CS计算机代考程序代写 Java compiler Chapter 8.

Chapter 8.
The Ultimate Superclass: Object
2020-2021
COMP2396 Object-Oriented Programming and Java
Dr. T.W. Chim (E-mail: twchim@cs.hku.hk)
Department of Computer Science, The University of Hong Kong
1

Polymorphism in Action
— OceanwasaskedtobuildasimpleDog-specificlist (pretending that we do not have the ArrayList class for the moment)
— Hecameupwiththefollowingdesign — Usearegulararraywithafixedlengthto
— Useanint valuetostoretheindexofthe next available position in the Dog array
— Implementanadd()methodforaddingaDogobjecttothe array at the next available position. When the array is full, the add() method will simply do nothing
— Implementaget() methodforretrievingaDogobjectfrom the array. If the index argument is out of bound, simply return null
MyDogList
Dog[] dogs int nextIndex
add(Dog d) get(int index)
store the Dog objects
2

Polymorphism in Action
— Below is Ocean’s first version of the Dog-specific list
public class MyDogList {
private Dog[] dogs = new Dog[5]; private int nextIndex = 0;
public void add(Dog d) {
if (nextIndex < dogs.length) { dogs[nextIndex] = d; System.out.println("Dog added at " + nextIndex); nextIndex++; } } public Dog get(int index) { if (index >= 0 && index < dogs.length) { return dogs[index]; } else { return null; } } } 3 Polymorphism in Action — Cameasnosurprise,therewasaminorchangetothe specification that the list should keep Cats too — Oceanhadafewoptionshere: — 1. Makeaseparateclass,MyCatList,tostoreCatobjects 2. Makeasingleclass,MyDogAndCatList,thatkeeps2 different arrays as instance variables, and has 2 different add() methods (i.e., addDog() and addCat()) and 2 different get() methods(i.e.,getDog()andgetCat()) 3. MakeaheterogeneousMyAnimalListclassthatcanstore any kind of Animal subclasses The first 2 options are quite clunky, while the 3rd option sounds the best (more generic) 4 Polymorphism in Action — BelowisOcean’srevisedversionoftheDog-specificlist public class MyAnimalList { private Animal[] animals = new Animal[5]; private int nextIndex = 0; public void add(Animal a) { if (nextIndex < animals.length) { Don’t panic. We are not making a new Animal object from the abstract Animal class, but a new array object of type Animal! } 5 animals[nextIndex] = a; System.out.println("Animal added at " + nextIndex); nextIndex++; } } public Animal get(int index) { if (index >= 0 && index < animals.length) { return animals[index]; } else { return null; } } — Example Polymorphism in Action public class AnimalTestDrive { public static void main(String[] args) { MyAnimalList list = new MyAnimalList(); list.add(new Dog()); list.add(new Cat()); } } — Sampleoutput Animal added at 0 Animal added at 1 What about non-Animals? Why not make the class generic enough to store anything? To achieve this, we need a class above Animal, one that is the superclass of everything! 6 Polymorphism in Action — EveryclassinJavaextendstheObjectclass — Objectclassisthemotherofallclasses(i.e.,it is the superclass of everything!) — Anyclassthatdoesnotexplicitlyextend another class, implicitly extends the Object class — Example — DogextendsCanine,andCanineextendsAnimal — SinceAnimaldoesnotexplicitlyextendanother class, it implicitly extends Object — HenceDogextendsObject(indirectly) Animal food hunger location makeNoise( ) eat() sleep() roam() Canine roam() Dog makeNoise() eat() 7 Polymorphism in Action — WiththeObjectclassbeingthesuperclassofeverything, it is possible to create a list that can store anything! ArrayList add(Object elem) Adds the object parameter to the list remove(int index) Removes the object at the index parameter remove(Object elem) Removes this object (if it is in the ArrayList) contains(Object elem) Returns true if there is a match for the object parameter isEmpty() Returns true if the list has no element indexOf(Object elem) Returns the index of the object parameter or −1 size() Returns the number of elements currently in the list get(int index) Returns the object currently at the index parameter ... 