结论

假设有

1. class A implements Interface
2. class B extends A
3. class C extends A implements Interface.

那么:

• B和C都可以向上转型为Interface
• B和C在不覆盖接口定义的方法时都会用父类的实现
• 在反射方法getInterfaces()中, A和C类都会有Interface, 但是B不会有Interface

验证

Viewable接口:

package interfaces;

/**
 * Created by xiaofu on 17-11-5.
 */
public interface Viewable {
    void view();
}

BasePhoto类:

package interfaces;

/**
 * Created by xiaofu on 17-11-5.
 */
public class BasePhoto implements Viewable {

    @Override
    public void view() {
        System.out.println("viewing base photo");
    }
}

Selfie类:

package interfaces;

/**
 * Created by xiaofu on 17-11-5.
 */
public class Selfie extends BasePhoto {

    // 可以直接覆盖父类实现的接口方法, 而不用再在class声明中再写implement Viewable了
    @Override
    public void view() {
//        super.view();
        System.out.println("viewing my selfie");
    }
}

Landscape类:

package interfaces;

/**
 * Created by xiaofu on 17-11-5.
 * 父类已经实现了Viewable接口
 */
public class LandscapePhoto extends BasePhoto implements Viewable {

    @Override
    public void view() {
        System.out.println("viewing landscape photo");
    }
}

Main类:

package interfaces;

/**
 * Created by xiaofu on 17-11-5.
 */
public class Main {

    public static void testImplementation(){
        Viewable basePhoto = new BasePhoto();
        Viewable selfie = new Selfie();
        Viewable landscape = new LandscapePhoto();
        basePhoto.view();
        selfie.view();
        landscape.view();
        // 输出
        // viewing base photo
        // viewing my selfie
        // viewing landscape photo
    }

    public static void testRelection(){
        BasePhoto basePhoto = new BasePhoto();
        BasePhoto selfie = new Selfie();
        BasePhoto landscape = new LandscapePhoto();
        System.out.println("basePhoto has Viewable interface: " + hasInterface(basePhoto));
        System.out.println("selfie has Viewable interface: " + hasInterface(selfie));
        System.out.println("landscape has Viewable interface: " + hasInterface(landscape));
        // 输出
        // basePhoto has Viewable interface: true
        // selfie has Viewable interface: false
        // landscape has Viewable interface: true
    }

    private static boolean hasInterface(BasePhoto photo){
        Class<?>[] interfaces = photo.getClass().getInterfaces();
        for(Class<?> c: interfaces)
            if (c.equals(Viewable.class))
                return true;
        return false;
    }

    public static void main(String[] args) {
        testImplementation();
        testRelection();
    }

}

如果实现的接口有类型参数

该类情况, 子类要么不声明implements直接Override父类中该接口的方法, 要么声明的时候只能用和父类一样的类型参数.
原因如下(原链接):

A class may not at the same time be a subtype of two interface types which are different parameterizations of the same generic interface (§9.1.2), or a subtype of a parameterization of a generic interface and a raw type naming that same generic interface, or a compile-time error occurs.
This requirement was introduced in order to support translation by type erasure (§4.6).