9: Polymorphism. OOP = abstraction + inheritance + polymorphism

Transcription

1 9: Polymorphism OOP = abstraction + inheritance + polymorphism 1

2 Substitutability A Circle Shape A Square A Line 2

3 Extensibility A Circle Shape A Square A Line A Triangle 3

4 Interface & Implementation Shape Establishes a common interface Circle Square Line Implementations of the common interface 4

5 Upcasting Shape s = new Circle(); Cast "up" the inheritance diagram Shape Circle Reference to a Circle object Square 5 Line

6 Why Upcast? All true OOP programs have some upcasting somewhere, even if it s implicit. Upcasting allows the isolation of type-specific details from the bulk of the code. This is a form of decoupling. Code becomes simpler to write and simpler to read. Most important, changes in type do not propagate changes in code. 6

7 A problem? Shape s = new Circle(); You might think s-> calls this since s is a Shape pointer Shape We want it to call this since s actually points to a circle Circle Square Line 7

8 Early or Static Binding Invocations methodb(); Definitions: Different Signatures void methoda() { methoda(); void methodb() { methodc(); void methodc() { Function calls compile to jumps to object code addresses. 8

9 Late or Dynamic Binding Shape s = new Square(); Shape t = new Line(); Shape u = new Circle(); Invocations s.; Circle Definitions: Same Signatures Circle. { t.; Square Square. { u.; Line Line. { Virtual Method Table (VMT) 9

10 Shapes... class Shape { public void { public void { class Circle extends Shape { public void { System.out.println("Circle."); public void { System.out.println("Circle."); class Square extends Shape { public void { System.out.println("Square."); public void { System.out.println("Square."); class Triangle extends Shape { public void { System.out.println("Triangle."); public void { System.out.println("Triangle."); 10

11 ...Shapes class RandomShapeGenerator { private Random rand = new Random(); public Shape next() { switch (rand.nextint(3)) { default: case 0: return new Circle(); case 1: return new Square(); case 2: return new Triangle(); public class Shapes { private static RandomShapeGenerator gen = new RandomShapeGenerator(); public static void main(string[] args) { Shape[] s = new Shape[9]; for (int i = 0; i < s.length; i++) s[i] = gen.next(); for (Shape shp : s) shp.; Circle. Circle. Circle. Circle. Square. Square. Triangle. Circle. Square. Circle. Square. Square. Circle. Triangle. Circle. Triangle. Square. Square. Triangle. Triangle. Circle. Triangle. Circle. Circle. Square. Circle. Triangle. 11

12 Instrument Extensibility void play() String what() void adjust Wind void play() String what() void adjust Percussion void play() String what() void adjust Stringed void play() String what() void adjust Wodwind void play() String what() Brass void play() void adjust 12

13 Music... public enum Note { MIDDLE_C, C_SHARP, B_FLAT; // Etc. class Instrument { void play(note n) { print("instrument.play() " + n); String what() { return "Instrument"; void adjust() { print("adjusting Instrument"); class Wind extends Instrument { void play(note n) { print("wind.play() " + n); String what() { return "Wind"; void adjust() { print("adjusting Wind"); class Percussion extends Instrument { void play(note n) { print("percussion.play() " + n); String what() { return "Percussion"; void adjust() { print("adjusting Percussion"); 13

14 ..Music... class Stringed extends Instrument { void play(note n) { print("stringed.play() " + n); String what() { return "Stringed"; void adjust() { print("adjusting Stringed"); class Brass extends Wind { void play(note n) { print("brass.play() " + n); void adjust() { print("adjusting Brass"); class Woodwind extends Wind { void play(note n) { print("woodwind.play() " + n); String what() { return "Woodwind"; 14

15 ...Music public class Music { public static void tune(instrument i) { i.play(note.middle_c); public static void tuneall(instrument[] e) { for(instrument i : e) tune(i); public static void main(string[] args) { Instrument[] orchestra = { new Wind(), new Percussion(), new Stringed(), new Brass(), new Woodwind() ; tuneall(orchestra); Wind.play() MIDDLE_C Percussion.play() MIDDLE_C Stringed.play() MIDDLE_C Brass.play() MIDDLE_C Woodwind.play() MIDDLE_C 15

