7: Polymorphism. Interface & Implementation. Hands-on Java Seminar 2000 Bruce Eckel page 1. A Circle.

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1 Chart 7: Polymorphism A Circle A Square A Line Substitutability 1 A Circle A Square A Line A Triangle Extensibility 2 Interface & Implementation Establishes a common interface Circle Square Line Implementations of the common interface 3 Hands-on Java Seminar 2000 Bruce Eckel page 1

2 Upcasting s = new Circle(); Cast up the inheritance diagram Reference to Circle object Circle Square Line 4 Why Upcast? All true OOP programs have some upcasting somewhere even if it s implicit Upcasting allows us to isolate typespecific details from the bulk of your code, i.e. decoupling Code is simpler to write and read Most important: changes in type do not propagate changes in code 5 You might think s. calls this since s is a We want it to call this since s actually points to a Circle A problem? s = new Circle(); Circle Square Line 6 Hands-on Java Seminar 2000 Bruce Eckel page 2

3 Early Function Call Binding... function call function call function call address address address function body 7 Dynamic Binding in Java... s.draw( ) s.draw( ) s.draw( ) Dynamic Binding Mechanism Same signatures: Circle draw( ) Square draw( ) Line draw( ) 8 //: c07:s.java // Polymorphism in Java. Example class { void { void { class Circle extends { void { System.out.println("Circle."); void { System.out.println("Circle."); class Square extends { void { System.out.println("Square."); void { System.out.println("Square."); class Triangle extends { void { System.out.println("Triangle."); void { System.out.println("Triangle."); 9 Hands-on Java Seminar 2000 Bruce Eckel page 3

4 Example cont. public class s { public static rand() { switch((int)(math.random() * 3)) { default: case 0: return new Circle(); case 1: return new Square(); case 2: return new Triangle(); public static void main(string args[]) { s[] = new [9]; // Fill up the array with shapes: for(int i = 0; i < s.length; i++) s[i] = rand(); // Make polymorphic method calls: for(int i = 0; i < s.length; i++) s[i].; 10 Output: Circle. Triangle. Circle. Circle. Circle. Square. Triangle. Square. Square. 11 Extensibility Instrument Wind Percussion Stringed Woodwind Brass 12 Hands-on Java Seminar 2000 Bruce Eckel page 4

5 Example Example cont. Example cont. class Instrument { public void play() { System.out.println("Instrument.play()"); public String what() { return "Instrument"; public void adjust() { class Wind extends Instrument { public void play() { System.out.println("Wind.play()"); public String what() { return "Wind"; public void adjust() { 13 class Percussion extends Instrument { public void play() { System.out.println("Percussion.play()"); public String what() { return "Percussion"; public void adjust() { class Stringed extends Instrument { public void play() { System.out.println("Stringed.play()"); public String what() { return "Stringed"; public void adjust() { 14 class Brass extends Wind { public void play() { System.out.println("Brass.play()"); public void adjust() { System.out.println("Brass.adjust()"); class Woodwind extends Wind { public void play() { System.out.println("Woodwind.play()"); public String what() { return "Woodwind"; 15 Hands-on Java Seminar 2000 Bruce Eckel page 5

6 Example cont. public class Music3 { // Doesn't care about specific type: static void tune(instrument i) { i.play(); //... static void tuneall(instrument[] e) { for(int i = 0; i < e.length; i++) tune(e[i]); public static void main(string[] args) { Instrument[] orchestra = new Instrument[5]; int i = 0; // Upcasting during addition to the array: orchestra[i++] = new Wind(); orchestra[i++] = new Percussion(); orchestra[i++] = new Stringed(); orchestra[i++] = new Brass(); orchestra[i++] = new Woodwind(); tuneall(orchestra); ///:~ 16 Output: Wind.play() Percussion.play() Stringed.play() Brass.play() Woodwind.play() 17 Abstract classes & abstract methods (can have data and definitions) abstract Instrument abstract ; { /* */ abstract ; Cannot create instances extends extends extends Must define all abstract methods to create instances (or class is still abstract) Wind Percussion Stringed extends Woodwind extends Brass 18 Hands-on Java Seminar 2000 Bruce Eckel page 6

7 Example An abstract Instrument Some methods and data may be defined abstract class Instrument { int i; // storage allocated for each public abstract void play(); public String what() { return "Instrument"; public abstract void adjust(); Rest of the code is the same This maps with C++, may have been an early design Interfaces introduced in the next lecture 19 Constructors & Polymorphism Order of constructor calls class Meal { Meal() { System.out.println("Meal()"); class Bread { Bread() { System.out.println("Bread()"); 20 class Cheese { Cheese() { System.out.println("Cheese()"); class Lettuce { Lettuce() { System.out.println("Lettuce()"); class Lunch extends Meal { Lunch() { System.out.println("Lunch()"); class PortableLunch extends Lunch { PortableLunch() { System.out.println("PortableLunch()"); 21 Hands-on Java Seminar 2000 Bruce Eckel page 7

