Thinking in Java中上溯造型的疑问

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);
}
}