理解多态

//为什么要上转型

//Note.java

package c07;

public enum  Note{middleC,cSharp,cFlat;}

//Music.java

package c07;

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


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


class Wind2 extends Instrument {
public void play(Note n) {
System.out.println("Wind2.play()      "+n);
}
}


public class Music {
public static void tune(Instrument i) {
// ...
i.play(Note.middleC);//关键是这里 i    play  
}
public static void main(String[] args) {
Wind flute = new Wind();
tune(flute); // Upcasting
Wind2 flute2 = new Wind2();
tune(flute2); // Upcasting
}
} ///:~

//不用上转型，很麻烦

//: Music2.java
//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);
 }//相对于上转型是多余的
 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);
 }
} ///:~