8 Example cont. public class Sandwich extends PortableLunch { Bread b = new Bread(); Cheese c = new Cheese(); Lettuce l = new Lettuce(); Sandwich() { System.out.println("Sandwich()"); public static void main(string[] args) { new Sandwich(); Meal() Lunch() PortableLunch() Bread() Cheese() Lettuce() Sandwich() 22 Order of Initialization 1) The base-class constructor is called. This step is repeated recursively such that the very root of the hierarchy is constructed first, followed by the nextderived class, etc., until the mostderived class is reached 2) Member initializers are called in the order of declaration 3) The body of the derived-class constructor is called 23 Polymorphism & Constructors Inside constructor, overridden method is used! This can produce incorrect results Overridden method assumes all parts of derived class have been initialized But you re still in the base part of the constructor Derived constructor hasn t been called yet! 24 Hands-on Java Seminar 2000 Bruce Eckel page 8

9 Example Example cont. abstract class Glyph { abstract void ; Glyph() { System.out.println("Glyph() before "); ; System.out.println("Glyph() after "); 25 class RoundGlyph extends Glyph { int radius = 1; RoundGlyph(int r) { radius = r; System.out.println( "RoundGlyph.RoundGlyph(), radius = " + radius); void { System.out.println( "RoundGlyph., radius = " + radius); public class PolyConstructors { public static void main(string[] args) { new RoundGlyph(5); 26 Actual Initialization Operations Glyph() before RoundGlyph., radius = 0 Glyph() after RoundGlyph.RoundGlyph(), radius = 5 1) The storage allocated for the object is initialized to binary zero before anything else happens 2) The base-class constructors are called as described previously. At this point, the overridden draw( ) method is called, which discovers a radius value of zero, due to step one 3) Member initializers are called in the order of declaration 4) The body of the derived-class constructor is called 27 Hands-on Java Seminar 2000 Bruce Eckel page 9

10 Example Is a Circle Square Line Subtyping : re-using the interface Cannot extend the interface Allows full substitutability Talks to Pure Inheritance vs. Extension Circle, Square, message Line, or new type of Useful void f() void g() MoreUseful void f() void g() void u() void v() void w() Is like a Assume this represents a big interface Extending the interface Subclassing : re-using the implementation. Prevents full substitutability Talks to Useful object message 28 Useful part More useful part Downcasting & Run-Time Type Identification (RTTI) Normally try to do everything with upcasting If you extend the class, you must downcast to access extended methods Java casts are always checked at run-time (safe) Upcast: always safe Useful void f() void g() MoreUseful void f() void g() void u() void v() void w() Downcast: must be checked 29 class Useful { public void f() { public void g() { class MoreUseful extends Useful { public void f() { public void g() { public void u() { public void v() { public void w() { Extends the interface 30 Hands-on Java Seminar 2000 Bruce Eckel page 10

11 Example cont. Example cont. Example public class RTTI { public static void main(string args[]) { Useful x[] = { new Useful(), new MoreUseful() ; x[0].f(); x[1].g(); // Compile-time: method not found in Useful: //! x[1].u(); ((MoreUseful)x[1]).u(); // Downcast/RTTI ((MoreUseful)x[0]).u(); // Exception thrown 31 //: c07:transmogrify.java // Dynamically changing the behavior of // an object via composition. abstract class Actor { abstract void act(); class HappyActor extends Actor { public void act() { System.out.println("HappyActor"); class SadActor extends Actor { public void act() { System.out.println("SadActor"); class Stage { Actor a = new HappyActor(); void change() { a = new SadActor(); void go() { a.act(); public class Transmogrify { public static void main(string[] args) { Stage s = new Stage(); s.go(); // Prints "HappyActor" s.change(); s.go(); // Prints "SadActor" ///:~ Also called the State Pattern Hands-on Java Seminar 2000 Bruce Eckel page 11

12 Return to Home Page Summary: The Key to OOP Abstraction Inheritance OOP Polymorphism Not just creating types, but proper behavior in all situations Allows decoupling of code from specific type it s acting on: easy reading, writing & extensibility 34 Click Here to Return to Home Page 35 Hands-on Java Seminar 2000 Bruce Eckel page 12