Java 语法 : Who can an interface be an object?

Question

我目前正在研究观察者模式，但遇到了一些困惑。

当我读到这行代码时:

IObserverSubscribe user1= new ConcreteObserverYoutubeUser();

我认为接口(interface) IObserverSubscribe user1 正在创建和实例化 new ConcreteObserverYoutubeUser()。这让我有点困惑，因为通常声明的同一个类也在实例化。是不是应该这样:

IObserverSubscribe user1= new IObserverSubscribe();

为什么接口(interface)装饰可以实例化另一个类？

完整代码如下:

主要内容:

package observerpattern;

public class ObserverPattern {

    /**
     * The observer pattern is a software design pattern in which
     *  an object, called the subject, maintains a list of its dependents,
     *  called observers, and notifies them automatically of any 
     *  state changes, usually by calling one of their methods.
     *  It is mainly used to implement distributed event handling systems.
     *  The Observer pattern is also a key part in the familiar
     *  Model View Controller (MVC) architectural pattern. 
     */
    public static void main(String[] args) {

        SubjectYouTubeChannel sytc= new SubjectYouTubeChannel();// create youtube channel
        IObserverSubscribe user1= new ConcreteObserverYoutubeUser();
        IObserverSubscribe user2= new ConcreteObserverYoutubeUser();
        IObserverSubscribe moderator1= new ConcreteObserverYoutubeModerator();

        sytc.Subscribe(user1);
        sytc.Subscribe(user2);
        sytc.Subscribe(moderator1);
        sytc.Unsubscribe(user2);

        sytc.notifySubscribers();


    }

}

主题:

package observerpattern;

import java.util.ArrayList;
import java.util.List;
import observerpattern.IObserverSubscribe;

public class SubjectYouTubeChannel {    
    private List<IObserverSubscribe> subscribers = new ArrayList<IObserverSubscribe>(); 
    public void Subscribe(IObserverSubscribe ios){
        subscribers.add(ios);       
    }   
    public void Unsubscribe(IObserverSubscribe ios){        
        subscribers.remove(ios);
    }   
    public void notifySubscribers(){        
        for(IObserverSubscribe ios : subscribers ){         
            ios.Notify();           
        }       
    }
}

界面观察者:

package observerpattern;

public interface IObserverSubscribe {

    void Notify();

}

具体观察者:

package observerpattern;

public class ConcreteObserverYoutubeUser implements IObserverSubscribe{

    @Override
    public void Notify() {
        System.out.println("User watches video, comments, ect");

    }

}

Answer 1

new IObserverSubscribe();

这是非法的，因为您无法实例化接口(interface) - 根据定义，它们没有任何...它们方法的定义。

IObserverSubscribe user1= new ConcreteObserverYoutubeUser();

这里的左边表示 user1 是 atleast IObserverSubscribe 类型，即指向一个实例，其类型实现了接口(interface).

另一方面，右侧实际上创建具体类型的实例并分配给user1。这是可能的，因为具体类型实现了接口(interface)。