《java编程思想》第九章 接口

接口和内部类为我们提供了一种将接口与实现分离的更加结构化的方法。

9.1抽象类和抽象方法

通用接口建立起一种基本形式，以此表示所有导出类的共同部分。

抽象方法：仅有声明没有方法体。

abstract void f();

包含抽象方法的类叫做抽象类，如果一个类包含一个或多个抽象方法，这个类必须被限制为抽象类。

如果从一个抽象类继承，并想创建该新类的对象，那么就必须为基类中的所有抽象方法提供方法定义。

//: interfaces/music4/Music4.java
// Abstract classes and methods.
package interfaces.music4;
import polymorphism.music.Note;
import static net.mindview.util.Print.*;

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
*///:~

他们可以使类的抽象性明确起来，并告诉用户和编译器打算怎样来使用它们。

9.2接口

interface关键字产生一个完全抽象的类，它根本就没有提供任何具体实现。接口只提供形式，而未提供任何实现。

一个接口表示“所有实现了该特定接口的类都一样”。所以接口用来建立类与类之间的协议。

接口中的方法即使不加public默认也是public的。

//: interfaces/music5/Music5.java
// Interfaces.
package interfaces.music5;
import polymorphism.music.Note;
import static net.mindview.util.Print.*;

interface Instrument {
  // Compile-time constant:
  int VALUE = 5; // static & final
  // Cannot have method definitions:
  void play(Note n); // Automatically 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
*///:~

无论将其向上转型为instrument的普通类、抽象类、接口都可以，因为行为都是相同的。

9.3完全解耦

策略模式：一个根据所传递的对象的不同而具备不同行为的方法：

//: interfaces/classprocessor/Apply.java
package interfaces.classprocessor;
import java.util.*;
import static net.mindview.util.Print.*;

class Processor {
  public String name() {
    return getClass().getSimpleName();
  }
  Object process(Object input) { return input; }
}	

class Upcase extends Processor {
  String process(Object input) { // Covariant return
    return ((String)input).toUpperCase();
  }
}

class Downcase extends Processor {
  String process(Object input) {
    return ((String)input).toLowerCase();
  }
}

class Splitter extends Processor {
  String process(Object input) {
    // The split() argument divides a String into pieces:
    return Arrays.toString(((String)input).split(" "));
  }
}	

public class Apply {
  public static void process(Processor p, Object s) {
    print("Using Processor " + p.name());
    print(p.process(s));
  }
  public static String s =
    "Disagreement with beliefs is by definition incorrect";
  public static void main(String[] args) {
    process(new Upcase(), s);
    process(new Downcase(), s);
    process(new Splitter(), s);
  }
} /* Output:
Using Processor Upcase
DISAGREEMENT WITH BELIEFS IS BY DEFINITION INCORRECT
Using Processor Downcase
disagreement with beliefs is by definition incorrect
Using Processor Splitter
[Disagreement, with, beliefs, is, by, definition, incorrect]
*///:~

复用代码的第一种方法，客户端程序员遵循接口来编写类：

//: interfaces/interfaceprocessor/StringProcessor.java
package interfaces.interfaceprocessor;
import java.util.*;

public abstract class StringProcessor implements Processor{
  public String name() {
    return getClass().getSimpleName();
  }
  public abstract String process(Object input);
  public static String s =
    "If she weighs the same as a duck, she's made of wood";
  public static void main(String[] args) {
    Apply.process(new Upcase(), s);
    Apply.process(new Downcase(), s);
    Apply.process(new Splitter(), s);
  }
}	

class Upcase extends StringProcessor {
  public String process(Object input) { // Covariant return
    return ((String)input).toUpperCase();
  }
}

class Downcase extends StringProcessor {
  public String process(Object input) {
    return ((String)input).toLowerCase();
  }
}

class Splitter extends StringProcessor {
  public String process(Object input) {
    return Arrays.toString(((String)input).split(" "));
  }	
} /* Output:
Using Processor Upcase
IF SHE WEIGHS THE SAME AS A DUCK, SHE'S MADE OF WOOD
Using Processor Downcase
if she weighs the same as a duck, she's made of wood
Using Processor Splitter
[If, she, weighs, the, same, as, a, duck,, she's, made, of, wood]
*///:~

适配器中的代码接受所拥有的接口，产生需要的接口：

//: interfaces/interfaceprocessor/FilterProcessor.java
package interfaces.interfaceprocessor;
import interfaces.filters.*;

class FilterAdapter implements Processor {
  Filter filter;
  public FilterAdapter(Filter filter) {
    this.filter = filter;
  }
  public String name() { return filter.name(); }
  public Waveform process(Object input) {
    return filter.process((Waveform)input);
  }
}	

public class FilterProcessor {
  public static void main(String[] args) {
    Waveform w = new Waveform();
    Apply.process(new FilterAdapter(new LowPass(1.0)), w);
    Apply.process(new FilterAdapter(new HighPass(2.0)), w);
    Apply.process(
      new FilterAdapter(new BandPass(3.0, 4.0)), w);
  }
} /* Output:
Using Processor LowPass
Waveform 0
Using Processor HighPass
Waveform 0
Using Processor BandPass
Waveform 0
*///:~

