为什么尽管无法实例化接口，我们仍可以创建接口实例变量？

Question

I have following Interface declared

public interface MyInterface {

    void do_it_now();
} 

The I can do

public class MainClass{

    public static void main(String[] args) {
        MyInterface mainClass = new MyInterface() {

            @Override
            public void do_it_now() {


            }
        };

    }

}

Now My Question regarding the above code is by definition Interface can't be instantiated Whey we are allowed have a instance variable Type of Interface in java. What is the meaning of new MyInterface() line.

I want to know whats going on under the hood.

I have also gone through the post quite slimier to my question. but answer is not quite satisfactory to me .

Please don't give negative feedback or block my account if you found my question stupid post comment I will remove it.

Answer 1

Being able to have a variable of an interface type allows you to assign to that variable an instance of any class that implements that interface. Then you can use that variable to execute interface methods of that instance without caring about the specific implementation being used. It makes your code much more general, since you can switch to a different implementation of the interface without changing the code that uses the variable of the interface type.

Answer 2

You are making an "Anonymous Class" in the second code block. This means that it creates a class that implements the interface or class that you write. Its basically a short hand for making a subclass that implements the interface (MyInterface)

Answer 3

I want to know whats going on under the hood.

Consider the code below, assuming the interface MyInterface defined in your question.

Two inner classes are defined; the first class is anonymous (having no name) and the second class is named MyClass . Each of these two classes implements MyInterface.

// declare variable of type MyInterface
MyInterface myVariable;

// assign the variable to an instance of anonymous class that implements MyInterface
myVariable = new MyInterface() { 
    @Override
    public void do_it_now() {
    }
};

// define a named class that implements MyInterface
class MyClass implements MyInterface { 
    @Override
    public void do_it_now() {
    }
}

// assign the variable to an instance of named class that implements MyInterface
myVariable = new MyClass();

What's going on under the hood is, the java compiler compiles new MyInterface() {...}; into a separate class file with a name like $1.class , just like it compiles MyClass into a separate class file with a name like MyClass$1.class .