java定義接口的關鍵字interface，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,它允許創建者確定方法名.參數列表和返回類型,但沒有任何方法體,接口只提供了形式,而未提供任何具體實現

一個接口表示:"所有實現了該特定接口的類看起來都像這樣".接口被用來建立類與類之間的協議(某些面向對象語言用關鍵字protocol來完成這一功能.)

要想創建一個接口,需要用interface關鍵字代替class關鍵字,就像類一樣,可以在interface前添加public關鍵字(但僅限于該接口在與其同名的文件中被定義),如果不加public,則只有包訪問權限,這樣就只能在同一個包內可用,接口也可以包含域,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接口(或者是一組接口),需要使用implements關鍵字,它表示"interface只是它的外貌,但是現在我要聲明它如何工作的."除此之外,它看起來很像繼承

恰當的原則是優先選擇類而不是接口,從類開始,如果接口的必要性變的非常明確,那么就進行重構.

//:?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?可以聲明域,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
*///:~

繼承和接口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{ //繼承和接口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
*///:~

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