将接口从具体实现中解耦，使得接口可以应用于多种不同的具体实现，因此代码也就更具有可复用性。

9.4Java中的多重继承

继承和实现接口同事使用的时候，继承必须放在前边：

使用接口的核心原因：可以向上转型为多个基类型。

第二个原因：防止客户端程序员创建该类的对象，确保仅仅建立一个对象。

//: interfaces/Adventure.java
// Multiple interfaces.

interface CanFight {
  void fight();
}

interface CanSwim {
  void swim();
}

interface CanFly {
  void fly();
}

class ActionCharacter {
  public void fight() {}
}	

class Hero extends ActionCharacter
    implements CanFight, CanSwim, CanFly {
  public void swim() {}
  public void fly() {}
}

public class Adventure {
  public static void t(CanFight x) { x.fight(); }
  public static void u(CanSwim x) { x.swim(); }
  public static void v(CanFly x) { x.fly(); }
  public static void w(ActionCharacter x) { x.fight(); }
  public static void main(String[] args) {
    Hero h = new Hero();
    t(h); // Treat it as a CanFight
    u(h); // Treat it as a CanSwim
    v(h); // Treat it as a CanFly
    w(h); // Treat it as an ActionCharacter
  }
} ///:~

9.5通过继承来扩展接口

//: interfaces/HorrorShow.java
// Extending an interface with inheritance.

interface Monster {
  void menace();
}

interface DangerousMonster extends Monster {
  void destroy();
}

interface Lethal {
  void kill();
}

class DragonZilla implements DangerousMonster {
  public void menace() {}
  public void destroy() {}
}	

interface Vampire extends DangerousMonster, Lethal {
  void drinkBlood();
}

class VeryBadVampire implements Vampire {
  public void menace() {}
  public void destroy() {}
  public void kill() {}
  public void drinkBlood() {}
}	

public class HorrorShow {
  static void u(Monster b) { b.menace(); }
  static void v(DangerousMonster d) {
    d.menace();
    d.destroy();
  }
  static void w(Lethal l) { l.kill(); }
  public static void main(String[] args) {
    DangerousMonster barney = new DragonZilla();
    u(barney);
    v(barney);
    Vampire vlad = new VeryBadVampire();
    u(vlad);
    v(vlad);
    w(vlad);
  }
} ///:~

组合多个接口时候，注意接口中的方法名重复问题

9.6适配接口

接口吸引人的特点，允许同一个接口具有多个不同的具体实现。

//: interfaces/RandomWords.java
// Implementing an interface to conform to a method.
import java.nio.*;
import java.util.*;

public class RandomWords implements Readable {
  private static Random rand = new Random(47);
  private static final char[] capitals =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
  private static final char[] lowers =
    "abcdefghijklmnopqrstuvwxyz".toCharArray();
  private static final char[] vowels =
    "aeiou".toCharArray();
  private int count;
  public RandomWords(int count) { this.count = count; }	
  public int read(CharBuffer cb) {
    if(count-- == 0)
      return -1; // Indicates end of input
    cb.append(capitals[rand.nextInt(capitals.length)]);
    for(int i = 0; i < 4; i++) {
      cb.append(vowels[rand.nextInt(vowels.length)]);
      cb.append(lowers[rand.nextInt(lowers.length)]);
    }
    cb.append(" ");
    return 10; // Number of characters appended
  }
  public static void main(String[] args) {
    Scanner s = new Scanner(new RandomWords(10));
    while(s.hasNext())
      System.out.println(s.next());
  }
} /* Output:
Yazeruyac
Fowenucor
Goeazimom
Raeuuacio
Nuoadesiw
Hageaikux
Ruqicibui
Numasetih
Kuuuuozog
Waqizeyoy
*///:~






//: interfaces/RandomDoubles.java
import java.util.*;

public class RandomDoubles {
  private static Random rand = new Random(47);
  public double next() { return rand.nextDouble(); }
  public static void main(String[] args) {
    RandomDoubles rd = new RandomDoubles();
    for(int i = 0; i < 7; i ++)
      System.out.print(rd.next() + " ");
  }
} /* Output:
0.7271157860730044 0.5309454508634242 0.16020656493302599 0.18847866977771732 0.5166020801268457 0.2678662084200585 0.2613610344283964
*///:~




//: interfaces/AdaptedRandomDoubles.java
// Creating an adapter with inheritance.
import java.nio.*;
import java.util.*;

public class AdaptedRandomDoubles extends RandomDoubles
implements Readable {
  private int count;
  public AdaptedRandomDoubles(int count) {
    this.count = count;
  }
  public int read(CharBuffer cb) {
    if(count-- == 0)
      return -1;
    String result = Double.toString(next()) + " ";
    cb.append(result);
    return result.length();
  }	
  public static void main(String[] args) {
    Scanner s = new Scanner(new AdaptedRandomDoubles(7));
    while(s.hasNextDouble())
      System.out.print(s.nextDouble() + " ");
  }
} /* Output:
0.7271157860730044 0.5309454508634242 0.16020656493302599 0.18847866977771732 0.5166020801268457 0.2678662084200585 0.2613610344283964
*///:~

