1. © 2006 Pearson Education Inheritance 1 of 31 INHERITANCE Class Hierarchies Extending Objects Abstract Methods Abstract Classes Overriding Methods protected

2. © 2006 Pearson Education Inheritance 2 of 31 Everyone wants a car nowadays Remember the CSMobile? ­ sufficient for driving around town But what about camping trips? - need a van, a vehicle that’s big enough to hold more passengers, and all the camping gear And what if you want to look sharp? ­ need to get places in a hurry ­ like driving with the top down ­ So, need a convertible sportscar that moves fast

3. © 2006 Pearson Education Inheritance 3 of 31 Similarities and Differences What do these three vehicles have in common? ­ they’re all cars! all can move all have an engine all have doors all have one driver all hold a number of passengers What about these three vehicles is different? ­ the sportscar: convertible top, 2 doors, moves really fast, holds small number of people ­ the van: high top, 5 doors (one of which slides open), moves at moderate speed, holds large number of people ­ the CSMobile: normal top, 2 doors, moves slowly, holds moderate number of people

4. © 2006 Pearson Education Inheritance 4 of 31 Inheritance models “is a” relationships ­ object “is an” other object if it can behave in the same way ­ Inheritance uses similarities and differences to model groups of related objects Where there’s Inheritance, there’s an Inheritance Hierarchy of classes ­ Mammal “is an” Animal ­ Cat “is a” Mammal ­ transitive: a Cat “is an” Animal, too We can say: ­ Reptile, Mammal and Fish “inherit from” Animal ­ Dog, Cat, and Moose “inherit from” Mammal Inheritance Animal Reptile Mammal Fish Cat Moose Dog

5. © 2006 Pearson Education Inheritance 5 of 31 Inheritance, even in the car industry! Wow! What does this have to do with cars? ­ a SportsCar “is a” Car ­ a CSMobile “is a” Car ­ you get the picture... We call this a tree diagram, with Car as the “root” and SportsCar, CSMobile, Van as “leaves” (yes, it’s an upside down tree...) How can this help Andy and the TAs? First let’s discuss some important facts about Inheritance Car Van CSMobile SportsCar

6. © 2006 Pearson Education Inheritance 6 of 31 Superclasses and Subclasses Inheritance is a way of: ­ organizing information ­ grouping similar classes ­ modeling similarities between classes ­ creating a taxonomy of objects Animal is called superclass ­ or base class or parent class ­ in car example, Car is called superclass Fish is called subclass ­ or derived class or child class ­ in car example, SportsCar is subclass Any class can be both at same time ­ e.g., Mammal is superclass of Moose and subclass of Animal Can only inherit from one superclass in Java ­ otherwise, no limit to depth or breadth of class hierarchy ­ C++ allows a subclass to inherit from multiple superclasses (error-prone)

7. © 2006 Pearson Education Inheritance 7 of 31 Inheriting Capabilities and Properties Subclass inherits all public capabilities of its superclass ­ if Animal s eat and sleep, then Reptile s, Mammal s and Fish eat and sleep ­ if Car s move, then SportsCar s move! ­ anything superclass can do, subclass can do Subclass specializes its superclass ­ by adding new methods, overriding existing methods, and defining “abstract” methods declared by parent ­ we’ll see this in a few slides! Superclass factors out capabilities common to its subclasses ­ subclasses described by differences from superclass As a general pattern, subclasses: ­ inherit public capabilities (methods) ­ inherit private properties (instance variables) but do not have access ­ so can subclasses inherit variables that they CAN access?

8. © 2006 Pearson Education Inheritance 8 of 31 protected Instance Variables Answer: subclasses inherit protected variables and can access them! A variable that is declared protected by a superclass becomes part of the inheritance ­ variable becomes available for subclasses to access as if it were their own ­ in contrast, if an instance variable is declared private by a superclass, its subclasses won’t have access to it superclass could still provide protected access to its private instance variables via accessor and mutator methods How can I decide between private and protected ? ­ use private if you want an instance variable to be encapsulated by superclass e.g., doors, windows, spark plugs ­ use protected if you want an instance variable to be available for subclasses to change (and you don’t want to make the variable more generally accessible via accessor/mutator methods) e.g., engine, so that subclasses can soup it up!

