interface這個關鍵字產生一個完全抽象的類,它根本就沒有提供任何具體的實現,它允許建立者確定方法名.引數列表和返回型別,但沒有任何方法體,介面只提供了形式,而未提供任何具體實現

一個介面表示:"所有實現了該特定介面的類看起來都像這樣".介面被用來建立類與類之間的協議(某些面嚮物件語言用關鍵字protocol來完成這一功能.)

要想建立一個介面,需要用interface關鍵字代替class關鍵字,就像類一樣,可以在interface前新增public關鍵字(但僅限於該介面在與其同名的檔案中被定義),如果不加public,則只有包訪問許可權,這樣就只能在同一個包內可用,介面也可以包含域,但是這些域都隱式地是static和final的

 要讓一個類遵循某個特定介面(或者是一組介面),需要使用implements關鍵字,它表示"interface只是它的外貌,但是現在我要宣告它如何工作的."除此之外,它看起來很像繼承

//: interfaces/music5/Music5.java
// Interfaces.
package object;
import static net.mindview.util.Print.*;
enum Note{
    MIDDLE_C,MIDDLE_D,MIDDLE_F
}
interface Instrument {
  // Compile-time constant:
  int
 
 VALUE = 5; // static & final 可以宣告域,但這些域都隱式地是static和final的
  // Cannot have method definitions:
  void play(Note n); // Automatically public //自動的是public
  void adjust();
}

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

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

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

class Brass extends Wind {
  public String toString() { return "Brass"; }
}    

class Woodwind extends Wind {
  public String toString() { return "Woodwind"; }
}

public class Music5 {
  // 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
*///:~

繼承和介面可以在同一個類同時使用

//: polymorphism/Sandwich.java
// Order of constructor calls.
package ch08;
interface FastFood{
    void show();
}
class Meal {
  Meal() { System.out.println("Meal()"); }
}

class Bread {
  Bread() { System.out.println("Bread()"); }
}

class Cheese {
  Cheese() { System.out.println("Cheese()"); }
}

class Lettuce {
  Lettuce() { System.out.println("Lettuce()"); }
}

class Lunch extends Meal {
  Lunch() { System.out.println("Lunch()"); }
}

class PortableLunch extends Lunch {
  PortableLunch() { System.out.println("PortableLunch()");}
}

public class Sandwich extends PortableLunch implements FastFood{ //繼承和介面可以在同一個類同時使用
  private Bread b = new Bread();
  private Cheese c = new Cheese();
  private Lettuce l = new Lettuce();
  public void show()
  {
      System.out.println("pushing your sandwich order");
  }
  public Sandwich()
  {
      System.out.println("Sandwich()"); 
      show();
  }
  public static void main(String[] args) {
    new Sandwich();
  }
} /* Output:
Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()
pushing your sandwich order

*///:~