9.7接口中的域

放入接口中的任何域都自动是static和final的。

接口中的域不是接口的一部分，它们的值被存储在该接口的静态存储区域中。

9.8嵌套接口

//: interfaces/nesting/NestingInterfaces.java
package interfaces.nesting;

class A {
  interface B {
    void f();
  }
  public class BImp implements B {
    public void f() {}
  }
  private class BImp2 implements B {
    public void f() {}
  }
  public interface C {
    void f();
  }
  class CImp implements C {
    public void f() {}
  }	
  private class CImp2 implements C {
    public void f() {}
  }
  private interface D {
    void f();
  }
  private class DImp implements D {
    public void f() {}
  }
  public class DImp2 implements D {
    public void f() {}
  }
  public D getD() { return new DImp2(); }
  private D dRef;
  public void receiveD(D d) {
    dRef = d;
    dRef.f();
  }
}	

interface E {
  interface G {
    void f();
  }
  // Redundant "public":
  public interface H {
    void f();
  }
  void g();
  // Cannot be private within an interface:
  //! private interface I {}
}	

public class NestingInterfaces {
  public class BImp implements A.B {
    public void f() {}
  }
  class CImp implements A.C {
    public void f() {}
  }
  // Cannot implement a private interface except
  // within that interface's defining class:
  //! class DImp implements A.D {
  //!  public void f() {}
  //! }
  class EImp implements E {
    public void g() {}
  }
  class EGImp implements E.G {
    public void f() {}
  }
  class EImp2 implements E {
    public void g() {}
    class EG implements E.G {
      public void f() {}
    }
  }	
  public static void main(String[] args) {
    A a = new A();
    // Can't access A.D:
    //! A.D ad = a.getD();
    // Doesn't return anything but A.D:
    //! A.DImp2 di2 = a.getD();
    // Cannot access a member of the interface:
    //! a.getD().f();
    // Only another A can do anything with getD():
    A a2 = new A();
    a2.receiveD(a.getD());
  }
} ///:~

9.9接口与工厂

接口是实现多重继承的途径，而生成遵循某个接口的对象的典型方式就是工厂方法设计模式。

工厂方法的结构：

//: interfaces/Factories.java
import static net.mindview.util.Print.*;

interface Service {
  void method1();
  void method2();
}

interface ServiceFactory {
  Service getService();
}

class Implementation1 implements Service {
  Implementation1() {} // Package access
  public void method1() {print("Implementation1 method1");}
  public void method2() {print("Implementation1 method2");}
}	

class Implementation1Factory implements ServiceFactory {
  public Service getService() {
    return new Implementation1();
  }
}

class Implementation2 implements Service {
  Implementation2() {} // Package access
  public void method1() {print("Implementation2 method1");}
  public void method2() {print("Implementation2 method2");}
}

class Implementation2Factory implements ServiceFactory {
  public Service getService() {
    return new Implementation2();
  }
}	

public class Factories {
  public static void serviceConsumer(ServiceFactory fact) {
    Service s = fact.getService();
    s.method1();
    s.method2();
  }
  public static void main(String[] args) {
    serviceConsumer(new Implementation1Factory());
    // Implementations are completely interchangeable:
    serviceConsumer(new Implementation2Factory());
  }
} /* Output:
Implementation1 method1
Implementation1 method2
Implementation2 method1
Implementation2 method2
*///:~

如果不是用工厂方法，代码就必须在某处指定将要创建的service的确切类型，以便调用合适的构造器。

//: interfaces/Games.java
// A Game framework using Factory Methods.
import static net.mindview.util.Print.*;

interface Game { boolean move(); }
interface GameFactory { Game getGame(); }

class Checkers implements Game {
  private int moves = 0;
  private static final int MOVES = 3;
  public boolean move() {
    print("Checkers move " + moves);
    return ++moves != MOVES;
  }
}

class CheckersFactory implements GameFactory {
  public Game getGame() { return new Checkers(); }
}	

class Chess implements Game {
  private int moves = 0;
  private static final int MOVES = 4;
  public boolean move() {
    print("Chess move " + moves);
    return ++moves != MOVES;
  }
}

class ChessFactory implements GameFactory {
  public Game getGame() { return new Chess(); }
}	

public class Games {
  public static void playGame(GameFactory factory) {
    Game s = factory.getGame();
    while(s.move())
      ;
  }
  public static void main(String[] args) {
    playGame(new CheckersFactory());
    playGame(new ChessFactory());
  }
} /* Output:
Checkers move 0
Checkers move 1
Checkers move 2
Chess move 0
Chess move 1
Chess move 2
Chess move 3
*///:~

9.10总结

接口时一种重要的工具，防止滥用。