9. © 2006 Pearson Education Inheritance 9 of 31 Inheritance Example Student inheritance hierarchy: ­ Student as baseclass ­ CollegeStudent as Student ’s subclass ­ CSstudent as subclass of CollegeStudent Student has one capability (or method) ­ study() which works by: going home, opening a book, and reading 50 pages. Student CollegeStudent CSstudent

10. © 2006 Pearson Education Inheritance 10 of 31 Inheritance Example (cont.) CollegeStudent “is a” Student, so inherits study() ­ But it overrides the study() method to work by: going to the library, studying for a midterm, and doing a problem set Finally the CSstudent also knows how to study() (it study() s the same way a CollegeStudent does) ­ However adds two capabilities: gitDown() and gitFunky() public void gitDown() { // Code to party } public void gitFunky() { // Code to do awesome CS dance } Each subclass specializes its superclass ­ Student knows how to study(), so all subclasses in hierarchy know how to study() ­ But the CollegeStudent does not study() the same way a Student does ­ And the CSstudent has some capabilities that neither Student nor CollegeStudent have ( gitDown() and gitFunky() )

11. © 2006 Pearson Education Inheritance 11 of 31 Inheritance as Form of Abstraction Root of class hierarchy is most general object because it is superclass to every other object in hierarchy ­ can always say much more about how a subclass behaves than how its superclass behaves ­ e.g., can say more about how a Van behaves than how a Car behaves!

12. © 2006 Pearson Education Inheritance 12 of 31 Inherit This! 5 things you might find in an Inheritance Hierarchy: 1) superclass is too general to declare all behavior, so each subclass adds its own behavior 2) superclass legislates an abstract behavior and therefore delegates implementation to subclasses ­ (see upcoming move method) 3) superclass specifies behavior, subclasses inherit behavior ­ (see upcoming startEngine method) 4) superclass specifies behavior, subclasses choose to override behavior ­ just because a subclass inherits a method, doesn’t mean that it must act in the same way as its superclass ­ subclass can choose to reject its superclass’ implementation of any method and “do it my way” 5) superclass specifies behavior, subclasses choose to override behavior in part ­ called partial overriding ­ you’ll learn this next lecture!

13. © 2006 Pearson Education Inheritance 13 of 31 Abstract Methods What if we know that all subclasses should have some capability, yet we don’t know how those subclasses will implement it? ­ e.g., all Car s know how to move, but each moves differently There would be no code that could be written for that capability that would apply to every subclass Methods for which no implementation makes sense should be abstract ­ abstract means it has no definition, only declaration ­ declaration highlights important features of method - its name, class of instance it returns (or none), and number and class of its parameters (if any) this information comprises what is known as the method’s “signature”

14. © 2006 Pearson Education Inheritance 14 of 31 Abstract Methods, continued Reserved word abstract is modifier that says every subclass has specific capability, but superclass is unaware of implementation of that capability ­ superclass defines policy for subclasses to implement ­ guarantees some subclass down in the hierarchy will implement method Although abstract methods have no implementation, they are not the same as empty methods ­ can’t instantiate an abstract class, i.e., a class that has one or more abstract methods ­ can instantiate a class with an empty method ­ most empty methods that you’ll see will be constructors ­ empty methods still need curly braces, abstract methods must not have curly braces, instead they must end in a semi-colon public void emptyMethod() { } abstract public void abstractMethod(); Any class which contains an abstract method must itself be declared abstract

15. © 2006 Pearson Education Inheritance 15 of 31 Abstract Classes Possible definition of Car ­ all Car s can move, ­ but superclass does not say how ­ superclass simply declares abstract methods, subclass must define all abstract methods in order to become concrete Abstract classes cannot be instantiated Once subclass has defined all abstract methods it can be instantiated ­ it is concrete! public abstract class Car { // define to provide moving behavior abstract public void move(); } // This will not compile. Car myCar = new Car(); note the semicolon!

