Java中的多态性

//Inheritance & upcasting

class Note {
	private int value;

	private Note(int val) {
		value = val;
	}

	public static final Note middleC = new Note(0), cSharp = new Note(1), cFlat = new Note(2);
} // Etc.

class Instrument {
	public void play(Note n) {
		System.out.println("Instrument.play()");
	}
}

// Wind objects are instruments
// because they have the same interface:
class Wind extends Instrument {
	// Redefine interface method:
	public void play(Note n) {
		System.out.println("Wind.play()");
	}
}

public class Music {
	public static void tune(Instrument i) {
		// ...
		i.play(Note.middleC);
	}

	public static void main(String[] args) {
		Wind flute = new Wind();
		tune(flute); // Upcasting
	}
}

//Overloading instead of upcasting
class Note2 {
	private int value;

	private Note2(int val) {
		value = val;
	}

	public static final Note2 middleC = new Note2(0), cSharp = new Note2(1), cFlat = new Note2(2);
} // Etc.

class Instrument2 {
	public void play(Note2 n) {
		System.out.println("Instrument2.play()");
	}
}

class Wind2 extends Instrument2 {
	public void play(Note2 n) {
		System.out.println("Wind2.play()");
	}
}

class Stringed2 extends Instrument2 {
	public void play(Note2 n) {
		System.out.println("Stringed2.play()");
	}
}

class Brass2 extends Instrument2 {
	public void play(Note2 n) {
		System.out.println("Brass2.play()");
	}
}

public class Music2 {
	public static void tune(Wind2 i) {
		i.play(Note2.middleC);
	}

	public static void tune(Stringed2 i) {
		i.play(Note2.middleC);
	}

	public static void tune(Brass2 i) {
		i.play(Note2.middleC);
	}

	public static void main(String[] args) {
		Wind2 flute = new Wind2();
		Stringed2 violin = new Stringed2();
		Brass2 frenchHorn = new Brass2();
		tune(flute); // No upcasting
		tune(violin);
		tune(frenchHorn);
	}
}

//Polymorphism in Java
class Shape {
	void draw() {
	}

	void erase() {
	}
}

class Circle extends Shape {
	void draw() {
		System.out.println("Circle.draw()");
	}

	void erase() {
		System.out.println("Circle.erase()");
	}
}

class Square extends Shape {
	void draw() {
		System.out.println("Square.draw()");
	}

	void erase() {
		System.out.println("Square.erase()");
	}
}

class Triangle extends Shape {
	void draw() {
		System.out.println("Triangle.draw()");
	}

	void erase() {
		System.out.println("Triangle.erase()");
	}
}

public class Shapes {
	public static Shape randShape() {
		switch ((int) (Math.random() * 3)) {
		default: // To quiet the compiler
		case 0:
			return new Circle();
		case 1:
			return new Square();
		case 2:
			return new Triangle();
		}
	}

	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] = randShape();
		// Make polymorphic method calls:
		for (int i = 0; i < s.length; i++)
			s[i].draw();
	}
}

//An extensible program
import java.util.*;

class Instrument3 {
	public void play() {
		System.out.println("Instrument3.play()");
	}

	public String what() {
		return "Instrument3";
	}

	public void adjust() {
	}
}

class Wind3 extends Instrument3 {
	public void play() {
		System.out.println("Wind3.play()");
	}

	public String what() {
		return "Wind3";
	}

	public void adjust() {
	}
}

class Percussion3 extends Instrument3 {
	public void play() {
		System.out.println("Percussion3.play()");
	}

	public String what() {
		return "Percussion3";
	}

	public void adjust() {
	}
}

class Stringed3 extends Instrument3 {
	public void play() {
		System.out.println("Stringed3.play()");
	}

	public String what() {
		return "Stringed3";
	}

	public void adjust() {
	}
}

class Brass3 extends Wind3 {
	public void play() {
		System.out.println("Brass3.play()");
	}

	public void adjust() {
		System.out.println("Brass3.adjust()");
	}
}

class Woodwind3 extends Wind3 {
	public void play() {
		System.out.println("Woodwind3.play()");
	}

	public String what() {
		return "Woodwind3";
	}
}

public class Music3 {
	// Doesn't care about type, so new types
	// added to the system still work right:
	static void tune(Instrument3 i) {
		// ...
		i.play();
	}

	static void tuneAll(Instrument3[] e) {
		for (int i = 0; i < e.length; i++)
			tune(e[i]);
	}

	public static void main(String[] args) {
		Instrument3[] orchestra = new Instrument3[5];
		int i = 0;
		// Upcasting during addition to the array:
		orchestra[i++] = new Wind3();
		orchestra[i++] = new Percussion3();
		orchestra[i++] = new Stringed3();
		orchestra[i++] = new Brass3();
		orchestra[i++] = new Woodwind3();
		tuneAll(orchestra);
	}
}