Question

In Java, if I have an interface:

public interface MyInterface{
}

Then MyInterface implementation is:

class MyClass implements MyInterface {
    public MyClass(int a) {
    }
}

So what I mean is that if a user wants to do declare a MyInterface instance with constructor:

MyInterface mine = new MyInterface(2);

then it is not possible right?

Answer 1

MyInterface mine = new MyInterface(2);

then it is not possible right?

That’s right. You can never do something like

MyInterface mine = new MyInterface(2);

After new you have to pick a class that implements the interface^(*), such as MyClass:

MyInterface mine = new MyClass(2);

Why?
You can think of an interface as a property of a class. An analogy would be an adjective, such as “Red”. It makes perfect sense to create, say, a red ball (new RedBall()) or a red car (new RedCar()), but just creating “red” (new Red()) doesn’t make sense (“red what??”).

^{(*) You can create anonymous classes that implement the interface on the fly by doing new MyInterface() { ... } but technically speaking you’re still instantiating a class.}