16. © 2006 Pearson Education Inheritance 16 of 31 Code: Let’s make some Car s! (1 of 3) public abstract class Car { // properties of all Cars private Door _driverDoor; // protected, because subclasses // might want to soup up their Engine protected Engine _engine; // more properties go here public Car() { _driverDoor = new Door(); // initialize other variables here } // notice the semicolon at the end of the // abstract method declaration! This is // important! public abstract void move(); public void startEngine() { // code to start up engine } // more methods go here } // end of class Car What’s the difference between move and startEngine?

17. © 2006 Pearson Education Inheritance 17 of 31 Code: Let’s make some Car s! (2 of 3) - Now that we are defining move(), it looks like any other method public class Van extends Car { public Van() { super(); } public void move() { // code to move at a moderate speed } } // end of class Van public class CSMobile extends Car { public CSMobile() { super(); } public void move() { // code to just pitter along! } } // end of class CSMobile Note that neither Van nor CSMobile include the startEngine method. Think about what this means…

18. © 2006 Pearson Education Inheritance 18 of 31 Code: Let’s make some Car s! (3 of 3) public class SportsCar extends Car { private ConvertibleTop _top; // more properties here public SportsCar() { super(); _top = new ConvertibleTop(); // initialize other variables here } public void move() { // code to move really fast } public void startEngine() { // code to start engine with the // gusto of a SportsCar! Vroom! } } // end of class SportsCar

19. © 2006 Pearson Education Inheritance 19 of 31 Syntax: Declaring/Defining a Subclass To declare subclass of Car, we say that it extends superclass public class SportsCar extends Car Now, let’s look at constructor definition for SportsCar : public SportsCar() { super(); _top = new ConvertibleTop(); } What’s going on inside SportsCar ’s constructor?

20. © 2006 Pearson Education Inheritance 20 of 31 Superclass Constructors SportsCar is responsible for initializing instance variables it defines: _top = new ConvertibleTop(); ­ ConvertibleTop is a property specific to SportsCar s, so it is initialized in the constructor for SportsCar Likewise, Car is responsible for initializing its instance variables. ­ some are private and encapsulated ­ some are protected, so subclasses can use them! So... must call SportsCar ’s constructor and Car ’s constructor ­ to make sure any inherited instance variables are initialized! How can I do that? Always have subclass constructor call superclass constructor! Note that the abstract superclass, Car, isn’t explicitly instantiated and only a subclass uses the superclass’ constructor

21. © 2006 Pearson Education Inheritance 21 of 31 Syntax: super class Constructors Use reserved word super to call superclass constructor ­ it is as if superclass’ name is super and you are calling its constructor (i.e., calling method with same name as class) public SportsCar() { super(); // rest of constructor elided } You’ll always use this pattern: ­ call to superclass constructor must be first line of subclass constructor ­ calling superclass constructor anywhere else is compile-time error because initializing subclass may presuppose initialized superclass ­ then initialize any subclass instance variables In fact, is exactly the default constructor Java provides every class automatically ­ don’t have to call super(); but a good habit { super(); }

22. © 2006 Pearson Education Inheritance 22 of 31 super and Parameters In our example, Car takes no parameters in its constructor public Car() nothing between the parentheses! What if Car had a constructor that needed parameters to be passed to it? public Car(Driver driver) this Car expects a Driver ! Then SportsCar needs to pass Car ’s constructor a Driver ­ so either SportsCar needs to create a Driver, or needs to take one as parameter public SportsCar(Driver driver) this SportsCar expects a Driver !

