abstract是声明抽象类和抽象方法的关键字

包含抽象方法的类叫抽象类,如果一个类中包含一个或多个抽象方法,该类必须被限定为抽象的,否则编译器会报错,抽象类不可创建对象,创建抽象类的对象编译器会报错

  1. //: interfaces/music4/Music4.java
  2. // Abstract classes and methods.
  3. package object;
  4. import static net.mindview.util.Print.print;
  7. enum Note {
  8.     MIDDLE_C,MIDDLE_E,MIDDLE_F,
  9. }
  10. abstract class Instrument {
  11.   private int i; // Storage allocated for each
  12.   public abstract void play(Note n);
  13.   public String what() { return "Instrument"; }
  14.   public abstract void adjust();
  15. }
  17. class Wind extends Instrument {
  18.   public void play(Note n) {
  19.     print("Wind.play() " + n);
  20.   }
  21.   public String what() { return "Wind"; }
  22.   public void adjust() {}
  23. }
  25. class Percussion extends Instrument {
  26.   public void play(Note n) {
  27.     print("Percussion.play() " + n);
  28.   }
  29.   public String what() { return "Percussion"; }
  30.   public void adjust() {}
  31. }
  33. class Stringed extends Instrument {
  34.   public void play(Note n) {
  35.     print("Stringed.play() " + n);
  36.   }
  37.   public String what() { return "Stringed"; }
  38.   public void adjust() {}
  39. }    
  41. class Brass extends Wind {
  42.   public void play(Note n) {
  43.     print("Brass.play() " + n);
  44.   }
  45.   public void adjust() { print("Brass.adjust()"); }
  46. }
  48. class Woodwind extends Wind {
  49.   public void play(Note n) {
  50.     print("Woodwind.play() " + n);
  51.   }
  52.   public String what() { return "Woodwind"; }
  53. }    
  55. public class Music4 {
  56.   // Doesn\'t care about type, so new types
  57.   // added to the system still work right:
  58.   static void tune(Instrument i) {
  59.     // ...
  60.     i.play(Note.MIDDLE_C);
  61.   }
  62.   
  63.   static void tuneAll(Instrument[] e) 
  64.   {
  65.     for(Instrument i : e)
  66.       tune(i);
  67.   }    
  68.   public static void main(String[] args) {
  69.     // Upcasting during addition to the array:
  70.     Instrument[] orchestra = {
  71.       new Wind(),
  72.       new Percussion(),
  73.       new Stringed(),
  74.       new Brass(),
  75.       new Woodwind()
  76.     };
  77.     tuneAll(orchestra);
  78.   }
  79. } /* Output:
  80. Wind.play() MIDDLE_C
  81. Percussion.play() MIDDLE_C
  82. Stringed.play() MIDDLE_C
  83. Brass.play() MIDDLE_C
  84. Woodwind.play() MIDDLE_C
  85. *///:~

 

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