Better Living in Objectville
Better Living in Objectville Adriano Cruz [email protected] Departamento de Ciˆ encia da Computa¸c˜ ao Instituto de Matem´ atica UFRJ April 2, 2015 Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 1 / 67 Section Summary 1 Inheritance 2 Members 3 Polymorphism 4 Exercise Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 2 / 67 Starting What have all four classes in common? Square +rotate() +playSound() Adriano Cruz (DCC-IM-UFRJ) Circle +rotate() +playSound() Triangle +rotate() +playSount() 07 - Inheritance and Polymorphism Amoeba +rotate() +playSound() April 2, 2015 3 / 67 Super Class Common features Shape +rotate() +playSound() Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 4 / 67 Super Class and Sub Classes Shape is a superclass. Others a subclasses. All Together Shape +rotate() +playSound() Square Adriano Cruz (DCC-IM-UFRJ) Circle Triangle 07 - Inheritance and Polymorphism Amoeba April 2, 2015 5 / 67 Amoebas are different! Amoeba class override the rotate() and playSound() methods. Overriding means that a subclass redefines one of its inherited methods. All Together Shape +rotate() +playSound() Square Circle Triangle Amoeba +rotate() +playSound() Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 6 / 67 An Example Doctors Superclass Doctor One instance variable One method +worksAtHospital: boolean +treatPatient(): void Surgeon FamilyDoctor +treatPatient(): void +makeIncision(): void Overrides the inherited treatPatient() method Adds one new method Adriano Cruz (DCC-IM-UFRJ) Subclasses +makesHouseCalls: boolean +giveAdvice(): void Adds one new instance variable Adds one new method 07 - Inheritance and Polymorphism April 2, 2015 7 / 67 Implementing the example p u b l i c c l a s s Doctor { boolean worksAtHospital; v o i d t r e a t P a t i e n t () { // perform a checkup } } p u b l i c c l a s s F a m i l y D o c t o r e x t e n d s Doctor { boolean makesHouseCalls; v o i d giveAdvice () { // give Advice } } p u b l i c c l a s s Surgeon e x t e n d s Doctor { v o i d t r e a t P a t i e n t () { // perform surgery } v o i d m a k e I n c i s i o n () { // uhh } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 8 / 67 Questions? Doctor +worksAtHospital: boolean +treatPatient(): void Surgeon +treatPatient(): void +makeIncision(): void FamilyDoctor +makesHouseCalls: boolean +giveAdvice(): void 1 How many instance variables does Surgeon have? 2 How many instance variables does FamilyDoctor have? 3 How many methods does Doctor have? 4 How many methods does Surgeon have? 5 How many methods does FamilyDoctor have? 6 Can a FamilyDoctor do treatPatient()? 7 Can a FamilyDoctor do makeIncision()? Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 9 / 67 New problem Design a simulation program that lets the user simulate a group of animals in an environment. We have a list of animals to be simulated, but not all. Animals will move around, doing whatever it is that each particular type is programmed to do. We need to allow programmers to add new kinds of animals. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 10 / 67 Steps Step 1: Look for objects that have common attributes and behaviours. Step 2: What do these six objects have in common? Step 3: How are these objects related? Step 4: Define the inheritance tree? Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 11 / 67 Instance Variables picture – the file name representing the .jpeg of the animal. food – the type of food the animal eats. hunger – an int representing the hunger level of the animal. boundaries – values representing the height and width of the space the animal will roam around in. location – coordinates where the animal is. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 12 / 67 Methods makeNoise() – noise the animal makes. eat() – behaviour for when the animal finds his preferred food. sleep() – behaviour for when the animal is considered asleep. roam() – behaviour for when the animal is not sleeping or eating. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 13 / 67 Diagram Animal +picture +food +hunger +boundaries +location +makeNoise() +eat() +sleep() +roam() Lion Hippo Adriano Cruz (DCC-IM-UFRJ) Tiger Dog 07 - Inheritance and Polymorphism Cat Wolf April 2, 2015 14 / 67 Are they all equal? Do all animals eat the same way? Which methods should we override? Does a dog and a lion make the same noise? Do they eat the same things? Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 15 / 67 Modified Diagram Animal +picture +food +hunger +boundaries +location +makeNoise() +eat() +sleep() +roam() Wolf +makeNoise() +eat() Lion +makeNoise() +eat() Hippo +makeNoise() +eat() Adriano Cruz (DCC-IM-UFRJ) Tiger +makeNoise() +eat() Dog +makeNoise() +eat() 07 - Inheritance and Polymorphism Cat +makeNoise() +eat() April 2, 2015 16 / 67 More opportunities for abstraction Wolf and dog? Lion, tiger and cat? Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 17 / 67 Modified once again Animal +picture +food +hunger +boundaries +location +makeNoise() +eat() +sleep() +roam() Feline +roam() Canine +roam() Hippo Tiger +makeNoise() +eat() +makeNoise() +eat() Cat Wolf Lion +makeNoise() +eat() Adriano Cruz (DCC-IM-UFRJ) Dog +makeNoise() +eat() +makeNoise() +eat() 07 - Inheritance and Polymorphism +makeNoise() +eat() April 2, 2015 18 / 67 Which method? Animal Wolf w = new Wolf(); w.makeNoise(); w.roam(); +picture +food +hunger +boundaries +location +makeNoise() +eat() +sleep() +roam() w.eat(); w.sleep(); Canine +roam() The lowest one wins! When calling a method the most specific version of the method is used. The JVM starts walking up the inheritance tree at the invoked class. Wolf +makeNoise() +eat() Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 19 / 67 Sharpen your Pencil Find relationships that make sense. Not everything can be connected to something else. Class Musician Rock Star Fan Bass Player Concert Pianist Superclasses Adriano Cruz (DCC-IM-UFRJ) Subclasses 07 - Inheritance and Polymorphism April 2, 2015 20 / 67 IS-A and HAS-A When one class inherits from another, the subclass extends the superclass. Triangle IS-A Shape. Cat IS-A Feline. Surgeon IS-A Doctor. Bathroom IS-A Tube or Bathroom HAS-A Tube? If X extends Y, X IS-A Y . The relationship IS-A works in only one direction. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 21 / 67 IS-A and HAS-A The IS-A test works anywhere in the inheritance tree. Canine extends Animal. Wolf extends Canine. Wolf extends Animal. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 22 / 67 Some Words A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class). Excepting Object, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 23 / 67 Be Warned! Constructors are not members A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 24 / 67 Dumb Questions! Q: What if the superclass wants to use the subclass version of the method? A: It is not possible. A superclass will not necessarily know about any of its subclasses. Q: What if I want to use both the superclass and my overriding version of the a method? A: It is possible. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 25 / 67 Using superclass method p u b l i c v o i d roam () { s u p e r . roam () ; // the i n h e r i t e d v e r s i o n // now my roam stuff . } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 26 / 67 What to do? DO: use inheritance when one class is a more specific type of a superclass. DO: consider using inheritance when you have behaviour that sould be shared among multiple classes of the same type. DO NOT: use inheritance just so that you can reuse code from another class. DO NOT: use inheritance if the subclass and superclass do not pass the IS-A test. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 27 / 67 Section Summary 1 Inheritance 2 Members 3 Polymorphism 4 Exercise Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 28 / 67 Members? Methods and instance (nonlocal) variables are collectively known as members. You can modify a member with both access and nonaccess modifiers. Usually methods and variable members are given access control in exactly the same way. Members can use four access control levels: default public protected private A class can use just two: default public Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 29 / 67 Members: Who gets what? What does it mean for code in one class to have access to a member of another class? Can a method in one class have access a member of another class? This means to use the dot (.) operator. Can a subclass inherit a member of its superclass? Which, if any, members of a superclass can be accessed through inheritance? Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 30 / 67 Members: Who gets what? A subclass inherits members of the superclass. Members include instance variables and methods. A superclass can choose whether or not it wants a subclass to inherit a particular member by the level of access the particular member is given. From the most restrictive to the least: private default protected public Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 31 / 67 public members A public member means that all other classes, regardless of the package they belong to, can access the member (assuming the class itself is visible). pac kage book ; // different packages i m p o r t abrave .