8 The Ultimate Superclass: Object Object — Everyclassyouwriteinheritsallthe methods of the Object class — Theclassesyouhavewritteninherit methods you do not even know you have — Belowaresomeofthemethodsofthe Object class that you may be interested in — equals() — hashCode() — getClass() — toString() boolean equals(Object o) int hashCode() Class getClass() String toString() ... YourClassHere 9 The Ultimate Superclass: Object — Theequals()method — Comparestheobjectparameterwiththecurrentobject,and returns true if they are thesame — Implementsthemostdiscriminatingpossibleequivalence relation on objects, i.e., for any non-null reference values x and y, this method returns true if and only if both x and y are referencing the same object — Youshouldoverridethismethodifyouwouldliketotest whether 2 objects are equal in the sense that they contain the same information (note that you should also override the hashCode() method as well in this case) public boolean equals(Object obj) 10 The Ultimate Superclass: Object — Example Dog d = new Dog(); Cat c = new Cat(); if (d.equals(c)) { System.out.println("true"); } else { System.out.println("false"); } — Sampleoutput false 11 The Ultimate Superclass: Object — ThehashCode()method — Returnsahashcodevalue(integer)forthecurrentobject (e.g., to be used with hash tables) — Bydefinition,if2objectsareequalaccordingtothe equals() method, then calling the hashCode() method on each of the 2 objects must produce the same integer result — Example: — Sampleoutput: 2018699554 Typically, the Object class implements this method by converting the internal address of the object into an integer public int hashCode() Dog d = new Dog(); System.out.println(d.hashCode()); 12 The Ultimate Superclass: Object public final Class getClass() — ThegetClass()method — ReturnsaClassobjectthatrepresentstheruntimeclassof the current object — Thekeywordfinal meansthismethodcannotbe overridden — Example: — Sampleoutput: Dog d = new Dog(); System.out.println(d.getClass()); class Dog 13 The Ultimate Superclass: Object — The toString() method — Returnsastringthat“textuallyrepresents”thecurrent object — Recommendedthatallsubclassesoverridethismethod — Example: — Sampleoutput Dog@7852e922 In the Object class implementation, this string is composed of the class name, the at-sign character @ and the unsigned hexadecimal representation of the hash code of the current object public String toString() Dog d = new Dog(); System.out.println(d.toString()); 14 Using Object as a Polymorphic Type If polymorphic types are so good, why don’t we just make all our methods take and return Object references? — Thatdefeatsthewholepointof‘type-safety’,oneofJava’s greatest protection mechanisms for your code — RecallthatJavaisastrongly-typedlanguage — Thecompilercheckstomakesurethattheobjectonwhicha method is being called is actually capable of responding — Inotherwords,onecancallamethodonanobjectreference only if the class of the reference type actually has that particular method 15 Using Object as a Polymorphic Type — Examples FamilyDoctor d = new FamilyDoctor(); Doctor workAtHospital treatPatient() d.treatPatient(); d.giveAdvice(); d is a FamilyDoctor reference and FamilyDoctor has a treatPatient() method and a giveAdvice() method Doctor d = new FamilyDoctor(); d.treatPatient(); d is a Doctor reference and Doctor FamilyDoctor makesHouseCalls giveAdvice() Doctor d = new FamilyDoctor(); d.giveAdvice(); This won’t compile! d is a Doctor reference but Doctor does not have a giveAdvice() method has a treatPatient() method 16 Using Object as a Polymorphic Type — UsingObjectasapolymorphictypehasapricetopay — ConsideranArrayListdeclaredtoholdDogobjects,i.e., ArrayList
— WhenyouputanobjectintoanArrayList,itgoesin
as a Dog, and comes out as a Dog — Example:
ArrayList myDogArrayList = new ArrayList(); Dog aDog = new Dog();
myDogArrayList.add(aDog);
Dog d = myDogArrayList.get(0);
The get() method returns a Dogreference 17

Using Object as a Polymorphic Type
— NowconsideranArrayListdeclaredtoholdObject objects, i.e., ArrayList