1. week111 APCS-AB: Java Inheritance November 17, 2005

2. week112 Checkpoint Design Project - from Henry and Eric USB drives (and check-out form to be signed by parents)

3. week113 Nested classes A class can be declared inside another class The nested class is a member of the enclosing class, so it can use the enclosing class’s instance variables & methods, even if they are private But enclosing class can only use data in the nested class if the data is public  (In this case, it is okay to have data public, because only the enclosing class would be able to see it) A class should only be nested inside another if it makes sense by the design of the objects The static modifier can be applied to a nested class A nonstatic nested class is called an inner class  An inner class is associated with each instance of the enclosing class

4. week114 Making a Zoo If we wanted to model a zoo, what kind of objects would we need? What would those objects do? (i.e. what are the instance variables?) What would those objects know? (i.e. what are the methods?)

5. week115 Our zoo In order to not duplicate lots of information that is shared by our data, could be maybe set up a hierarchy of objects similar to the Taxonomy that we’ve learned about in Biology class? Animal Mammal Reptile Canine Primate Dog Wolf

6. week116 Of course we can… we just need some Inheritance

7. week117 Inheritance Inheritance allows us to do just that !  We can describe higher level objects (like Animal or Mammal) and have characteristics (or actions) defined there that are true for any object beneath it in the hierarchy  This saves us from repeating tons of code! The high level class is the parent class (aka superclass or base class) The derived (lower level) classes is the child class (aka subclass)

8. week118 Inheritance Form of software reuse in which classes are created by absorbing an existing class’s data and methods  and then you can embellish them with new or modified capabilities Allows you to define a very general class and later define more specialized classes by adding new details

9. week119 So you have a hierarchical relationship -- now what?? You must define the Superclass: public class Animal{ String name; int size; public void eat(){ //do something } } Then you define the the Subclass, which needs to extend the Superclass (using the Java keyword extends). The subclass has everything in the superclass, plus anything else you want to add! public class Dog extends Animal{ public void bark(){ //do something } }  Note: if fido is an object of type Dog, you can call fido.eat() and java will recognize the method as valid

10. week1110 IS-A Relationship Inheritance is always an IS-A relationship  To make sure you are designing your classes properly and making use of inheritance, ask yourself: Does SubClass IS-A Superclass make sense? Does Dog IS-A Animal make sense? Does Janitor IS-A Employee make sense? Does Triangle IS-A ThreeDimensionalShape make sense?

11. week1111 Designing with Inheritance Make sure you think out the relationships between all the objects and what makes sense Practice Abstraction  Focus on the big picture and commonalities between objects, rather than on implementation details

12. week1112 Another Sample Hierarchy Shape TwoDimensionalShape ThreeDimensionalShape Triangle Square Circle Tetrahedron Cube Sphere

13. week1113 Book Example (From Ch 8)  Book protected int pages = 1500; setPages(int numPages) int getPages()  Dictionary (is-a Book) int definitions = 52500; Double computRatio() setDefintions(int numDefinitions) getDefintions() public class Words{ Dictionary webster = new Dictionary(); webster.getPages(); webster.getDefinitions(); webster.computeRatio(); }

14. week1114 How it works The Book code is needed to create the definition of Dictionary, but we never need a Book object Inheritance is a one-way street  Dictionary can use methods/variables of Book, but not the other way around

15. week1115 What is the protected qualifier? The protected keyword is the best encapsulation that still permits inheritance  If a superclass declares its data or methods protected it means that any subclasses can access the data It also means that any thing in that package can also access the data Private superclass members can only be changed by a subclass by using non-private accessor methods

16. week1116 Methods in the Superclass The Superclass can define methods and indicate data that all the Subclasses should have But what if the subclass wants to behave differently than the superclass  It can override the superclass’s method  Overriding is kind of like overloading What do you think the difference is?

17. week1117 Overloading public class myClass{ public myClass(){ } public myClass(int x, int y){ } public void move(){ } public void move(int x){ } Overloading = Two methods with the same name, But different method signatures

18. week1118 Overriding Superclass has a method: public void rotate(){ System.out.println(“Spin Right”); } Subclass wants to redefine method: public void rotate(){ System.out.println(“Spin Left”); } Overriding = Method with the same name But different functionality in a superClass and subClass

19. week1119 Overriding We’ve already been using Overriding!  Every class in Java extends from java.lang.Object  This class defines a toString method (which by default returns the internal representation of the object)  When we make our toString methods, we override the default version and return something more meaningful What do we do if we want to use part of the superclass’s method (or part of its constructor)?  Use the super reference

20. week1120 The super reference The super reference refers to an object’s direct superclass (one level up in the hierarchy tree) super() refers to the Superclass’s constructor super.doStuff() would refer to the Superclass’s doStuff method super.x would refer to the Superclass’s variable named x (assuming a protected variable)

21. week1121 Constructors in inheritance Constructors are not inherited from the superclass at all. If you want to access the superclass constructors, you must use super()

22. week1122 A little example class Animal { String name; String noise; public Animal(String name, String noise){ this.name = name; this.noise = noise; } public void movement (){ System.out.println(“Moving”); }

23. week1123 A little example class Dog extends Animal { int mySize; public Dog(String name, String noise, int size){ super(name,noise); mySize = size; } public Dog(){ super(“Fido”, “Woof”); } public void movement (){ System.out.println(“Walking”); }

24. week1124 Sudoku How’s it working?  Any questions? Due Friday…

25. week1125 APCS-AB: Java Polymorphism, Abstract and Interfaces November 18, 2005

26. week1126 Polymorphism If we say that every class in Java extends (inherits) from Object, that means that everything IS-A Object. Inheritance allows us to use the more general category to refer to all the subclasses.  Use the generic superclass Animal to refer to all the different animal subclasses This allows you to have really flexible code  Its cleaner, more efficient, easier to develop & easier to extend!