*; c l a s s Too { p u b l i c s t a t i c v o i d main ( String [] args ) { Blob o = new Blob () ; // accessible , public class Blob and method testIt o . testeIt () ; } } pac kage cert ; // different packages p u b l i c c l a s s Blob { p u b l i c v o i d testIt () { System . out . println ( " blob " ) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 32 / 67 public members If the member of the superclass is declared public the subclass inherits that member regardless of whether both classes are in the same package. pac kage cert ; p u b l i c c l a s s Roo { p u b l i c String doRooThings () { r e t u r n " fun " ; } } pac kage notcert ; i m p o r t cert . Roo ; c l a s s Cloo e x t e n d s Roo { p u b l i c v o i d testCloo () { System . out . println ( doRooThings () ) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 33 / 67 public members The doRooThings() method is invoked without having to preface it with a reference. The Roo class declares the doRooThings() member as public. A subclass of Roo can call its own inherited doRooThings() method. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 34 / 67 private members private members can not be accessed by code in any class other than the class in which the private member was declared. pac kage cert ; p u b l i c c l a s s Roo { p r i v a t e String doRooThings () { r e t u r n " fun " ; } } pac kage notcert ; i m p o r t cert . Roo ; c l a s s UseARoo { p u b l i c v o i d testIt () { Roo r = new Roo () ; // OK Roo is public // Compiler error // doRoothings is private System . out . println ( r . doRooThings () ) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 35 / 67 private members and Inheritance What about a subclass that tries to inherit a private member of its class? It is not possible! pac kage cert ; p u b l i c c l a s s Roo { p r i v a t e String doRooThings () { r e t u r n " fun " ; } } pac kage cert ; // same package c l a s s Cloo e x t e n d s Roo { p u b l i c v o i d testCloo () { // compiler error System . out . println ( doRooThings () ) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 36 / 67 Protected and Default Members Protected and default access control levels are almost identical. A protected member can be accessed (through inheritance) by a subclass even if the subclass is in a different package. A default member (think package level) may be accessed only if the class accessing the member belongs to the same package. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 37 / 67 Default Members Illegal access different packages. pac kage certification ; p u b l i c c l a s s OtherClass { v o i d testIt () { // default access System . out . println ( " OtherClass " ) ; } } pac kage somethingelse ; i m p o r t certification . OtherClass ; c l a s s AccessClass { s t a t i c p u b l i c v o i d main ( Strint [] args ) { OtherClass o = new OtherClass () ; o . testIt () ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 38 / 67 Protected Members Legal access through inheritance. pac kage certification ; p u b l i c c l a s s Parent { p r o t e c t e d i n t x = 9; } } pac kage other ; // other package i m p o r t certification . Parent ; c l a s s Child e x t e n d s Parent { p u b l i c v o i d testIt () { // x is inherited System . out . println ( " x is " + x ) ; // no problem } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 39 / 67 Determining Access Visibility From the same class From any class in the same package From a subclass in the same package From a subclass outside the same package From any non-subclass class outside package Adriano Cruz (DCC-IM-UFRJ) Public Yes Yes Protected Yes Yes Default Yes Yes Private Yes No Yes Yes Yes No Yes Yes, through inheritance No No No No No Yes 07 - Inheritance and Polymorphism April 2, 2015 40 / 67 Access Modifiers There are three access modifiers: public, protected, private. However there are four access levels!!?? The fourth access level (default) is what you get when you don’t use any of the three modifiers. Every class, method and instance variable you declare has an access control, whether you declare or not. A class can be declared with only public or default access, the other two does not make sense for a class. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 41 / 67 Packages Java is a package-centric language. A package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer. It is assumed that you put all your classes into packages. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 42 / 67 Public Access A class declaration with the public keyword give all classes from all packages access to the public class. Do not forget that if a public class is in a differente package from the class you are writing, you will still need to import the public class. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 43 / 67 Example pac kage cert ; c l a s s Beverage ; pac kage exam . stuff ; i m p o r t cert . Beverage ; c l a s s Tea e x t e n d s ←֓ Beverage {} Tea will not compile because its superclass Beverage is in a different package and has default acess. pac kage cert ; p u b l i c c l a s s Beverage ; // OK now !! Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 44 / 67 Dumb Questions! Q: So what does inheritance give you? A: Get rid of duplicate code. When it is necessary to modify the code, there is only one place to update. Define a common protocol (methods etc) for a group of classes. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 45 / 67 Section Summary 1 Inheritance 2 Members 3 Polymorphism 4 Exercise Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 46 / 67 Creating an Object 1 2 3 4 5 6 7 8 9 0 myDog Dog object Step 2: new Dog(); Dog Step 1: Dog myDog Step 3: Dog myDog = new Dog(); Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 47 / 67 Polymorphism In the example the reference type (myDog) and the object are the same. With polymorphism they can be different. Animal myDog = new Dog() The reference type can be a superclass of the actual object type. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 48 / 67 Modified once again Animal +picture +food +hunger +boundaries +location +makeNoise() +eat() +sleep() +roam() Feline +roam() Canine +roam() Hippo Tiger +makeNoise() +eat() +makeNoise() +eat() Cat Wolf Lion +makeNoise() +eat() Adriano Cruz (DCC-IM-UFRJ) Dog +makeNoise() +eat() +makeNoise() +eat() 07 - Inheritance and Polymorphism +makeNoise() +eat() April 2, 2015 49 / 67 Example I.a p u b l i c c l a s s Animal { p u b l i c v o i d makeNoise () { System . out . println ( " Animal ! " ); } p u b l i c v o i d eat () { System . out . println ( " Animal eating . " ) ; } p u b l i c v o i d sleep () { System . out . println ( " Animal sleeping " ) ; } p u b l i c v o i d roam () { System . out . println ( " Animal roaming . " ) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 50 / 67 Example I.b p u b l i c c l a s s Canine e x t e n d s Animal { p u b l i c v o i d roam () { System . out . println ( " I am a Canine roaming . " ) ; } } p u b l i c c l a s s Dog e x t e n d s Canine { p u b l i c v o i d makeNoise () { System . out . println ( " I am a Dog making noise . " ) ; } p u b l i c v o i d eat () { System . out . println ( " I am a Dog eating . " ) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 51 / 67 Example I.c p u b l i c c l a s s TestAnimal { p u b l i c s t a t i c v o i d main ( String [] args ) { Animal [] animals = new Animal [5]; animals [0] animals [1] animals [2] animals [3] animals [4] = = = = = new new new new new Dog () ; Cat () ; Wolf () ; Hippo () ; Lion () ; f o r ( Animal t : animals ) { System . out . println ( " Doing stuff " ) ; t . makeNoise () ; t . eat () ; t . sleep () ; t . roam () ; System . out . println () ; } } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 52 / 67 Example results Doing stuff I am a Dog making noise. I am a Dog eating. Animal sleeping I am a Canine roaming. Doing stuff I am a Cat making noise! I am a Cat eating. Animal sleeping I am a Feline roaming. Doing stuff I am a Wolf making noise. I am a Wolf eating. Animal sleeping I am a Canine roaming. Doing stuff I am a Hippo making noise. I am a Hippo eating. Animal sleeping Animal roaming. Doing stuff I am a Lion making noise! I am a Lion eating. Animal sleeping I am a Feline roaming. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 53 / 67 Polymorphic Arguments and Return Types It is possible to have polymorphic arguments and return types. It is possible to declare a reference variable of a supertype and assign a subclass object to it. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 54 / 67 Example II p u b l i c c l a s s Vet { p u b l i c v o i d giveShot ( Animal a ) { System . out . println ( " Giving shot . " ) ; a . makeNoise () ; } } p u b l i c c l a s s PetOwner { p u b l i c v o i d start () { Vet v = new Vet () ; Dog d = new Dog () ; Hippo h = new Hippo () ; v . giveShot ( d ) ; v . giveShot ( h ) ; } } p u b l i c c l a s s TestPetOwner { p u b l i c s t a t i c v o i d main ( String [] args ) { PetOwner p = new PetOwner () ; p . start () ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 55 / 67 Advantages With polymorphism it is possible to write code that does not have to change when new subclasses are introduced. A Vet can treat animals that extends the class Animal. Adriano Cruz (DCC-IM-UFRJ) A Giraffe extending the class Animal! 