music.java

《《Thinking in Java》中文版》包括：《Thinking in Java》和《ThinkInJava2000》

//: c07:music:Music.java 
// From 'Thinking in Java, 2nd ed.' by Bruce Eckel
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
// Inheritance & upcasting.

class Note {
  private int value;
  private Note(int val) { value = val; }
  public static final Note
    MIDDLE_C = new Note(0), 
    C_SHARP  = new Note(1),
    B_FLAT   = 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.MIDDLE_C);
  }
  public static void main(String[] args) {
    Wind flute = new Wind();
    tune(flute); // Upcasting
  }
} ///:~