27. week1127 In the world before inheritance We declared a reference variable  Dog myDog; We created the object and assigned it to the reference  myDog = new Dog();  Or in one step: Dog myDog = new Dog(); And the reference type and the object were the same (they were both Dog s)

28. week1128 Now… With polymorphism, the reference type and the object type can be different Animal myDog = new Dog(); The reference type can be a superclass of the actual object type.

29. week1129 Some code Animal [] animals = new Animal[5]; animals[0] = new Dog(); animals[1] = new Cat(); animals[2] = new Wolf(); animals[3] = new Hippo(); animals[4] = new Lion(); for(int i=0; i<animals.length; i++){ animals[i].eat(); animals[i].move(); }

30. week1130 What it does animals[i].eat(); animals[i].move(); When i is 0, we have a Dog and the Dog’s eat() method is called, but when i is 1, we have a Cat and the Cat’s eat method is called This works for any of the Animal-class methods The process of deciding which method (i.e. from which object) to use at run time is called dynamic (or late) binding

31. week1131 What it means Write code using polymorphic arguments, declare the method parameter as a superclass type – then you can pass in any subclass object at runtime  So you can write your code, pass it off to someone else, and they can add all the new subclass types – your methods will still work

32. week1132 Rules for overriding When you override a method, you are agreeing to fulfill the contract (the method specification)  Return type and parameters (number and type) need to be exactly the same as the overridden method in the superclass  The method can’t be less accessible (but it can be more) So the subclass method can’t be private if the superclass method was public

33. week1133 Polymorphism in action The Java Library is bursting with polymorphism  Tons of methods with generic and abstract arguments and return types  Collections, classes, & methods in the library work with all the classes you create (classes that the creators of Java had no idea you were going to write!) Because everything is an Object (there is an implicit extension)

34. week1134 What’s in Object class? “Class Object is the root of the class hierarchy. Every class has Object as a superclass. All objects, including arrays, implement the methods of this class.” The mega superclass has:  boolean equals()  Class getClass()  int hashCode()  String toString()  Object clone()  And more…

35. week1135 Abstract

36. week1136 Abstract Classes Classes that can’t be instantiated Animal class is good for inheritance and polymorphism, but we can’t really have a generic Animal object  So restrict it by declaring it an abstract class  Inheritance still works, so the subclasses still benefit from the Animal definition, we’re just enforcing the fact that you can’t create an object of type Animal

37. week1137 Concrete Classes When you are designing inheritance structure – some classes are specific enough to be instantiated  These are the concrete ones

38. week1138 Abstract Methods If a method must be overriden – make it abstract  You’ll get a compile time error if you don’t write the method in the subclass An abstract method has no body, it exists only for polymorphism  The first concrete class in the inheritance tree must implement all abstract methods An abstract method has to be in an abstract class (but that class can contain non-abstract methods as well)

39. week1139 Interfaces

40. week1140 Interfaces An Interface is a 100% abstract class, it can’t be instantiated Another way to take advantage of polymorphism – another contract that objects can fulfill Let’s explore this a little more to see why it is necessary

41. week1141 Two superclasses? Why do you think we have interfaces instead of having the base class extend two superclasses?  (Why can multiple inheritance be a bad thing?) Digital Recorder burn() CDBurner burn() DVDBurner burn() ComboDrive

42. week1142 Multiple Inheritance Leads to ambiguity  “The Deadly Diamond of Death”  In our example: which burn method would ComboDrive inherit? Don’t want the language to have to support/encode special rules to deal with ambiguity Ambiguity is bad in programming!

43. week1143 And therefore Java has interfaces instead of multiple inheritance Gives you the benefits of multiple inheritance without ever putting you in the situation of the Deadly Diamond of Death All methods in an interface are abstract  Nothing is inherited, the subclass must implement the methods and the JVM will never get confused about which version of an inherited method it was supposed to call

44. week1144 Another interface benefit You can now have classes from different inheritance trees implement the same interface Animal Canine DogLionCat Feline Wolf Robot RoboDogAgent Pet

45. week1145 Interfaces in JavaAPI Comparable Iterator List Collection EventListener ErrorHandler And on and on and on  (They’re listed in italics in the API docs)

46. week1146 Exercise – Draw it! public class Gamma extends Delta implements Epsilon{} public interface Epsilon{} public interface Beta {} public class Alpha extends Gamma implements Beta {} public class Delta {}

47. week1147 Other Stuff

48. week1148 null and this references null  Represents a reference that does not point to a valid object  So we can use null for objects that do not exist, but not primitives this  The way for an object to reference itself  In a method, it can be used to refer to instance variables and other methods in the object

49. week1149 Aliases Having multiple names for the same object in memory Car car1 = new Car(“Audi”); Car car2 = new Car(“Ford”); car2 = car1; All references to the object that was originally references by car2 are now gone – oops no more Ford! Both car1 and car2 point to the same object! == operator compares the references and sees if they are aliases of each other

50. week1150 Wrapper Classes Remember primitive data types are not objects  But maybe you want the primitive as an equivalent object Java provides wrapper classes for all primitives  Integer, Float, Double, Character, Long Integer n = new Integer(42); int i = n.intValue();  These classes also have useful constants & static methods Integer.MAX_VALUE Integer.MIN_VALUE Integer.parseInt(“42”);//returns an int

51. week1151 Checkpoint Sudoku  Where are you with it? Do you need more time? This weekend  Study: Note - inheritance stuff is in Chapters 8 & 9 (although like all the chapters in the book, we haven’t done everything in those chapters  (I’ll try to catch up on grading!) Monday - a little inheritance lab of some sort Test on Tuesday