[j] = temp; } } } Delegate-parameter Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

  50. Delegate-parameterExample classstudent { publicstringname; publicintcredits; publicdiak(stringnewname, intnewcredits) { name = newname; credits = newcredits; } publicoverridestringToString() { returnname + " - " + credits; } } student[] group= newstudent[] { newstudent("Első Egon", 52), newstudent("Második Miksa", 97), newstudent("Harmadik Huba", 10), newstudent("Negyedik Néró", 89), newstudent("Ötödik Ödön", 69) }; Szabó Zsolt, Óbudai Egyetem, 2011 szabo.zsolt@nik.uni-obuda.hu

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