Java abstract 关键字

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

//: interfaces/music4/Music4.java
// Abstract classes and methods.
package object;
import static net.mindview.util.Print.print;


enum Note {
    MIDDLE_C,MIDDLE_E,MIDDLE_F,
}
abstract class Instrument {
  private int i; // Storage allocated for each
  public abstract void play(Note n);
  public String what() { return "Instrument"; }
  public abstract void adjust();
}

class Wind extends Instrument {
  public void play(Note n) {
    print("Wind.play() " + n);
  }
  public String what() { return "Wind"; }
  public void adjust() {}
}

class Percussion extends Instrument {
  public void play(Note n) {
    print("Percussion.play() " + n);
  }
  public String what() { return "Percussion"; }
  public void adjust() {}
}

class Stringed extends Instrument {
  public void play(Note n) {
    print("Stringed.play() " + n);
  }
  public String what() { return "Stringed"; }
  public void adjust() {}
}    

class Brass extends Wind {
  public void play(Note n) {
    print("Brass.play() " + n);
  }
  public void adjust() { print("Brass.adjust()"); }
}

class Woodwind extends Wind {
  public void play(Note n) {
    print("Woodwind.play() " + n);
  }
  public String what() { return "Woodwind"; }
}    

public class Music4 {
  // Doesn't care about type, so new types
  // added to the system still work right:
  static void tune(Instrument i) {
    // ...
    i.play(Note.MIDDLE_C);
  }
  
  static void tuneAll(Instrument[] e) 
  {
    for(Instrument i : e)
      tune(i);
  }    
  public static void main(String[] args) {
    // Upcasting during addition to the array:
    Instrument[] orchestra = {
      new Wind(),
      new Percussion(),
      new Stringed(),
      new Brass(),
      new Woodwind()
    };
    tuneAll(orchestra);
  }
} /* Output:
Wind.play() MIDDLE_C
Percussion.play() MIDDLE_C
Stringed.play() MIDDLE_C
Brass.play() MIDDLE_C
Woodwind.play() MIDDLE_C
*///:~