07 - Inheritance and Polymorphism April 2, 2015 56 / 67 Dumb Questions! Q: If I do not have access to the source code of a class, can I use subclass to do that? To extend the class and override the method with my own better code? A: Yes. That is another advantage of OO. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 57 / 67 Dumb Questions! Q: Can you extend any class? Or is it like class members where if the class is private you can’t inherit it? A: There is no private class, except in a special case called inner class, which we will discuss later. There are three things that can prevent a class from being subclassed. 1 The first is access control. A class can be made non-public when it is not declared as public. A non-public class can be subclassed only by classes in the same package (later). 2 When the keyword modifier final is used. A final class means it is the end of the inheritance line. 3 If a class has only private constructors (later) it can not be subclassed. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 58 / 67 Dumb Questions! Q: Why would we want a final class? A: Typically we do not want. However when it is necessary to guarantee that the methods will work the way they were designed. The Java API class has many final classes. For instance the String class. Q: Can you make a method final, without making the whole class final? A: Yes, mark the method with the final modifier. The method is protected. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 59 / 67 Keeping the contract Rules for overriding! The methods are the contract! Arguments must be the same, and return types must be compatible. The method can not be less accessible. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 60 / 67 Keeping the contract Appliance Appliance +turnOn(): boolean +turnOff(): boolean +turnOn(): boolean +turnOff(): boolean Toaster Toaster +turnOn(level:int): boolean -turnOn(): boolean This is not an override. It is a legal overload! Adriano Cruz (DCC-IM-UFRJ) It is not legal. The access was restricted. Indicated by the - signal (private) 07 - Inheritance and Polymorphism April 2, 2015 61 / 67 Overloading Overloaded methods MUST change the argument list. Method overloading is nothing more than having two methods with the same name but different argument lists. It is possible to write multiple versions of a method with different arguments lists. Overloaded methods CAN change the return type. The return types can be different, as long as the arguments are different. It is not possible to change only the return type. Overloaded methods CAN change the access modifier. It is possible to change the access levels in any direction. Overloaded methods CAN declare new or broader checked exceptions. later. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 62 / 67 Overloading It is possible to overload a method in a subclass. The subclass inherits the method and overloads it. Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 63 / 67 Overload Example c l a s s Multiplier { p u b l i c i n t multiply ( i n t x, i n t y) { ret u rn x * y; } p u b l i c d o u b l e m u l t i p l y( d o u b l e x , d o u b l e y ) { ret u rn x * y; } } p u b l i c c l a s s testMultiplier { p u b l i c s t a t i c v o i d main ( String [] args ) { M u l t i p l i e r a = new M u l t i p l i e r () ; i n t r e s u l t I n t = a . m u l t i p l y (2 , 3) ; d o u b l e r e s u l t D o u b l e = a . m u l t i p l y (2.0 , 3.0) ; } } Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 64 / 67 Section Summary 1 Inheritance 2 Members 3 Polymorphism 4 Exercise Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 65 / 67 Mixed Messages What is the output of the program? class A { i n t ivar = 7; v o i d m1 () { System . out . println ( " A ' s m1 , " ); } v o i d m2 () { System . out . println ( " A ' s m2 , " ); } v o i d m3 () { System . out . println ( " A ' s m3 , ") ; } } c l a s s B extends A { v o i d m1 () { System . out . println ( " B ' s m1 , " ); } } c l a s s C extends B { v o i d m3 () { System . out . println ( "C ' s m3 , " +(←֓ ivar +6) ) ; } } 1 p u b l i c c l a s s Mixed2 { p u b l i c s t a t i c v o i d main ( String [ ] ←֓ args ) { A a new A () ; B b new B () ; C c new C () ; A a2 = new C () ; /* Incluir os candidatos aqui */ } } (a) (b) (c) (d) b . m1 () ; c . m2 () ; a . m3 () ; c . m1 () ; c . m2 () ; c . m3 () ; a. m1 () ; b. m2 () ; c. m3 () ; a2 . m1 () ; a2 . m2 () ; a2 . m3 () ; Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 66 / 67 . The End Adriano Cruz (DCC-IM-UFRJ) 07 - Inheritance and Polymorphism April 2, 2015 67 / 67
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