What does it mean to say that an interface is also a type?

Question

I think this means that through the concept of polymorphism, a variable type can be declared as an interface type. For example: if Animal is an interface, you can specify the following:

Animal simba = new Lion(); 

Do I understand this correctly? Thanks for any help.

Answer 1

Each object has a type (and value). * There are many types: primitive types (e.g. int ), class types (e.g. string ), enumerations, arrays (and maybe I forget about some).

The term “interface” refers to a particular type of class type: it is a class that does not have member objects (safe constants) and only public methods, all of which are abstract. **

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

_{*) It is clear that the type says: "What is the structure of this," and the meaning says "what is the content." Type 5 is int , and its value ... well, 5.}

_{**) This allows you to inherit from several interfaces, while in Java it is impossible to inherit from several common classes.}

Answer 2

An interface can act like superclass in Java, because it can take the type of its implementation.

In your example, in particular, you define Animal as the interface that implements Lion .

Because of this, you can create an Animal type Lion . The code will 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 right, however, in saying that this is an example of polymorphism .