说接口也是一种类型是什么意思？

Question

I think that it means that via the concept of polymorphism, a variable's type can be declared to be that of an interface. For ex: if Animal is an interface, you could code the following:

Animal simba = new Lion();

Am I correct in my understanding of this? Thanks for any help.

Answer 1

An interface can act similarly to a superclass in Java, in that it can take on the type of its implementations.

In your example, specifically, you define Animal as an interface which Lion implements.

Because of this, you can create an Animal of type Lion . The code would look like this:

interface Animal {
  //do interfacing stuff
}

class Lion implements Animal {
  public Lion() {
    //...
  }
  //do implementing stuff
}

class Driver {
  Animal simba = new Lion(); // This works because of polymorphism.
}

You are correct, though, in saying that this is an example of polymorphism .

Answer 2

Every object has a type (and a value).* There are many types: primitive types (like int ), class types (like string ), enums, arrays (and perhaps I'm forgetting some).

The term "interface" refers to a particular sort of class type: it is a class which has no member objects (safe constants) and only public methods, all of which are abstract .**

So: an interface is a special sort of class, which in turn is a special sort of type. So interfaces are types. In your example, both Animal and Lion are types, and one happens to be convertible to the other.

_{*) Loosely, the type says "what is the structure of this", and the value says "what is the content of this". The type of 5 is int , and its value is... well, 5.}

_{**) This enables you to inherit from multiple interfaces , while it is not possible in Java to inherit from multiple general classes.}