Upcasting. Taking an object reference and treating it as a reference to its base type is called upcasting.

Transcription

1 7. Polymorphism 1

2 Upcasting Taking an object reference and treating it as a reference to its base type is called upcasting. class Instrument { public void play () { public class Wind extends Instrument { public void breathe () { public class Music { public static void main (String[] args) { Wind flute = new Wind(); flute.play (); // inheritance flute.breathe (); Instrument instr = flute; // upcasting instr.play (); instr.breathe (); // Error!! Wind horn = instr; // Error!! Wind horn = (Wind) instr; // downcasting 2

3 Upcasting Casting from a derived type to a base type moves up on the inheritance diagram, so it s commonly referred to as upcasting. Upcasting is always safe because you re going from a more specific type to a more general type. You can also perform the reverse of upcasting, called downcasting, but this involves a dilemma that is the subject of Chapter 10. 3

4 Upcasting public class Wind extends Instrument { System.out.println("Wind.play() " + n); public class Stringed extends Instrument { System.out.println(" Stringed.play() " + n); public class Brass extends Instrument { System.out.println( Brass.play() " + n); public class Music2 { public static void tune (Wind w) { i.play (Note.MIDDLE_C); public static void tune (Stringed s) { s.play (Note.MIDDLE_C); public static void tune (Brass b) { b.play (Note.MIDDLE_C); public static void main(string[] args) { Wind flute = new Wind(); Stringed violin = new Stringed(); Brass frenchhorn = new Brass(); // Method overloading tune (flute); tune (violin); tune (frenchhorn); 4

5 Upcasting public class Wind extends Instrument { System.out.println("Wind.play() " + n); public class Stringed extends Instrument { System.out.println(" Stringed.play() " + n); public class Brass extends Instrument { System.out.println( Brass.play() " + n); public class Music { public static void tune (Instrument i) { i.play (Note.MIDDLE_C); public static void main(string[] args) { Wind flute = new Wind(); Stringed violin = new Stringed(); Brass frenchhorn = new Brass(); // Upcasting & Polymorphism tune (flute); tune (violin); tune (frenchhorn); 5

6 Method-call binding Connecting a method call to a method body is called binding. When binding is performed before the program is run (by the compiler and linker, if there is one), it s called early binding. late binding means that the binding occurs at run time, based on the type of object. Late binding is also called dynamic binding or run-time binding. All method binding in Java uses late binding unless the method is static or final (private methods are implicitly final). 6

7 Producing the right behavior Example: Shape Upcasting : Shape s = new Circle (); If you call one of the base-class methods s.draw (); Then, the proper Circle.draw( ) is called because of late binding (polymorphism). 7

8 Producing the right behavior class Shape { void draw () { void erase () { public class Circle extends Shape { void draw () { System.out.println ( Circle.draw() ); void erase () { System.out.println ( Circle.erase() ); public class Triangle extends Shape { void draw () { System.out.println ( Triangle.draw() ); void erase () { System.out.println ( Triangle.erase() ); public class Square extends Shape { void draw () { System.out.println ( Square.draw() ); void erase () { System.out.println ( Square.erase() ); 8

9 Producing the right behavior 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 ShapesTest { private static RandomShapeGenerator gen = new RandomShapeGenerator(); public static void main(string[] args) { Shape[] s = new Shape[9]; // Fill up the array with shapes: for(int i = 0; i < s.length; i++) s[i] = gen.next(); // Make polymorphic method calls: for(int i = 0; i < s.length; i++) s[i].draw(); 9

10 Abstract classes In all the instrument examples, the methods in the base class Instrument were always dummy methods. You create an abstract class when you want to manipulate a set of classes through this common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism. If you have an abstract class like Instrument, objects of that class almost always have no meaning. 10

11 Java provides a mechanism for doing this called the abstract method. This is a method that is incomplete; it has only a declaration and no method body. abstract void f () ; A class containing abstract methods is called an abstract class. If a class contains one or more abstract methods, the class itself must be qualified as abstract. You cannot create an object of an abstract class. If you inherit from an abstract class and you want to make objects of the new type, you must provide method definitions for all the abstract methods in the base class. 11

12 abstract class Instrument { private int i; // Storage allocated for each public abstract void play (Note n); public String what() { return "Instrument"; public abstract void adjust(); class Wind extends Instrument { System.out.println("Wind.play() " + n); public String what() { return "Wind"; public void adjust() { Wind class must implement play (Note n) and adjust ( ) methods. Otherwise, it makes a compile-time error. 12