23. © 2006 Pearson Education Inheritance 23 of 31 super and Parameters (continued) public SportsCar(Driver driver) { super(driver); } this SportsCar gets a Driver and passes it up to its superclass’ constructor! Note again that only a subclass will invoke the abstract superclass’ constructor and pass it its parameters What happens if you forget to call the superclass constructor here? ­ Java automatically calls super(); for you ­ But... latest definition for Car doesn’t have a constructor that takes no parameters ­ So... Compile Error. Java complains that there is no constructor declared public Car()

24. © 2006 Pearson Education Inheritance 24 of 31 Overriding (Redefining) Methods Subclasses can override, i.e., redefine inherited methods: ­ can choose to reject their superclass’ implementation of any method ­ this means they can write their own definitions for how to respond to messages sent to them ­ Car declares a startEngine() method that subclasses can either inherit (not change) or override (change) public class SportsCar extends Car { // declarations and constructor elided public void startEngine() { // code to start engine with the gusto of // a SportsCar! Vroom! } } Definition of startEngine replaces the one in superclass ­ must be declared identically (in name and parameter list!) — check documentation ­ otherwise, nothing special about declaration

25. © 2006 Pearson Education Inheritance 25 of 31 How does Java call methods? We can use Van in the same way we would use Car ­ because it behaves like the superclass in that it responds to the same messages How does Java determine which method to call when move() message is sent to instance? ­ this is where class of instance is important ­ when instance of Van is created, its class determines which methods will be called

26. © 2006 Pearson Education Inheritance 26 of 31 Method Resolution First, Java sees if instance’s class defines the method; if so, calls it If not, Java “walks up class inheritance tree” from subclass to superclass until it either ­ finds method, in which case it calls inherited method ­ doesn’t find method; this is compile-time error (sending message for which there is no method) This process is called method resolution. ­ for Van ’s startEngine(), Java calls Car ’s definition ­ for Van ’s move(), Java would call Van ’s definition Car move startEngine Van move Object (all classes that you write are automatically subclasses of Java’s Object class)

27. © 2006 Pearson Education Inheritance 27 of 31 Inheritance: Common Applications What are some common applications of Inheritance in programming? ­ extending GUI (Graphical User Interface) components, such as buttons, sliders, pull-down menus to do things that are useful to your program PushButton (class written by us) defines a release() method to do nothing (i.e. PushButton ’s release() is an empty method) ­ PushButton is a general class that knows how to respond to being clicked on, an “event” ­ when user clicks on PushButton, its release() method is called automatically by Java’s “event dispatcher” We could extend PushButton in order to redefine the release() method to do something useful! ­ like controlling a SportsCar or a CSMobile or a Van !

28. © 2006 Pearson Education Inheritance 28 of 31 public class VanButton extends PushButton { private Van _van; /** * Not so clean to have button contain car, * so we associate button with car in * constructor. */ public VanButton(Van myVan) { super( "Van" ); _van = myVan; } public void release() { _van.move(); } } // end of class VanButton Typical constructor, except for "Van" ­ "Van" is called a string ­ strings are words, phrases, or any other sequence of alphanumeric characters ­ more on strings later Let’s move some Car s (1 of 2)

29. © 2006 Pearson Education Inheritance 29 of 31 Let’s move some Car s (2 of 2) public class SportsCarButton extends PushButton { private SportsCar _sportsCar; public SportsCarButton(SportsCar mySC) { super( "SportsCar" ); _sportsCar = mySC; } public void release() { _sportsCar.move(); } } // end of class SportsCarButton public class CSMobileButton extends PushButton { } // end of class CSMobileButton Can you fill this class O in?

30. © 2006 Pearson Education Inheritance 30 of 31 Making the Program public class CarApp extends wheels.users.Frame { private Van _van; private VanButton _vanButton; private CSMobile _csMobile; private CSMobileButton _csMobileButton; private SportsCar _sportsCar; private SportsCarButton _sportsButton; public CarApp() { super(); _van = new Van(); _vanButton = new VanButton(_van); _csMobile = new CSMobile(); _csMobileButton = new CSMobileButton(_csMobile); _sportsCar = new SportsCar(); _sportsButton = new SportsCarButton(_sportsCar); // code to put buttons in different // locations on the screen } public static void main(String[] argv) { CarApp app = new CarApp(); } }// end of class CarApp remember to associate each button with the Car it’s expecting!

31. © 2006 Pearson Education Inheritance 31 of 31