明确表示类实现接口

Question

在Java和类似的语言中，必须显式地说一个类实现一个接口的真正原因是什么？

想象一下隐式实现：

interface Flyer { void fly(); }

public class Duck { public void fly() {...} }

public class Plane { public void fly() {...} }

public class Hoe { void hangAround() {...} }

void startFlying(Flyer flyer) {...}

void race() {
    ...
    startFlying(duck);  // OK
    startFlying(plane); // OK
    startFlying(hoe);   // Compilation error.
}

interface Runner { void run(); }

public class Guy { public void run(); }
public class Gal { public void run(); }

void startRunning(Runner r) {...}

void race() {
    startRunning(guy);            // OK
    startRunning(gal);            // OK
    startRunning(runnableThread); // OK
}

public class Guy **implements Runner **{ public void run(); }
public class Gal **implements Runner **{ public void run(); }

粗体（或**和**之间的文字）是你必须付出的代价

void race() {
    startRunning(guy);            // OK
    startRunning(gal);            // OK
    startRunning(runnableThread); // Compilation error!
}

注意编译错误，以便在测试前看到问题。

如果打算在startRunning（）中也使用runnableThread，则必须更加明确地执行（有足够的时间来意识到您在做什么）。

public class Airbus {
    void takeOff() {...}
    void land() {...}
    Passenger[] getPassengers() {...}
}

public class Ladybug {
    void takeOff() {...}
    void land() {...}
}
public class Duck {
    void takeOff() {...}
    void land() {...}
    Passenger[] getPassengers() {...}
}

public interface Aircraft {
    void takeOff();
    void land();
    Passenger[] getPassengers();
}

public void fly(Aircraft aircraft) {
    aircraft.takeOff();
    for (Passenger p : aircraft.getPassengers()) {...}
    aircraft.land();
}

public void airport() {
    fly(airbus_a380); // OK
    fly(ladybug);     // Compilation error, does not match Aircraft requirements.
    fly(duck);        // OK
}

public interface Lander {
    void land();
}

public void landAtMeadow(Lander lander) {...}

public void meadow() {
    landAtMeadow(airbus_a380); // OK
    landAtMeadow(duck);        // OK
    landAtMeadow(ladybug);     // OK
}

public interface Lander { void landAtLimitedArea(int size); }

Answer 1

Java是一种强类型化语言，因此必须在编译时验证将实例分配给接口类型化变量。这只能通过显式声明的接口实现来完成。