CS102 – OOP_2

1. CS102 – OOP_2 Inheritance, Polymorphism, Interfaces & Abstract classes. David Davenport

2. Abstract Classes & Methods Motivation Do not want to include instances of common (base) class in polymorphic collection Want guarantee all objects in a polymorphic collection include certain methods

3. {Rectangle} {Rectangle} {Circle} Get into Shapes… • A collection of Rectangles & Circles? Ensure instances of TwoDShape cannot be added to collection TwoDShape Rectangle Circle picture { ?? } { TwoDShape[] }

4. University People… • Abstract vs. Concrete classes Person Staff Student Academic Non-Academic UnderGrad Grad Secretarial Security Cleaning

5. Get into Shapes… • Compute area of collection of shapes Ensure instances of TwoDShape cannot be added to collection Need guarantee that all future classes implement getArea() TwoDShape +getArea() Rectangle +getArea() Circle +getArea() Triangle Polygon RightAngled Isosceles Equilateral

6. Abstract Classes (& methods) • Abstract classes provide • common parent which cannot be instantiated • a guarantee sub-classes already haveor must implement certain methods • Create with keyword “abstract” e.g. public abstract class Media {…} (sub-class extends abstract class as normal) • Can include • Properties (& constructors!) • Implemented methods • Abstract methods (create with keyword “abstract” & no body) e.g. public abstract double getDuration();

7. The MusicCD Collection Object toString(), clone(), … title, artist, price, tracks getTitle(), getArtist(), getPrice(), getDuration() MusicCD CDCollection Set of MusicCD getValue() getTotalDuration() DiscountCD Discount setDiscount(-) getPrice()

8. The Media Hierarchy Object toString(), clone(), … Media price, title, getPrice(), getTitle() {abst} getDuration() Library Set of Media getValue() getTotalDuration() DVD MusicCD artist, tracks getDuration() Video length director getDuration() DiscountCD discountsetDiscount(-) getPrice()

9. The Media Class Declare as abstract - cannot be instantiated public abstract class Media { String title; double price; public Media( String title, double price) { this.title = title; this.price = price; } public String getTitle() { return title; } public double getPrice() {return price; } public void setPrice( double newPrice) { price = newPrice; } public abstract int getDuration(); public String toString() { return title + "\t" + getPrice() + "\n"; } } Can includeconstants, properties constructors!, implemented & abstract methods abstract method – look, no body! Class must be abstract if it includes abstract methods

10. The Video Class // Video // David, 23/3/02 public class Video extends Media { int length; String director; public Video( String title, String director, int length, int price) { super( title, price); this.length = length; this.director = director; } public int getDuration() { return length; } public String getDirector() { return director; } public String toString() { return "VIDEO " + super.toString(); } } Sub-class simply extends “abstract” parent as normal Calls parent constructor Implements required “abstract” method Overrides & reuses parent method

11. Interfaces Motivation Provides a form of multiple-inheritance Guarantee specified methods exist Powerful design approach!

12. Interfaces • An interface is the boundary between two systemsthrough which they connect/communicate • Often standardised Advantage: can change either side without affecting the other! SystemA SystemB interface

13. Java Interfaces • Declare with public interface X {…} • restricted to constants & abstract methods only • cannot be instantiated • guarantees any implementing classhas specified methods (else cannot be instantiated) • Classes extend one class & implement interfaces e.g. public class U extends V implements X,Y,Z {…} • Can view as a special form of class so class U is_a V, is_a X, is_a Y, is_a Z i.e. a form of multiple inheritance

14. Simple example… Define interfacewith abstract method public interface Pointable { public boolean contains( int x, int y); } public class Circle extends TwoDShape implements Pointable { int radius; public Circle( int radius) { super(); this.radius = radius; } public int getRadius() { return radius; } public boolean contains( int x, int y) { // set result true iff x,y inside circle... return result; } } Define classthat implements interface Required method...won’t compile if omitted

15. Another example… • Multiple inheritance? SmartPhone is_a Phone is_a Computer is_a NetworkedDevice Brain is_a Computer is_a BiologicalDevice GasCooker is_a Cooker is_a GasDevice Notebook is_a Computer is_a ElectricalDevice is_a NetworkedDevice ElectricCooker is_a Cooker is_a ElectricalDevice Microwave is_a Cooker is_a ElectricalDevice