16 Constructors & Polymorphism class Meal { Meal() { print("meal()"); class Bread { Bread() { print("bread()"); class Cheese { Cheese() { print("cheese()"); public class Sandwich extends PortableLunch { private Bread b = new Bread(); private Cheese c = new Cheese(); private Lettuce l = new Lettuce(); public Sandwich() { print("sandwich()"); public static void main(string[] args) { new Sandwich(); class Lettuce { Lettuce() { print("lettuce()"); class Lunch extends Meal { Lunch() { print("lunch()"); class PortableLunch extends Lunch { PortableLunch() { print("portablelunch()"); Meal() Lunch() PortableLunch() Bread() Cheese() Lettuce() Sandwich() 16

17 Order of Initialization 1. The base-class constructor is called. This step is repeated recursively so that the very root of the hierarchy is constructed first, followed by the nextderived class, etc., until the most-derived class is reached. 2. Member initializers are called in the order of declaration. 3. The body of the derived-class constructor is called. 17

18 Constructors & Polymorphism... Inside a constructor overridden versions of polymorphic methods are used. This behavior is potentially problematic: The overridden method assumes that all parts of the derived class have been initialized. But in a base-class constructor, they haven t because... the derived-class constructor hasn t been called yet. 18

19 ...Constructors & Polymorphism class Glyph { void { print("glyph."); Glyph() { print("glyph() before "); ; print("glyph() after "); class RoundGlyph extends Glyph { Glyph() before private int radius = 1; RoundGlyph., radius = 0 RoundGlyph(int r) { Glyph() after radius = r; RoundGlyph.RoundGlyph(), radius = 5 print ("RoundGlyph.RoundGlyph(), radius = " + radius); void { print ("RoundGlyph., radius = " + radius); public class PolyConstructors { public static void main(string[] args) { new RoundGlyph(5); 19

20 Initialization Operations The order of initialization is: 1. Storage allocated for the object is initialized to binary zero. 2. Base-class constructors are called. Any methods they call are resolved through polymorphism. 3. Member initializers are called in the order of declaration. 4. The body of the derived-class constructor is called. The lesson: constructors should call final methods only, if at all. 20

21 Pure Inheritance vs. Extension Shape Useful f() g() MoreUseful Line Is a Shape f() g() u() v() w() Is like a Useful Subtypting: re-using the interface Cannot extend the interface Allows full substitutability Subclassing: reusing the implementation Prevents full substitutability Talking to a Shape Circle, Square, Line or new type of Shape Talking to a Useful Useful part MoreUseful part 21

22 Downcasting & RTTI Normally one should try to do everything with upcasting. Useful f() g() If the class is extended, one must downcast to access the extended methods. Java casts are always checked at runtime. Upcast: always safe MoreUseful f() g() u() v() w() Downcast: must be checked 22

23 Downcasting class Useful { public void f() { public void g() { class MoreUseful extends Useful { public void f() { public void g() { public void h() { public void i() { public void j() { public class RTTI { public static void main(string[] args) { Useful u = new Useful(), m = new MoreUseful(); u.f(); m.g(); m.h(); ((MoreUseful)m.h(); ((MoreUseful)u.h(); Extends the interface Compile error RTTI Runtime 23

24 Polymorphism & Composition class Actor { public void act() { class HappyActor extends Actor { public void act() { print("happyactor"); class SadActor extends Actor { public void act() { print("sadactor"); Composition gives flexibility that can t be acheived through inheritance. 24 Behavior may be changed through a change in state. class Stage { private Actor actor = new HappyActor(); public void change() { actor = new SadActor(); public void performplay() { actor.act(); public class Transmogrify { public static void main(string[] args) { Stage stage = new Stage(); stage.performplay(); stage.change(); stage.performplay();

25 Summary: The Key to OOP Abstraction OOP Inheritance Polymorphism OOP is not just about creating types, but ensuring their proper behavior in all situations. Client code is decoupled from the specific types upon which it acts. Programs should be easy to write, read and extend. 25