《OnJava》-- 接口

// interfaces/PureInterface.java
// Interface only looked like this before Java 8
public interface PureInterface {
    int m1(); 
    void m2();
    double m3();
}

• 在 Java 8之前我们可以这么说：interface 关键字产生一个完全抽象的类，没有提供任何实现。我们只能描述类应该像什么，做什么，但不能描述怎么做，即只能决定方法名、参数列表和返回类型，但是无法确定方法体。接口只提供形式，通常来说没有实现，尽管在某些受限制的情况下可以有实现。
• Java 8 中接口稍微有些变化，因为 Java 8 允许接口包含默认方法和静态方法。

使用关键字 interface 而不是 class 来创建接口。和类一样，需要在关键字 interface 前加上 public 关键字（但只是在接口名与文件名相同的情况下），否则接口只有包访问权限，只能在接口相同的包下才能使用它。

接口同样可以包含属性，这些属性被隐式指明为 static 和 final。

使用 implements 关键字使一个类遵循某个特定接口（或一组接口），它表示：接口只是外形，现在我要说明它是如何工作的。除此之外，它看起来像继承。

// interfaces/ImplementingAnInterface.java
interface Concept { // Package access
    void idea1();
    void idea2();
}

class Implementation implements Concept {
    @Override
    public void idea1() {
        System.out.println("idea1");
    }
    
    @Override
    public void idea2() {
        System.out.println("idea2");
    }
}

你可以选择显式地声明接口中的方法为 public，但是即使你不这么做，它们也是 public 的。所以当实现一个接口时，来自接口中的方法必须被定义为 public。否则，它们只有包访问权限，这样在继承时，它们的可访问权限就被降低了，这是 Java 编译器所不允许的。

默认方法

interface InterfaceWithDefault {
    void firstMethod();
    void secondMethod();
    
    default void newMethod() {
        System.out.println("newMethod");
    }
}

// onjava/Operations.java
package onjava;
import java.util.*;

public interface Operations {
    void execute();
    
    static void runOps(Operations... ops) {
        for (Operations op: ops) {
            op.execute();
        }
    }
    
    static void show(String msg) {
        System.out.println(msg);
    }
}

// interface/Machine.java
import java.util.*;
import onjava.Operations;

class Bing implements Operations {
    @Override
    public void execute() {
        Operations.show("Bing");
    }
}

class Crack implements Operations {
    @Override
    public void execute() {
        Operations.show("Crack");
    }
}

class Twist implements Operations {
    @Override
    public void execute() {
        Operations.show("Twist");
    }
}

public class Machine {
    public static void main(String[] args) {
        Operations.runOps(
        	new Bing(), new Crack(), new Twist());
    }
}

// interfaces/Adventure.java
// Multiple interfaces
interface CanFight {
    void fight();
}

interface CanSwim {
    void swim();
}

interface CanFly {
    void fly();
}

// 基类
class ActionCharacter {
    public void fight(){}
}

// 继承基类，实现多个接口
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 staic 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
    }
}

// interfaces/Factories.java
interface Service {
    void method1();
    void method2();
}

interface ServiceFactory {
    Service getService();
}

class Service1 implements Service {
    Service1() {} // Package access
    
    @Override
    public void method1() {
        System.out.println("Service1 method1");
    }
    
    @Override
    public void method2() {
        System.out.println("Service1 method2");
    }
}

class Service1Factory implements ServiceFactory {
    @Override
    public Service getService() {
        return new Service1();
    }
}

class Service2 implements Service {
    Service2() {} // Package access
    
    @Override
    public void method1() {
        System.out.println("Service2 method1");
    }
    
    @Override
    public void method2() {
        System.out.println("Service2 method2");
    }
}

class Service2Factory implements ServiceFactory {
    @Override
    public Service getService() {
        return new Service2();
    }
}

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 Service1Factory());
        // Services are completely interchangeable:
        serviceConsumer(new Service2Factory());
    }
}

// interfaces/Games.java
// A Game framework using Factory Methods
interface Game {
    boolean move();
}

interface GameFactory {
    Game getGame();
}

class Checkers implements Game {
    private int moves = 0;
    private static final int MOVES = 3;
    
    @Override
    public boolean move() {
        System.out.println("Checkers move " + moves);
        return ++moves != MOVES;
    }
}

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

class Chess implements Game {
    private int moves = 0;
    private static final int MOVES = 4;
    
    @Override
    public boolean move() {
        System.out.println("Chess move " + moves);
        return ++moves != MOVES;
    }
}

class ChessFactory implements GameFactory {
    @Override
    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());
    }
}