16. One soln with interfaces Computer ElectricalDevice Cooker Brain Notebook Microwave ElectricCooker GasCooker

17. Another soln with interfaces Computer ElectricalDevice Cooker Brain Notebook Microwave ElectricCooker GasCooker

18. Interfaces… • as a form of multiple-inheritance public class U extends V implements X, Y, Z {U} V a; a = new V(); a = new U(); X b; b = new X(); b = new U(); b = a; b = (U) a; Y c; c = new U(); c = b; {U} {X} {V} c {Y} {Y} {Z} {V} b {X} a {V}

19. Categorised Library… Simple: just add set/get category to base class… Object toString(), clone(), … Media price, title, getPrice() {abst} getDuration()set & getCategory() Library Set of Media getValue() getTotalDuration() listInCategory(cat) MusicCD artist, tracks getDuration() Video length director getDuration() DiscountCD discount setDiscount(-) getPrice()

20. The Categories Interface // Categories // David, 23/3/02 public interface Categories { public final String COMEDY = "COMEDY"; public final String ADVENTURE = "ADVENTURE"; public final String CRIME = "CRIME"; public final String NONFICTION = "NONFICTION"; public final String ROCK = "ROCK"; public final String POP = "POP"; public final String CLASSICAL = "CLASSICAL"; public String getCategory(); public void setCategory( String c); } Declare interface (instead of class) Can only include constants & abstract methods Methods abstract by default!

21. Media with Interface (1) Object toString(), clone(), … Categories Media price, title, getPrice() {abst} getDuration()set & getCategory() Library Set of Media getValue() getTotalDuration() listInCategory(cat) MusicCD artist, tracks getDuration() Video length director getDuration() DiscountCD discount setDiscount(-) getPrice()

22. The Media Class • public abstract class Media implements Categories { • // include all // properties, constructors & methods • // as before.. • // ********************************* • // & add properties & methods • // to implement Categories interface • String category; • public String getCategory() { • return category; • } • public void setCategory( String c) { • category = c; • } • // ********************************* • } Class implements one or more interfaces by adding any necessary code

23. Categories CatVideo setCategory() getCategory() Media with Interface (2) Object toString(), clone(), … Media price, title, getPrice() {abst} getDuration() Library Set of ?Categories? getValue() getTotalDuration() listInCategory(cat) Categories MusicCD artist, tracks getDuration()setCategory() getCategory() Video length director getDuration() DiscountCD discount setDiscount(-) getPrice()

24. Media with Interface (3) • Moral: Design interfaces first! (a) (b) IMedia Media ICategories Library - initially: set of IMedia - later: set of ICategories

25. Extending Interfaces (a) public interface IMedia { public int getPrice(); public int getDuration(); } public abstract classMedia implements IMedia { public int getPrice() {…} } public classLibrary { ArrayList<IMedia>() items; public int getValue () {…} public int getTotalDuration() {…} } public classVideo extends Media { public int getDuration() {…} } public interface ICategories extends IMedia { public String getCategory(); public void setCategory( String c); } public class CatVideo extends Video implements ICategories { public String getCategory() {…} public void setCategory( String c) {…} } public classLibrary { ArrayList<ICategories>() items; public int getValue () {…} public int getTotalDuration() {…} public void listInCategory( cat) {…} }

26. Media with Interface (3) • Moral: Design interfaces first! (a) (b) IMedia Media IMedia ICategories ICategories ICatMedia Library - initially: set of IMedia - later: set of ICategories Library - initially: set of IMedia - later: set of ICatMedia

27. Extending Interfaces (b) public interface IMedia { public int getPrice(); public int getDuration(); } public abstract classMedia implements IMedia { public int getPrice() {…} } public classLibrary { ArrayList<IMedia>() items; public int getValue () {…} public int getTotalDuration() {…} } public classVideo extends Media { public int getDuration() {…} } public interface ICatMedia extends IMedia, ICategories { } public interface ICategories { public String getCategory(); public void setCategory( String c); } public class CatVideo extends Video implements ICatMedia { public String getCategory() {…} public void setCategory( String c) {…} } public classLibrary { ArrayList<ICatMedia>() items; public int getValue () {…} public int getTotalDuration() {…} public void listInCategory( cat) {…} }

28. Interface notes • Can’t create objects of interface type, only a type that implements it • Interfaces can extend interfaces (but not classes) to form a hierarchy separate from class one • Start design with interfaces! • Java provides “instanceof” operator to test object type (but use very sparingly) • See also “getClass()”

29. Other notes… • Method access modifiersprotected vs. default(nothing specified) • “default” is package access only • “protected” is package, plus can be sub-classed from another package!